1 Bedrock geology

1.1 Introduction

The Newtonmore–Ben Macdui district (Figure 1) includes the southern parts of the Cairngorm and the Monadhliath mountains extending south across the Gaick plateau into the Forest of Atholl and upper Glen Tilt. It constitutes a rugged, sparsely populated terrain with rounded hills and ridges generally rising to 600–900 m OD. In the south-east of the district the quartzite ridges such as Beinn a’ Ghlo and Carn an Righ rise above 1000 m. Mountains only rise above 1200 m within the granitic Cairngorm massif, where Ben Macdui forms the highest summit at 1309 m. Newtonmore and Kingussie are the main centres of population and lie in the upper Spey valley which forms the conduit for the main road and rail links to Inverness.

The bulk of the rocks in the district are Neoproterozoic metasedimentary strata, deformed and metamorphosed during the Grampian Event of the Caledonian Orogeny at about 470 Ma and cut by Siluro-Devonian intrusive rocks. The oldest rocks, in the north-west of the district (Figure 1), are the Glen Banchor Subgroup which was also affected by a tectonothermal event at about 800 Ma.. Formerly the Glen Banchor succession of Smith et al. (1999), these rocks are now formally assigned subgroup status in the Badenoch Group. Badenoch Group rocks are overlain by the Dalradian Supergroup and there appears to be an orogenic unconformity between the two; although it is largely obscured by ductile shearing. The Glen Banchor Subgroup, comprising gneissose psammites and semipelites, has uncertain Knoydartian or Moinian affinities and its base is not exposed. Isotopic studies indicate that the comparable Dava Subgroup to the north was affected by pre-Caledonian (c. 840–800 Ma) events (Noble et al., 1996; Highton et al., 1999).

The Dalradian Supergroup (Harris et al., 1994) is an extensive, mainly metasedimentary, succession ranging from Neoproterozoic to Arenig in age (Soper et al., 1999, Tanner and Sutherland, 2007). It was deposited on the Laurentian continental margin during and following the break-up of the supercontinent Rodinia. Only the lower parts, i.e. the Grampian, Appin and Argyll groups, crop out in the district. The basal Grampian Group comprises a thick succession of siliciclastic deposits that are mainly psammitic, but with subsidiary intervals of semipelite. Locally, the preservation of sedimentary structures allows a coherent lithostratigraphy to be established. This lithostratigraphy has been extended across postulated depositional rift basins (Banks, 2005; Leslie et al., 2006), so that the equivalents of the Corrieyairack and Glen Spean subgroups can be identified to the south-east of the major Ericht–Laidon Fault (Figure 1).

The extensive Gaick Psammite Formation is the predominant component of the Glen Spean Subgroup and was deposited in a shallow marine environment. It now lies in a stack of recumbent Caledonian folds facing sideways to the south. The broad expanse of Gaick Psammite becomes more steeply inclined south-eastwards and passes stratigraphically up into the basal beds of the Appin Group near the major NE-trending Loch Tay Fault (Figure 1). This distinct lithological change is obscured by a high strain zone, the Boundary Slide, related to the earlier recumbent folding. To the south-east of the slide and the Loch Tay Fault, all the subgroups of the Appin Group are represented although the basal Lochaber Subgroup appears to be condensed and/or attenuated (Figure 1). The latter subgroup has been recognised in the Atholl area as the Glen Banvie Formation and farther north, in the Dee catchment as the Tom Anthon Mica Schist Formation. The succeeding Ballachulish Subgroup includes a sequence ranging from prodelta graphitic pelite through turbiditic into coarse delta-top quartzitic beds capped by phyllites and metalimestones. Substantial thicknesses of quartzite, which are more resistant to erosion, form mountainous ridges such as Beinn a’ Ghlo. The overlying Blair Atholl Subgroup (Figure 1) comprises graphitic pelites and metalimestones with cleaner semipelites, psammites and carbonates towards the top as deposition continued in more oxygenated waters. At the base of the Argyll Group in Glen Tilt, impersistent glacigenic boulder beds lie within the succeeding quartzitic Islay Subgroup. This subgroup includes the Schiehallion Quartzite as exposed in Glen Tilt, and Creag Leacach Quartzite in Glen Fearnach, both of which are relatively thin developments of the subgroup compared to the Schiehallion district. The overlying Easdale Subgroup begins with graphitic pelites indicating a gradual deepening and a deeper water character to the basin. The overlying calcareous schists and psammites signal a gradual shallowing.

As the basin continued to expand during latest Neoproterozoic times, basic volcanic rocks erupted and minor basic sills/dykes and granitic sheets were intruded. These rocks were all deformed during the Caledonian Orogeny, dominated by the Grampian (Ordovician) deformation phase, which formed the Gaick Fold Complex and the Tay Nappe (Leslie et al., 2006). The frequent changes in lithology and competence in the Appin and Argyll groups gave rise to the numerous folds accompanied by attenuation and sliding/slicing of the lithostratigraphy. The orogeny resulted in Barrovian regional metamorphism of the nappe pile and the rocks presently exposed lie essentially within the kyanite zone but few lithologies have suitable mineral compositions to produce metamorphic index minerals. Locally semipelites are migmatitic. The Caledonian Orogeny terminated with extensive calc-alkaline plutonic magmatism that produced the Cairngorm and Glen Tilt plutons and numerous minor intrusions during late Silurian to early Devonian time.

The concluding period of rapid uplift and erosion in Devonian times was accompanied by brittle movements focussed on the major NE-trending Ericht–Laidon Fault and Loch Tay Fault systems.

1.2 Previous work

The area covered by this explanation includes classic areas for the study of geology ever since 1785 when ‘the father of modern geology’ James Hutton made an excursion up Glen Tilt, (Figure 1) searching for proof of his ‘theory of the earth’ (Hutton, 1788, 1794; Craig et al., 1978; Stephenson, 1999). He had been invited by the Duke of Atholl to accompany him in the shooting season into Glen Tilt (Playfair, 1805). To the north-east of Forest Lodge Hutton found metasedimentary rocks invaded, veined and recrystallised by granite, indicating that the latter intruded as a hot magma. These observations attracted other geologists such as Lord Webb Seymour (1815) to the glen and MacCulloch (1816) went so far as to report ‘having blown up a considerable portion of the rock, I am enabled to say that it is of a laminated texture throughout’. These were already considered classic geological localities.

Nicol (1844; 1863) gave his overall opinion on Central Highland Geology. Later in the 19th century Murchison and Geikie (1861) traversed the Highlands and drew sections through Glen Tilt. They noted how the quartz-rocks would originally have been sandstones and granular limestones were once calcareous mudstones. Early in the 20th century the Geological Survey workers (Barrow et al., 1913) referred the bulk of the psammites (i.e. Grampian Group) to the ‘Moine Series’ and the quartzite, limestone and schist succession to a Perthshire or Central Highland Series, which was one thin sequence intensely interfolded. The ‘Newer’ granite masses of Glen Tilt and the Cairngorms were distinguished from ‘Older’, foliated biotite-granites or augen gneiss sheets.

Bailey (1925) recognised that the metasedimentary Banvie Burn succession is separate from the rest of the Perthshire succession. However, the quartzite (of the Perthshire succession) on Beinn a’ Ghlo was miscorrelated with that at Schiehallion (Bailey, 1925; Pantin, 1961). Thomas (1965; 1979; 1980) described the Iltay Boundary Slide between the psammitic Atholl Nappe and the Tay Nappe. The Atholl Nappe was rooted to the north-west in a primary steep belt near Newtonmore and folded over the secondary Drumochter Dome to the south-east. The presence of the Ballachulish Subgroup east of Glen Tilt was recognised (Smith and Harris, 1976) and the geology around the Beinn a’ Ghlo range was revised by Smith (1980).

The debate on the status of the gneissose rocks in the north-west of the area was started when Piasecki and van Breemen (1979) described an older Central Highland ‘Division’ basement and a younger cover (regarded an equivalent of the Grampian Group) separated by a ductile shear termed the Grampian Slide. Piasecki (1980) interpreted the slide as a deformed unconformity containing pegmatites dated at about 720 Ma (Knoydartian) and correlated part of the basement rocks with the Glenfinnan Group of the Moine Supergroup in the Northern Highlands. Subsequently the basement rocks were referred to as the Central Highland Migmatite Complex (Highton, 1992; Harris et al., 1994; Stephenson and Gould, 1995) but more recently they have been referred to the Glen Banchor (Figure 1) & (Figure 2) and Dava successions (Smith et al., 1999). The latter workers also described the Grampian Group successions in the Corrieyairack and Strathtummel basins either side of a Glen Banchor ‘basement high’.

Studies of the Cairngorm Pluton (Figures 1 & 4) were made by Harry (1965) and Harrison (1986, 1987). Harry (1965) concluded that the pluton was a stock-like intrusion and distinguished ‘Porphyritic Granite’ on Carn Ban Mor and beside Loch Einich from the ‘Main Granite’. Harrison (1986, 1987) recognised three textural facies within the Main Granite and four other phases within the pluton. The relationships of the granites and diorites in the Glen Tilt Pluton (Figure 1) have been described (Deer, 1938, 1950, 1953; Mahmood, 1986; Beddoe- Stephens, 1999). Beddoe-Stephens (1999) concluded that the biotite granodiorite/granite was emplaced early in the intrusive sequence and that localised melt remobilisation and back veining accompanied diorite emplacement.

The foliated ‘Older’granites, including the one east of upper Glen Tilt (Clachghlas), were considered to have been emplaced between D2 and D3 by Bradbury et al. (1976). More recent dating (Rodgers et al., 1989) and studies of the similar Ben Vuirich Granite (Tanner and Leslie, 1994: Tanner; 1996; Tanner et al., 2006) indicated that the analogous foliated granites are probably pre-tectonic intrusions.

Field and isotopic studies by Piasecki and van Breemen (1983) were carried out on pegmatites in shear zones within the Glen Banchor Subgroup (their Laggan Inlier of the Central Highland Division). They concluded that the quartz and pegmatite veins were segregations formed during ductile shearing under amphibolite-facies conditions. Muscovites from the veins in the Glen Banchor Subgroup yielded ages between about 750 and 700 Ma (Piasecki and van Breemen, 1983). The mineralogy and geochemistry of the shear zones associated with the Glen Banchor Subgroup were described by Hyslop and Piasecki (1999).

The recently published papers based on work by the geological survey have elucidated the basin architecture in the northern Grampian Highlands including the Glen Banchor Subgroup (Smith et al., 1999) and recognised that the Appin and Grampian groups onlap on to the Glen Banchor ‘high’ (Robertson and Smith, 1999). The timing and P-T conditions of the regional metamorphism have been discussed (Phillips et al., 1999). The lithostratigraphy and structure of the Gleann Fearnach area (Figure 3) has been described in a memoir on the adjacent Glen Shee district (Crane et al., 2002). They recognised that the Carn an Righ, Carn Dallaig, Glen Loch and Glen Lochsie slides were folded around the Meall Reamhar Synform (Figure 7). They also demonstrated that, as a result of regional metamorphism of the mafic rocks, garnet amphibolites in Gleann Mor and Gleann Bheag lay to the north of garnet-epidote amphibolites. Thermobarometry calculations on the garnet amphibolite indicated temperatures of 540–590°C at 8.6 Kbar.

In the Gaick area, the fold complex (Figure 5) & (Figure 6) in the Grampian Group psammites has been related to the Tay Nappe structure (Leslie et al., 2006). A sedimentological and provenance study of the Grampian Group in the northern Grampian Highlands (Banks, 2005) has described its depositional facies and established the links between the Corrieyairack and Strathtummel basins.

1.3 Lithostratigraphy

1.3.1 Glen Banchor Subgroup of the Badenoch Group

This metasedimentary succession, up to 1 km thick, including distinct gneissose and migmatitic lithologies (Smith et al., 1999), is exposed along Glen Banchor to the west of Newtonmore (Figure 2) and continues northwards on to the Tomatin Sheet 74W (British Geological Survey, 2004) and south-westwards on to the Dalwhinnie Sheet 63E (British Geological Survey, 2000a) as part of an antiformal structure. It was previously referred to the ‘Central Highland Division’ (Piasecki and van Breemen, 1979; 1983; Piasecki and Temperley, 1988) although its definition is not precisely the same. This is because the development of migmatitic and gneissose rocks depends largely on geochemical composition and cannot form the basis of discriminating basement-cover relationships. In Glen Banchor, Piasecki and Temperley (1988) proposed a tripartite division with a Lower Siliceous ‘group’, a Middle Pelitic ‘group’ and an Upper Psammitic ‘group’. They considered that the succession is separated from the Grampian Group by the Grampian Slide which incorporates sheared pegmatites dated at c. 750 Ma (Piasecki and van Breemen, 1983). However, since the base of the succession is not exposed and because the succession is highly tectonised and metamorphosed, its internal stratigraphy and true relationship with the Grampian Group are not known. No sedimentary structures are preserved within it, but despite the sheared and faulted relationship with the Grampian Group, the rocks are considered to be part of an older Neoproterozoic shallow marine succession unconformably overlain by the Grampian Group (Robertson and Smith, 1999; Banks, 2005). It was probably deposited before c. 850 Ma and the youngest concordant detrital zircon from it has been dated at 900 +/− 17 Ma (Cawood et al., 2003).

The succession comprises interlayered semipelite and K-feldspar-bearing banded psammite, some of which is distinctly gneissose. Subordinate quartzites are commonly associated with a transition from psammitic to semipelitic units within the succession (Figure 2). Although there are no way-up indicators, there appears to be a sequence from, structurally lowest, interbedded psammite and semipelite into striped and gneissose semipelite and quartzite followed by a psammitic unit.

The striped semipelites and psammites are well exposed on the northern flank of Glen Banchor, on Creag Liath [NH 665 005] and on Creag an h-Iolaire [NH 675 015], where interlayered psammite and semipelite, quartzite and striped semipelite crop out around a large synform plunging north-north-east.

In places the sequence contains migmatitic or flaggy and micaceous intervals. The subordinate non-gneissose, predominantly psammitic, rocks are K-feldspar rich and petrographically similar to the feldspathic psammites that dominate the Grampian Group (Phillips et al., 1999). Massive garnet-muscovite semipelite or schist intervals locally contain prominent burgundy-coloured garnets and pass into semipelite with thin layers of micaceous psammite (Plate 1).

Some semipelites contain coarse muscovite porphyroblasts and these have locally been found to contain relict kyanite. The principal foliation in the Glen Banchor Subgroup is generally a composite S0/S1 or, commonly in semipelitic lithologies, an S1/S2 foliation of biotite ± muscovite. In ductile shear-zones the earlier fabrics are transposed or replaced by a locally phyllonitic, muscovite-bearing, S2 shear-fabric. Sheared semipelites also contain fibrous sillimanite within foliae ((Plate 2), Sample no. S99663 Creag an Loin [NH 6997 0058]).

Trace amounts of fibrolite were also noted (S99692) in the highly foliated garnet-muscovite schistose semipelite from Allt Coire Shairaidh [NN 6698 9800].

In Coire an Eich [NN 6683 9980] massive white feldspathic quartzite with lenses of garnet- zoisite-tremolite-bearing calcsilicate rock as well as interbanded quartzite and siliceous psammite are exposed. Distinctive gneissose quartzites also contain K-feldspar and have a more massive appearance.

The Glen Banchor and Dava subgroups have been identified as basement to the Dalradian Supergroup (Smith et al., 1999) but the inferred unconformity is affected by ductile shearing. Deformation of the succession has been described in the structural section. Within these basement subgroups, blastomylonites developed along ductile shear-zones and are associated geochemically with deformed pegmatites (Hyslop and Piasecki, 1999). These pegmatites were thought to have formed during the ductile shearing at around 750 Ma (Rb-Sr muscovite ages, Piasecki and van Breemen, 1979) and are considered to have been formed during the Knoydartian Orogeny. Piasecki and van Breemen (1983) made a detailed study of tectonic schists with sheared pegmatite veins within the Glen Banchor Subgroup (their Central Highland Division) at a locality on Creag Shiaraidh [NN 663 978] in Glen Banchor (Figure 2) and another on Craig Blargie on Sheet 63W. At Creag Shiaraidh, muscovite books from deformed veins, quartz-feldspar aggregates and whole-rock tectonic schists were dated isotopically using Rb/Sr methods. The ages ranged from 739+/−8 Ma to 702+/−8 Ma with an overall regression age of 727+/−17 Ma. The micas from syn-D2 and late-D2 veins could not be separated isotopically and this was taken as support for the interpretation that the veins and the tectonic schists developed together due to interaction between the schists and fluids active in the slide zones (Piasecki and van Breemen, 1983). U-Pb analyses of monazites from large pegmatites in similar tectonic situations at A’ Bhuideanaich and Lochindorb to the north yielded ages of about 800 Ma (Noble et al., 1996). U-Pb dating of single zircon grains within Dava Subgroup migmatites near Slochd yielded an age of 840 +/−11 Ma (Highton et al., 1999) and this has been interpreted as evidence of a Knoydartian tectonothermal event, which did not affect the Dalradian Supergroup. Although the Glen Banchor Subgroup is cut by a Caledonian pegmatitic sheet-complex there is no record of earlier migmatite in Glen Banchor rocks, which are overprinted by younger Caledonian fabrics.

Studies of U-Pb isotopes in detrital zircons from the Glen Banchor Subgroup have shown that the source rocks can be grouped into several age ranges; at about 1800, about 1650 and a smaller group about 1100 Ma (Cawood et al., 2003). The youngest concordant detrital grain yielded an age of 900+/−17Ma. This detrital age range is similar to that of the Dava Subgroup and the Moine Supergroup. The detrital zircon provenance record is also similar to that within the overlying Grampian Group (Cawood et al., 2003) and so the two units may have had similar Proterozoic sources.

In the Newtonmore district only one small amphibolite plug (Figure 2) is recorded at [NN 653 974] intruding the succession (cf. Robertson and Smith, 1999). Since a similar body occurs within the Grampian Group nearby, these intrusions may relate to the extensional magmatic event at about 590 Ma (see Section 2.3). However, other small amphibolite bodies within the Glen Banchor Subgroup (Figure 2) on Sheet 63E (for example, at [NN 579 927]) contain the assemblage garnet-hornblende-plagioclase-ilmenite-titanite-quartz. These rocks have textural similarities to eclogitic amphibolites (Baker, 1986) and the garnets have coronas of plagioclase- hornblende without quartz. Quartz is abundant but is always separated from garnet coronas by a reaction rim of hornblende, and Baker (1986) suggested that the rock once contained the assemblage garnet-omphacite-quartz, indicative of an early high-pressure metamorphic event. Preliminary observations (Cuthbert, 2008) indicate that the amphibolite shared the migmatisation of the country-rock, hence the eclogite-facies metamorphism predates the migmatisation (about 840 Ma) and is younger than the youngest reliable detrital zircon age in the Glen Banchor Subgroup (900±17 Ma).

1.3.2 Dalradian Supergroup

This succession of metasedimentary rocks with subordinate metavolcanic rocks (Harris et al., 1994) occupies the majority of the Newtonmore–Ben Macdui district. Taking the Glen Banchor Subgroup in the north-west corner of the district (Figure 1) and (Figure 2) as basement to the supergroup (Smith et al., 1999), all the succeeding strata belong to the Dalradian Supergroup. The supergroup has an overall younging to the south-east, starting with the dominantly psammitic Grampian Group, passing through the shelfal limestones, quartzites and pelites of the Appin Group into the deeper water quartzites, pelites, calcareous semipelites and basic volcanic rock of the Argyll Group.

The supergroup represents part of a former extensive continental shelf succession which is thought to have started (Grampian Group) in a broad ensialic rift which opened north-eastwards to form a marine gulf. The Grampian Group is dominantly a deep-water turbiditic pile passing up into a mainly shallow marine shelf succession. The metasedimentary rocks belonging to the Appin and Argyll groups represent deposition on the north-west side of the widening gulf. The Appin Group can be subdivided into subgroups which have persistent character along the continental shelf; i.e. the Ballachulish, Blair Atholl and Islay subgroups. The Ballachulish Subgroup has a persistence of lithological type, which can be extended from the classic Dalradian succession in the Appin area north-east into Aberdeenshire and south-west to Donegal. The Blair Atholl Subgroup begins to show signs of instability in the depocentre and these become more emphatic in Islay Subgroup times. Deeper water deposits of the Easdale Subgroup (Argyll Group) are associated with extensional rifting.

The Dalradian Supergroup below the Ben Lawers Schist Formation in this district is thought to have been deposited in the Cryogenian Period and its boundary with the overlying Ediacaran is located within the Easdale Subgroup (at the base of the Cranford Limestone in Donegal, McCay et al., 2006).

The thickness of the Dalradian succession and the length of time that supposed continuous sedimentary deposition operated (pre-750 Ma to 470 Ma) have caused several authors to question whether there are not distinct stratigraphical unconformities (e.g. Prave, 1999) or even orogenic unconformities (e.g. Dempster et al., 2002) within the supergroup. This district continues to have potential for the study of these questions.

1.3.2.1 Grampian Group

The Grampian Group is a widespread conformable succession of psammites, feldspathic quartzites and semipelites. The group has been divided in this district into a lower Corrieyairack Subgroup and an upper Glen Spean Subgroup (Table 1) on the basis of a lithostratigraphy established in the western Grampian Highlands where the succession is more complete. The thick siliciclastic package was deposited in a series of intracratonic basins created during the attempted break-up of the supercontinent Rodinia (Banks, 2005). In the Newtonmore district, only a small part of the Grampian Group was deposited in the Corrieyairack depocentre, which lies to the north-west of the Glen Banchor palaeohigh. The bulk of the group was deposited in the Strathtummel Basin on the south-east side of the ‘high’.

Corrieairack Subgroup

The Corrieyairack Subgroup comprises psammitic and semipelitic formations deposited in deep water basins by turbiditic currents. It was defined in the Corrieyairack Basin and is a transgressive sequence of submarine fans that passes up gradually into cleaner psammites with minor semipelite and quartzite lenses. On the Newtonmore Sheet, the Creag Meagaidh Psammite, which crops out to the north of the Glen Banchor palaeohigh and west of the Craig Liath Fault (Figure 1), was deposited in the Corrieyairack Basin. The equivalent sedimentary formations to the south and east of the Glen Banchor ‘high’ were deposited in the Strathtummel Basin (Table 1).

West of the Craig Liath Fault

Creag Meagaidh Psammite Formation. In the Corrieyairack Basin to the north-west of the Glen Banchor palaeohigh, i.e. the north-west corner of Sheet 64W, this formation is the uppermost unit of the Corrieyairack Subgroup or the Corrieyairack ‘Turbidite Complex’ Deep Water Association of Glover et al. (1995). The formation comprises grey psammite and micaceous psammite with mica laminae and calcsilicate-rock lenses. It is dominated by monotonous sequences of grey, flaggy, fissile micaceous psammite in medium to thinly bedded (c. 10–15 cm thick) units that preserve normal grading into thin (c. 1 cm thick) schistose semipelite tops (Banks, 2005). Where observed, the grading is considered to reflect primary graded bedding. The formation is exposed on the slopes west of Allt Fionndrigh [NH 656 019] dipping steeply to the south-east. The total thickness of the formation is not exposed on Sheet 64W but it is estimated to be 750 m thick in the Dalwhinnie district.

The Fara Psammite Formation. Feldspathic psammites, correlated with the Fara Psammite Formation on Sheet 63E (Dalwhinnie), lie on the western margin of Sheet 64W, and west of the Creag Liath Fault. The Fara Psammite Formation comprises grey flaggy psammite and micaceous psammite in attenuated beds each now typically less than 10 cm thick as exposed on the adjacent Sheet 63E.

Mashie Semipelite Formation. This formation was established to the west of the Creag Liath Fault (on Sheet 63E), but is not exposed in this district. The semipelite is schistose to gneissose and in tectonic contact with the Glen Banchor Subgroup. It has a gradational boundary with the Fara Psammite Formation and is at least 100 m thick. It is of uncertain stratigraphical position but is probably part of the Corrieyairack Subgroup and considered to correlate with the Coire nan Laogh Semipelite Formation (Banks, 2005). It also could correlate with the Creag na Sanais Semipelite if part of the Fara Psammite belongs to the Glen Spean Subgroup.

Between the Craig Liath and Glen Truim/Ericht–Laidon faults

Torr na Truim Semipelite Formation. This formation is a dominantly schistose semipelite although locally gneissose and more massive (Leslie et al., 2003). It belongs to the Corrieyairack Subgroup but its base is cut out by faulting. It was considered by Banks (2005) to correlate with the Coire nan Laogh Semipelite Formation. It includes subordinate thin quartzite, psammite and micaceous psammite units, and intercalations of finer grained quartzose semipelites are common. Fine-grained psammite units 1–2 cm thick occur in several outcrops. Psammite units become thicker towards the top of the formation but the junction with the overlying Creag Dhubh Psammite Formation is poorly exposed. Where schistose semipelite is present, muscovite and biotite are common together with quartz and feldspar porphyroblasts. Small garnet porphyroblasts occur mainly in the more quartzose units.

The formation is well exposed in the type area around Torr na Truim [NN 681 954] where the schistose semipelite has an F3 crenulation-cleavage dipping 15°NE. In the forested area south and east of Mains of Glen Truim [NN 680 944], craggy outcrops of semipelite are commonly gneissose with small quartz and plagioclase segregations in a biotite-muscovite-quartz-feldspar ± garnet matrix. Thickness  750 m. According to Banks (2005) this formation comprisesis estimated as  mudstone facies M1 and M2. M1 comprises homogeneous mudstone units tens of metres thick suggestive of quiescent, low-energy sedimentation. M2 contains homogeneous mudstone units separated by quartzitic beds. No unequivocal sedimentary structures are preserved, although a fine, compositional banding may reflect original planar lamination. The inclusion of clean sands within the muddy facies indicates that either in-situ sediment was reworked by tidal/current activity or that reworked sands were washed in from adjacent environments. A nearshore shelf is a plausible depositional environment (Banks, 2005).

Creag Dhubh Psammite Formation. This formation belongs to the Corrieyairack Subgroup and typically comprises graded, thin-bedded to laminated psammite with micaceous psammite and semipelite interbeds. These mainly belong to a facies association (MS4 of Banks, 2005) in which sheet-like, massive muddy sandstones and graded muddy sandstones occur in laterally extensive homogeneous successions. The association includes small-scale thinning and thickening upwards cycles and is interpreted to form in depositional lobes of lower submarine fans. The sheet-like nature of the beds suggests a lack of significant sea-floor topography (Banks, 2005). Subordinate thinly bedded, graded, muddy sandstone-mudstone couplets (Facies association MS6 of Banks, 2005) are also laterally continuous. Their nature suggests deposition by weak/dilute turbidity currents in an interlobe or distal lobe-fringe setting.

In its type area near Creag Dhubh, the transitional junction with the stratigraphically underlying Torr na Truim Formation is observed at [NN 690 983], about 1.5 km to the north-east of Creag Dhubh cairn. In the north, the formation consists of a grey psammite with micaceous psammite layers showing grading to thin schistose semipelite tops. The overall thickness for the formation is estimated to be 700–1000 m. On Creag Dhubh there are massive amalgamated psammites, 5 m thick, and in the cliffs to the south [NN 671 958] are stacked, upward bed-thinning cycles with graded bedding and widespread calcsilicate lenses. North-east of Creag Dhubh summit and folded round it, is an interbanded interval [NN 685 980], up to 200 m thick, in which semipelite is more common than psammite towards the base of the predominantly psammite formation. Minor interbanded psammite and semipelite units are also mapped within the formation, e.g. at [NN 681 961].

The Creag Dhubh Psammite Formation crops out extensively between Cnocan na h-Oidche Uvie [NN 668 947] and Cruban Beag [NN 668 923]. South of Am Binnein [NN 667 910] the formation is displaced by the Ericht–Laidon Fault and west of Am Binnein the formation is structurally and stratigraphically overlain by the Creag na Sanais Semipelite Formation. North- east of Creag a’ Chrubain [NN 676 931] the lowest exposed beds are grey flaggy fine-grained psammites, 1–5 cm thick, with thin micaceous psammitic to semipelitic partings. Where the preservation state is sufficient, these are interpreted locally as graded beds with semipelitic tops. The semipelitic fraction is generally less than 1 cm thick and only rarely are semipelite bands or lenses up to 10–20 cm thick present. Minor thin fine-grained calcsilicate lenses are also present. Farther west up-slope, psammites with individual beds 10–40 cm thick are common; exceptionally the beds reach 70 cm thick. Amalgamated psammites up to 6 m thick are inferred, probably indicating channelled deposits. Beds generally dip moderately westwards but changes in younging direction indicate that tight to isoclinal folds with wavelengths of 400–500 m are present. Apart from the grading, little else in the way of primary sedimentary structure can be seen due to lack of clean exposures and the effects of deformation.

North-west of the Ericht–Laidon Fault, on the south-eastern slopes of Creag Bheag, near Kingussie, medium-bedded grey psammitic units grade up into thin semipelitic tops. This sequence youngs into structurally overlying quartzitic psammites with thin garnetiferous semipelites at the top of the formation at [NH 742 012] lying below a predominantly semipelitic unit now correlated with the Falls of Phones Semipelite Formation. The Pitmain Semipelite Member is schistose to gneissose semipelite with thin beds of psammite bound by interlayered units of striped quartzose psammite, semipelite and garnetiferous calcsilicate rock. It occurs on the northern margin of sheet 64W at [NH 726 022] interbedded within the Creag Dhubh Psammite Formation. The semipelite at [NH 745 015] north of Pitmain farm, near Kingussie, which was termed the Pitmain Semipelite is now correlated with the Falls of Phones Semipelite Formation (see below). This makes the use of the name Pitmain Semipelite Member for the unit within the Creag Dhubh Psammite Formation confusing and a change to a name such as the ‘Creag Mhor Semipelite Member’ (see the Tomatin sheet 74W) would be more appropriate.

South-east of the Ericht–Laidon Fault, south of Etteridge, interbedded grey fine-grained psammites, micaceous psammites, semipelites and pelites dip south-east structurally below the Falls of Phones Semipelite Formation. The interbedded sequence was formerly termed the Etteridge Lodge Psammite and Pelite Formation (Leslie et al., 2003) but has since been correlated with the Creag Dhubh Psammite Formation (Leslie et al., 2006). The turbiditic psammites exposed at Feshiebridge [just north-east of Sheet 64W at [NH 852 043]], and on Creag Dhubh [NN 824 997] about 2 km west of Glen Feshie, are also now assigned to the Creag Dhubh Psammite (Leslie et al., 2006). A good section through the upward change from graded, turbiditic psammite through the Falls of Phones Semipelite and into the quartz-rich psammites at the base of the Gaick Psammite Formation is exposed on Creag na Sroine [NN 840 969].

Glen Spean Subgroup

The Neoproterozoic Glen Spean Subgroup comprises distinctive semipelites and clean feldspathic and siliceous psammites which have a widespread distribution in the district (Figure 1). In the Newtonmore area, the subgroup consists of a thin semipelite formation below a much thicker, more widespread psammite formation. The semipelite is termed the Falls of Phones Semipelite Formation to the east of the Glen Truim and Ericht–Laidon faults (Table 1). West of these faults the Creag na Sanais Semipelite Formation is probably equivalent to the Falls of Phones Semipelite (Leslie et al., 2003) with the overlying Allt nan Biorag Member equivalent to the lower part of the Gaick Psammite Formation (Table 2). This relationship is most probably similar to that of the Clachaig Semipelite below the Inverlair Psammite in the Corrieyairack depocentre (Banks, 2005).

The bulk of the psammites to the north-west of the Loch Tay Fault on Sheet 64E and W belong to the Gaick Psammite Formation. These quartzofeldspathic rocks have a distinct biotite-rich lamination, which in areas of low strain, can be shown to reflect original bedding. In places this includes cross-bedding and indications of shallow water deposition. This persists throughout the subgroup in the Strathtummel Basin that lies to the south-east of the Glen Banchor high and extends up to the Loch Tay Fault. Quartzofeldspathic psammites interpreted as shallow-water deposits to the south-east of the Loch Tay Fault have not been assigned to a particular formation as they are interbedded with impersistant limestones and quartzites. These are atypical of the Gaick Psammite Formation and may reflect a passage into the previously recognised but now obsolete Strathtummel Subgroup of Treagus (2000) which has been offset sinistrally by the Loch Tay Fault.

Creag na Sanais Semipelite Formation. The Creag na Sanais Semipelite Formation consists of dark grey or brownish grey, gneissose semipelite containing local thin fine-grained biotite-rich and quartzose psammite interbeds. Its base is marked by the incoming of a coherent gneissose semipelite above thinly interbedded psammites and semipelites at the top of the Creag Dhubh Psammite Formation. The semipelite is coarsely foliated in places, and elongate quartz grains and feldspar porphyroblasts are wrapped by the biotite-and-muscovite-dominated foliation. Thin fine-grained biotitic and quartzose psammite beds contain scattered garnets (Leslie et al., 2003). The formation is well exposed in the type area on the slopes south of Creag na Sanais [NN 655 921] and the crags to the south at [NN 657 917].

The facies association is mudstone with quartzite but sedimentary structures are very limited. The formation is considered to represent a prodeltaic mud in a transition from deep water turbiditic to shelfal lithofacies which occurs above the Creag Dhubh Psammite Formation. This means that the Creag na Sanais Semipelite Formation is the lowermost formation in the Glen Spean Subgroup and probably correlates with the Falls of Phones Semipelite (Table 1) on the south-east side of the Ericht–Laidon Fault (Banks, 2005). West of Creag na Sanais, the overlying Allt na Biorag Member forms part of the Gaick Psammite Formation. Full resolution of the lithostratigraphy in this area will require some re-investigation of the Fara Psammite Formation in Sheet 63E (Dalwhinnie).

Falls of Phones Semipelite Formation. This semipelite formation lies to the south-east of the Ericht –Laidon Fault and was established at the type locality at the Falls of Phones on Allt Phoineis [NN 707 933]. It is also exposed near the A9 road at [NN 6776 9105] and on the slopes south-east of Etteridge Farm at [NN 6945 9237], where it is interbedded with thin psammite beds. The semipelitic unit at [NH 745 015] near Pitmain Farm is also correlated with this formation. The semipelite is mainly gneissose, but locally schistose with local lenses of micaceous psammite. It contains mainly biotite, quartz and feldspar with subordinate muscovite and garnet. The exposures near Pitmain Farm include subsidiary quartzose psammites, quartzites and calcsilicate rocks.

The formation is about 100 m thick and passes down transitionally into the Creag Dubh Psammite Formation. At the type locality, local channels of cross-bedded biotite-rich psammite indicate younging to the south-east where it is structurally and stratigraphically overlain by the Gaick Psammite Formation (Table 1), (Table 2). As with the Creag na Sanais Semipelite, the formation probably represents a prodeltaic mud.

(Table 1): Relationships of the subgroups, formations and members of the Grampian Group present on Sheet 64

Gaick Psammite Formation. This monotonous succession of flaggy laminated biotite psammitic rocks is extensively exposed from Glen Truim to the Loch Tay Fault. It belongs to the Glen Spean Subgroup (Table 1) as correlated from the Corrieyairack Basin into the Strathtummel Basin. The formation is widespread and has had several names (Table 2).

The formation comprises mainly thin or medium-bedded psammite, subordinate quartz-rich psammite and micaceous psammite with minor amounts of quartzite and semipelite. Calcsilicate lenses are small and scattered. The base of the formation is marked locally by a c. 10–20 m-thick unit of quartzitic psammite or quartzite. Good exposures of this unit occur on Creag na Sroine [NN 840 969]. This transitional passage towards its base reflects the change from deeper water to overlying deltaic deposits forming the bulk of the formation.

The banding or layering within the psammites reflects variation on 10–30 cm scale between the pale grey and darker grey quartzo-feldspathic layers but internal pelitic to semipelitic laminae are also common on a mm scale. This reflects the occurrence of biotite as disseminated flakes or concentrated into laminae between granoblastic quartz-plagioclase +/− K-feldspar layers. Garnet is a minor component in some lithologies. Muscovite and chlorite are rare and commonly products of retrograde metamorphism. A few quartzose psammites occur within the formation and in places, such as near Glen Derry [NO 038 996], quartzite interbeds are mapped. Small scale cross-bedding and rare graded beds within weakly deformed psammites indicate way-up of the succession but only a few cases of reliable way-up are preserved (Plate 3). In some examples, psammites become less micaceous upwards, in the same direction of younging as indicated by ripples or cross-bedding. This suggests deposition in shallow water with some current- winnowing of the sediment.

(Table 2) Stratigraphical jargon-buster for the Glen Spean Subgroup in the Strathtummel Basin (adapted from Banks et al., 2006; 2007)

Well-exposed sections in the Gaick Psammite occur along the Allt Bhran [NN 767 900] and [NN 770 897], where typical banded lithologies are exposed with examples of cross-bedded units. Also in Allt Bhran [NN 76559 90129], well-preserved sedimentary cross-lamination is folded by minor south-east-facing F2 structures (Leslie et al., 2006). In exposures around Loch an Duin [NN 729 803], flaggy psammites display rounded F2 fold hinges about subhorizontal axial planes. Other well-exposed sections lie in Allt a’ Chama/Edendon Water [NN 71 79] and Allt Gharbh Ghaig (Plate 4).

Overall the succession youngs to the south-east in a stack of recumbent folds which are structurally overlain by the Lochaber and Ballachulish subgroups near the ‘Boundary Slide’ which lies close to the Loch Tay Fault. Near the Loch Tay Fault minor impersistent interbeds of semipelite occur. The Glen Spean Subgroup psammite to the south-east of the Loch Tay Fault has not been included in the Gaick Psammite Formation as it is also associated with quartzite and limestone units (c.f. the Strathtummel Subgroup of Treagus (2000)).

Allt nam Biorag Psammite Member. This member (Table 1), (Table 2) lies to the east of the Craig Liath Fault where it forms the lower part of the Gaick Psammite Formation (Banks, 2005). It comprises quartzitic and quartzose psammites with minor calcsilicate lenses typical of the lower part of the Gaick Psammite Formation. Small lenses of gneissose semipelite have been mapped within the unit, which is cut out to the west by the Craig Liath Fault. The member is well exposed on Meall Ruigh nam Biorag [NN 655 905] south-east of Allt nam Biorag, in the type area. On this ridge, examples of cross-laminated units occur in quartzose psammites.

1.3.2.2 Appin Group

This group heralds a distinct change to alternating units and coarsening upwards cycles of metalimestone, pelite, semipelite, psammite and quartzite in the Lochaber, Ballachulish and Blair Atholl subgroups (Table 3). Several of the pelitic and calcareous formations are graphitic. The thickness of the group varies as it is partly diachronous with the Grampian Group but also onlaps onto the Glen Banchor ‘high’. In this district the majority of the group crops out to the south-east of the Loch Tay Fault (Figure 3).

Lochaber Subgroup

The Lochaber Subgroup is of variable facies and has been considered as a transition between the thick pile of psammites (Grampian Group) and the shelfal muds, carbonates and quartz sands of the Appin Group. The gross lithological contrast between the groups is most probably the reason for the development of the Boundary Slide in this district. The subgroup is mainly limited to the north-western side of the Loch Tay Fault on Sheet 64E where the Glen Banvie Formation has been established. It is affected by the Boundary Slide and invaded by the Glen Tilt Pluton so that its true lithological sequence is difficult to ascertain. Farther south-west, for instance near Crianlarich, the Lochaber Subgroup includes a basal quartzitic formation (Harris et al., 1994). Such a quartzite cannot be readily correlated in the Glen Tilt area and may not have been deposited or is strongly drawn out by the sliding (cf. Treagus, 2000). South-east of the Loch Tay Fault, the Tom Anthon Mica Schist Formation overlies the Grampian Group on the eastern margin of the sheet.

Glen Banvie Formation. The Glen Banvie Formation (Figure 3) is considered part of the Lochaber Subgroup established farther west (Treagus, 2000) and compares with the Loch Treig Schist and Quartzite or Leven Schist formations (Key et al., 1997). It comprises two members, the lower heterogeneous Forest Lodge Member and the structurally overlying, quartzitic Carn a’ Chlamain Member. No convincing way-up structures have been found, but the latter is considered the younger member on the grounds that the Grampian Group is older and structurally underlies the Forest Lodge Member. Dips are commonly steep to vertical within the formation, partly affected by the intrusion of the Glen Tilt Pluton, which also overprinted contact metamorphism on these metasedimentary rocks. The rocks were originally laid down as tidal shelf deposits in shallow open seas.

The formation was formerly termed the ‘Banvie Burn series’ (Bailey, 1925) and structurally overlies the monotonous Gaick Psammite Formation in the Glen Tilt area. Their junction appears to be sheared but the amount of metasedimentary succession cut out is probably minor. The Forest Lodge Member comprises a varied, commonly banded, succession of semipelites and micaceous psammites, calcsilicate rocks, hornblende schists and thin impure metalimestones (Plate 5). These pass northwards into calcsilicate rocks and pelites that are commonly schistose and biotitic with local opaques, including possible graphite. Most of the lithologies have been affected by contact metamorphism resulting from the Glen Tilt Pluton; some pelitic rocks contain late andalusite porphyroblasts. The member is well exposed at [NN 936 744] in the River Tilt, in its type area 350 m north-east of Forest Lodge. Interbedded pale grey metalimestone and dark grey semipelite are exposed at the latter locality, whereas pelites and interbedded calcsilicate rocks are exposed along Allt Glac na Conlaich [NN 940 768] to [NN 933 772]. Parallel laminated amphibole-rich schists, dominated by tremolite-actinolite, are probably para- amphibolites. The larger amphibolites and hornblende schist bodies (D), such as that exposed at [NN 942 755], are considered to be Neoproterozoic meta-igneous rocks.

The Carn a’ Chlamain Member includes mainly pure, pink or white quartzites that vary from massive to laminated. Some quartzite is described as almost glassy containing small grains of reddish feldspar (Barrow et al., 1913). This is likely to be the result of contact metamorphism by the Glen Tilt Pluton. The member is partly exposed in the type area between Carn a’ Chlamain [NN 915 758] and Conlach Mhor [930 768]. Subordinate banded biotite-bearing micaceous psammites are also associated with this member.

Tom Anthon Mica Schist Formation. Farther north at [NO 033 865], the isolated outlier of schist is correlated with the Tom Anthon Mica Schist established in the Braemar district (Upton, 1986). It appears to directly overlie Glen Spean Subgroup psammites (Figure 3) and therefore lies at the base of the Lochaber Subgroup. Exposures are poor in this district and blocks of biotite- muscovite schist and semipelite are taken to be representative of the formation. A biotite pelitic schist has been collected (N1196).

Aonach Beag Semipelite Formation. Cropping out to the north-west of the Glen Banchor ‘high’ on Sheet 64W, this formation belongs to the Appin Group (Robertson and Smith, 1999) or more specifically the Lochaber Subgroup (British Geological Survey, 2000a). The formation structurally and unconformably overlies the Glen Banchor Subgroup in the area north of Glen Banchor [NH 654 010] and forms the basal part of the Appin Group in the Geal-Charn–Ossian Steep Belt. The formation varies along strike from its type section on Sheet 63E (British Geological Survey, 2000a) and in this district it comprises schistose semipelite and micaceous psammite, which locally contains kyanite. Quartzite interbeds increase in abundance towards the top of the formation. Calcsilicate rocks are sparsely developed and generally comprise tremolite- actinolite schists. The formation is poorly exposed in the district and its top is cut out by the Aonach Beag Slide against the Creag Meagaidh Psammite Formation lying to the north-west.

Ballachulish Subgroup

The succeeding Ballachulish Subgroup (Table 3) has a more widespread outcrop than the Lochaber Subgroup but it is restricted to the south-east of the Loch Tay Fault (Figure 3). The severe faulting and sliding focussed towards the base of the Appin Group along Glen Tilt has cut out the equivalent of the Ballachulish Limestone on Sheet 64E (Ben Macdui). The overlying Glen Clunie Graphitic Schist Formation, is partly exposed and is the equivalent of the Ballachulish Slate Formation. This is succeeded in turn by the Beinn a’ Ghlo Transition, the An Socach Quartzite and the Glen Loch Phyllite and Limestone formations. The sequence represents euxinic carbonate, probably precipitates, and muds over which a deltaic sand body built out to become capped by carbonates, muds and sands, partly in cyclic siliciclastic/ clastic carbonate layers.

Glen Clunie Graphitic Schist Formation. This graphitic pelite formation (Figure 3) is partly exposed in the core of the antiform at Beinn a’ Ghlo [NN 962 735], immediately above the Glen Tilt Thrust between Carn Torcaidh and Allt Fheannach and in inliers around An Lochain [NN 984 777] and the confluence of the Allt Feith Lair and the Allt a’ Ghlinne Mhoir. The succession is locally laminated and becomes phyllitic where interbedded with siltstone or quartzite towards the top of the formation. Grey schists, that are locally only slightly graphitic, take on a spaced crenulation cleavage; elsewhere the rock has a fine-grained slaty appearance. Pyrite is common locally although disseminated, producing a rusty iron oxide coating on weathered surfaces. Towards the base of the section in Allt Fheannach [NN 9594 7590], local impure metalimestones contain green calcsilicate bands. The presence of metalimestones towards the base of the formation suggests a passage down into the equivalent of the Ballachulish Limestone (cf. Baddoch Burn Limestone of Upton, 1986) which is not exposed in this section due to faulting. Local beds of quartzite at [NN 9508 7476] indicate a clastic input into the muddy prodeltaic environment. The pelitic succession passes up into the Beinn a’ Ghlo Transition Formation (Smith and Harris, 1976). The main constituents of the schist are muscovite, biotite and quartz. Garnets are poorly developed and commonly altered to chlorite. Rare kyanite porhyroblasts are present together with minor plagioclase and iron oxides. The calcsilicate beds contain fibrous tremolite with carbonate minerals or pale green actinolite with quartz and biotite.

Beinn a’ Ghlo Transition Formation. This formation comprises interbedded psammites, quartzites, flaggy siltstones and graphitic pelites forming a passage up into the An Socach Quartzite Formation. Its type area is within the Beinn a’ Ghlo range (Figure 3) whence it takes its name. The lower part of the formation is well exposed in the tributary to Glas Leathad at [NN 965 737], in Coire Mhuirich [NN 965 754] and in the valley of An Lochain [NN 984 775] (Plate 6).

The upper part of the formation is exposed in Allt Coire a’ Chaisteil [NN 966 748], where there is a gradual increase upwards in the number of quartzite beds occurring within the formation. The quartzites tend to be massive, fine-grained and in places over 1 m thick. In Allt Fheannach [NN 960 752], where psammite and quartzite beds predominate over graphitic pelite, distinctively banded grey and orange-brown rocks consisting of 1–10 cm-thick interbeds of psammite and pelite are exposed. Many graded units are 15–20 m thick and have sharp bases yielding good younging information (Stephenson, 1990). Some of the fine-grained phyllitic intervals are laminated dark and pale grey on a mm scale. Cross-bedding, washouts, small-scale channels, slump structures and slump breccias have been recorded (Smith and Harris, 1976) which consistently young towards the An Socach Quartzite. Near the bottom of Allt Feith Ghuithsachain [NN 987 767], the formation youngs to the north-west and is separated from the An Socach Quartzite by the Carn an Righ Slide.

The depositional environment is considered to be one in which fine quartz sands were transported from an encroaching delta over the prodelta muds of the Glen Clunie Graphitic Schist Formation. In the semipelitic part of the formation, biotite and muscovite flakes are usually conspicuous together with small pyrite crystals. Some pelitic beds also contain small garnet and feldspar porpyroblasts.

An Socach Quartzite Formation. The An Socach Quartzite Formation (Figure 3) is equivalent to the Appin Quartzite of the south-west Grampian Highlands and continues north-eastwards to An Socach and The Cairnwell in the Braemar district (British Geological Survey, 1989). In the present district the quartzite forms the peaks and ridges of the Beinn a’ Ghlo range, as it is a resistant rock-type but the hill slopes are generally covered in quartzite scree or regolith. Quartzite is particularly well exposed on Carn nan Gabhar [NN 972 734], Carn an Righ [NO 028 772] and Stac na h-Iolair [NO 018 772].

Fresh quartzite is commonly white or pink to yellow-brown, massive to thin bedded, and locally coarse-grained to pebbly. Fine- to medium-grained quartzite is typically pure with only a minor content of iron oxides, titanite and rutile which tend to be concentrated in heavy mineral layers. The coarse-grained feldspathic variety weathers to have a porous appearance as exposed around Coire a’Chaisteil [NN 973 749] (Stephenson, 1995). This rock-type was described by Barrow (Geological Survey of the United Kingdom, 1902) as ‘porous quartzite’. Granular quartz and feldspar clasts are set in a pinkish quartzose matrix with a vitreous aspect. The pink and white feldspar clasts are generally from 2 to 4 mm in length. Feldspar grains comprise up to 15% of the volume of the quartzite and the commonest type is microcline, followed by plagioclase. A few small biotite and muscovite laths are scattered within the quartzite. Clear and milky quartz pebbles, 1 to 3 cm long, tend to be concentrated at the bases of some beds, although isolated pebbles are also scattered within quartzite beds. Sedimentary structures such as trough cross- bedding, grading and slumping are locally preserved e.g. on Carn nan Gabhar [NN 9704 7319].

Details of the formation around Carn an Righ [NO 029 773] were given by Crane et al. (2002, p.13–14). They described trough cross-bedding (Plate 7) and graded bedding south and west of Stac na h-Iolair [NO 019 772]. However, ‘pseudo cross-bedding’ was also noted (Crane et al., 2002) in deformed quartzite near Carn an Righ, where tight to isoclinal minor folds of hematite partings have only one limb preserved, juxtaposed against S2. On Carn an Righ, the quartzite is commonly platy due the proximity of tectonic slides. However, about 200 m to the south-east, well-preserved cross-bedding is present in the quartzite which passes up into the Glen Loch Phyllite and Limestone Formation. A fairly energetic depositional environment is considered to be the setting for these deltaic to shelfal sand bodies.

Glen Loch Phyllite and Limestone Formation. This varied succession, well exposed in the type area around Glen Loch (Figure 3), comprises mainly interbedded grey biotite-bearing or calcareous phyllitic semipelites and psammites and pale grey to white metalimestones. The well- exposed type section was established by Stephenson (1991) from the Allt Ruigh na Cuile area in Glen Loch [NN 985 756] to [NN 987 755] and detailed descriptions were given by Crane et al., (2002, p.14 -16).

Throughout the area, the base of the formation is marked consistently by a thin bed of pure white siliceous metalimestone that rests directly upon the underlying quartzite, incorporating sand grains presumably derived from that substratum. This bed seems to be preferentially exposed although it is rarely more than 1 metre in thickness. Several exposures occur in the hinge of the Meall Reamhar Synform e.g. at [NN 994 765]. Where it is not exposed, it is commonly marked by a line of small sink holes (e.g. NNE of Allt Ruigh na Cuile). It is arguably one of the most reliable stratigraphical markers in the sequence and, in areas of complex structure, is an invaluable indicator of the top of the An Socach Quartzite.

Throughout the formation in general, the metalimestones vary from thick white or pale grey medium-grained metalimestones to cream coloured dolomitic metalimestones. Fine-grained saccharoidal dolomitic metalimestones are exposed near Allt Ruigh na Diollaide [NN 985 730] and where the beds are slightly siliceous or marly [NN 9835 7300], tremolite laths develop in abundance. Distinctive thinly interbedded metalimestones and calcareous phyllitic semipelites were called ‘tiger rock’ by Bailey (1925), because dark ribs of semipelite alternate with orange- or yellow-weathered carbonate-rich bands (Plate 8). These sequences are commonly developed; for example, at [NN 9847 7353]. Locally psammites, quartzites and greenish calcsilicate rocks are present. Pelites become more common towards the top of the formation, which passes upwards stratigraphically into the Blair Atholl Subgroup. Exposures within the semipelites and psammites in Allt a’ Ghlinne Mhoir [NO 009 765] show textural and compositional grading that indicates younging to the south in near vertical beds.

The prominent banded pale- to medium-grey metalimestone present at the top, or near the top, of the formation is called the Gleann Mor Limestone Member (Crane et al., 2002) and is currently the only established member within the formation (Plate 9). Locally near the top of this member there are pelitic interbeds and at the base schistose graphitic pelite is present. The member varies from about 30 m thick south-west of Carn an Righ [NO 021 759] to 150 m on Creag an Loch [990 741], although this may be due to folding within the limestone. It is well exposed in a strike section along Allt a’ Ghlinne Mhoir [NO 010 765] to [NO 015 762].

In Allt Feith Guithsachain at [NN 9945 7655] a section through the formation starts with the basal 5 m-thick pure white, coarsely crystalline limestone resting directly on the An Socach Quartzite. Upstream, in a waterfall, banded grey, pink and white limestone or ‘tiger rock’ is exposed below grey and grey-green phyllitic psammite and semipelite with thin partings of garnet-biotite schist (Stephenson, 1990). A cream-coloured dolomitic metalimestone is mapped in this stream section below the Gleann Mor Limestone Member. Other examples of ‘tiger rock’ are exposed in Allt Coire Caseagallach at [NN 9841 7353] where biotitic phyllites are prominent, and north-east of Fealar Lodge, at the head of Allt Feith Lair [NO 0348 8305] and details are given in Stephenson (1990; 1991, 1995). The depositional environment, in which there was a sharp decrease in clastic input into areas of carbonate accumulation under oxidising conditions, is characteristic of a mixed clastic carbonate shoreline.

Blair Atholl Subgroup

In the Glen Tilt area, the Ballachulish Subgroup is conformably overlain by the Blair Atholl Subgroup, which comprises the graphitic Blair Atholl Dark Limestone and Dark Schist Formation succeeded by the non-graphitic Cnoc an Fhitich Semipelite Formation and Drumchastle Pale Limestone Formation. East of Glen Loch, adjacent to the Glen Shee district (Crane et al., 2002) lateral facies changes and interfingering of lithologies necessitates the establishment of different formations. The Sron nan Dias Pelite and Limestone Formation is approximately laterally equivalent to the Blair Atholl Dark Limestone and Dark Schist Formation and is succeeded by the Tulaichean Schist Formation. The overlying Gleann Beag Schist Formation can be correlated laterally with the Cnoc an Fhithich Banded Semipelite Formation (Table 3). The Gleann Beag Schist is divided into two members on Sheet 64E. The strata represent shelf and shoreline clastic carbonates and offshore muds and sands. The presence of graphite, locally towards the base of the subgroup, probably indicates some deposition as precipitates in euxinic conditions.

Blair Atholl Dark Limestone and Dark Schist Formation. This formation can be traced up Glen Tilt from its type area in Blair Atholl. A graphitic pelite or schistose semipelite usually lies at the base of the sequence followed by various units of mid-grey or blue-grey graphitic crystalline metalimestone with thin pelitic to semipelitic bands. The pelitic units tend to be more quartzose, more biotitic and less graphitic than the Glen Clunie Graphitic Schist and typically contain muscovite and some garnet as well as biotite.

South-west of Coire Rainich at [NN 952 753], the graphitic pelite contains local beds of quartzite. By way of contrast, in Coire Thorcaidh [NN 938 735], the graphitic, biotitic pelite includes grey metalimestone interbeds tens of metres thick, but the base of the formation is cut out by the Carn an Righ Slide. Towards the top of the formation in Allt Torcaidh, a thick unit of dark grey metalimestone contains some pelitic and psammitic interbeds and passes up into the Cnoc an Fhithich Banded Semipelite Formation. In the Blair Atholl area, a ‘Dark Schist’ unit can generally be distinguished lying stratigraphically below ‘Dark Limestone’; this distinction does not persist into the Ben Macdui district where the formation appears to be condensed.

Cnoc an Fhithich Banded Semipelite Formation. This succession is generally non-graphitic, interbedded quartzose semipelite and psammite, mainly banded, with local metalimestones. Towards the top, pale banded metalimestones include schistose biotite semipelite and green calcsilicate rocks. These correlate with the Drumchastle Pale Limestone Formation around Schiehallion (see Treagus, 2000). It appears that the Cnoc an Fhithich Banded Semipelite and Drumchastle Pale Limestone formations are combined in this district because the limestone sequence is intercalated with the semipelites and psammites. This may be due to facies changes in a condensed sequence as indicated by the thin Islay Subgroup succession which lies stratigraphically above in the south-eastern flank of Glen Tilt opposite Forest Lodge (Figure 3).

The quartzose psammites and semipelites are commonly interbedded on the scale of 1–10 cm, as exposed in Allt Coire Fhiann [NN 931 729] and Allt Torcaidh [NN 933 736]. Some of the semipelites are rich in muscovite and biotite. Some fine-grained pale grey-green calcareous schists occur in the transition into the metalimestone beds. Pale banded metalimestone with schistose biotite semipelite and green calcsilicate rock, occurs in two impersistent lenses at [NN 935 739] and [NN 939 745]. The most common calcsilicate mineral in the metacarbonate rocks is tremolite with minor white mica (?talc) and phlogopite. The depositional environment is considered to be a warm shallow coastal one in which silts and marls were deposited in lagoons, the sands on bars or spits and the carbonate, clastic or possibly stromatolitic, on banks or shoals.

Sron nan Dias Pelite and Limestone Formation. This formation comprises a varied succession of graphitic pelite and grey graphitic limestone which crops out in the area east of Glen Loch on Sheet 64E (Figure 3). The pelites are variably graphitic, commonly schistose and biotitic with local intercalations of semipelite. Towards the top, quartzite interbeds are present. The metalimestones are locally thinly interbedded or flaggy but usually the thicker units of dark to mid-grey medium-grained metalimestone, about 10 to 15 m thick, contain only thin ribs of pelite or semipelite (Plate 10). The metalimestones have characteristics in common with the Gleann Mor Limestone Member but are interbedded with pelites typical of the Blair Atholl Subgroup.

Two interbedded metalimestone units are recognised (Crane et al., 2002); exposed in Allt Choire na Moine [NO 018 785] and on the western slopes of Beinn Iutharn Mhor [NO 030 792]. The formation includes garnet and biotite-bearing graphitic pelites and semipelites with some calcsilicate rocks and the brown-weathering metacarbonate rocks on Sron nan Dias [NN 994 730] lie within schistose biotite pelite and semipelite. Boudinaged lenses of semipelite and pelite occur throughout the metacarbonate rock. On the slopes immediately east of Sron an Dias, conspicuous beds of brown-weathering metacarbonate rock occur within garnet-mica schists.

In thin section, the semipelites contain muscovite and biotite with some local hornblende, orientated in a good foliation within a matrix of fine-grained recrystallised quartz and plagioclase. Garnets are inclusion free and wrapped by the schistosity (Crane et al., 2002).

Tulaichean Schist Formation. Most of this formation consists of medium-bedded, schistose biotite-muscovite semipelite; subordinate laminated phyllitic semipelite and psammites include some graded beds and laminated metasiltstone. The schistose semipelites are mostly well foliated and abundantly garnetiferous. Minor garnet amphibolite, quartzite, calcite-bearing semipelite and calcsilicate rock are present. The main outcrop (Figure 3) lies in the Carn an t-Sionnaich [NO 01 75] area and a significant quartzite intercalation is mapped to the west at [NO 006 755], near the centre of the Meall Reamhar Synform. On the ridge [NO 246 751] about 1 km to the west of Carn an t-Sionnaich, a belt of laminated semipelitic to psammitic schist is sparsely garnetiferous (Plate 11). Calcsilicate rocks, such as those at [NN 9973 7473], tend to be associated with amphibolites and may be skarns. Such amphibolites are relatively common in the Tulaichean Schist Formation and are considered to be mafic metaigneous rocks, possibly intrusive or volcanic (see section 2.3). At a structurally lower level, at [NO 02 78], garnet-mica schist of the formation lies between the Sron nan Dias Pelite and Limestone Formation and the Carn an Righ Slide.

Structurally above the Glen Loch Slide on Meall Reamhar [NN 993 759], very large garnet porphyroblasts (Plate 12) occur within muscovite-biotite-amphibole schist associated with amphibolitic intrusive units in the hinge of the Meall Reamhar Synform.

Micaceous psammite and massive quartzite forms a significant part of the formation on the south-west limb of the Meall Reamhar Synform and is exposed west of Creag Leacagach [NO 000 728]. The micaceous psammite is typically fine-grained, grey and flaggy. It has a granoblastic texture composed mainly of quartz with some feldspar and scattered biotite. Biotite- rich foliae, heavy mineral concentrations and hematitic partings define compositional layering, which at one locality [NN 9997 7290] has preserved centimetre-scale trough cross-bedding (Crane et al., 2002) younging towards the centre of the Meall Reamhar Synform. A crude schistosity of probable S1 age is developed subparallel to the compositional layering in the thin biotitic layers. Bedding and S1 foliation are overprinted by the main S2 schistosity (Crane et al., 2002). The western margin of the quartzitic unit is marked by a broad zone of high D2 strain. The resulting finely laminated, blastomylonitic quartzites with down-dip rodding are part of the Glen Loch Slide-zone which lies within the Blair Atholl Subgroup (Crane et al., 2002).

The quartzitic unit at the base of the formation was included within the Killiecrankie Schist Formation on Sheet 55E (Institute of Geological Sciences, 1981) but it passes up into thin schistose semipelites typical of the Tulaichean Schist Formation and so is assigned to the Blair Atholl Subgroup (Goodman et al., 1997). The establishment of the Tulaichean Schist Formation above the Sron nan Dias Pelite and Limestone Formation, within the Blair Atholl Subgroup, highlights the problem of correlation with the adjacent Pitlochry district, where the equivalent formation was mapped as part of the Killiecrankie Schist in the Easdale Subgroup. An alternative solution is that the lower part of the Killiecrankie Schist on Sheet 55E is the upper part of the Blair Atholl Subgroup on Sheet 64E, and if the Schiehallion Quartzite was missing along strike, the upper Blair Atholl Subgroup would then be succeeded by Lower Easdale Subgroup (Stephenson, 1995). Another model in which the Tulaichean Schist equates to the Killiecrankie Schist in the lower Easdale Subgroup bounded by slides (cf. Bailey, 1925) has also been postulated (Stephenson, 1995).

Gleann Beag Schist Formation. This is the uppermost formation of the Blair Atholl Subgroup in the Glen Fearnach area (Table 3). The formation equates to the upper part of the Blair Atholl Subgroup (Crane et al., 2002). It `comprises a lower, Glen Lochsie Calcareous Schist Member and an upper, Glen Taitneach Schist Member.

The lower, calcareous member consists of dark graphitic calcareous schists and calcsilicate rocks interbedded with metacarbonate rock, psammite, graphitic pelite and garnetiferous semipelite. The brown-weathering metacarbonate layers are commonly dolomitic. The member crops out round the Meall Reamhar Synform near the head of Gleann Fearnach [NO 01 74] and is of the order of 250 m thick where least deformed. It is severely attenuated along the north-east side of the Meall Reamhar Synform against the Carn Dallaig Slide.

The upper, Glen Taitneach Schist Member is predominantly a black to silver-grey, and locally garnetiferous, graphitic pelite. Minor semipelite, metacarbonate rock and psammite interbeds are present. The siliciclastic component increases west of the Glen Shee district and at Gleann Fearnach [NO 010 747] the member grades into laminated micaceous psammites and pelites with some non-graphitic schistose semipelite. Metacarbonate-rock layers are common throughout the member, particularly near the base. In the centre of the Meall Reamhar Synform [NO 009 747], the member contains thin metacarbonate rock (marble) lenticles within graphitic schist and carbonate breccia with sulphides. As the graphite content of the member decreases westwards in Gleann Fearnach [NO 0091 7480] the metacarbonate layers are creamy white with only a few graphitic laminae. One of these dolomitic metacarbonate rocks contains grossular garnet in graphitic pelite laminae.

This member is commonly intensely folded and attenuated, probably due to its enhanced ductility as a result of its graphitic nature. Changes in thickness over short distances make it difficult to determine the depositional thickness of the member, but 200 to 300 m is considered to be a reasonable estimate (Crane et al., 2002).

1.3.2.3 Argyll Group

Islay Subgroup

In many areas, such as the type area at Schiehallion (Sheet 55W), this subgroup comprises a thick quartzite formation which can be mapped separately from an underlying glacigenic boulder bed formation. On the Ben Macdui sheet, where the subgroup is poorly developed, the boulder beds are impersistent and are included within the Schiehallion Quartzite Formation as an informal member. The stratigraphically equivalent quartzite adjacent to Sheet 56W (Glen Shee), where it is also associated with glacigenic boulder beds, has been assigned to the Creag Leacach Quartzite Formation (Crane et al., 2002). These fairly clean quartzites are considered to have been deposited on a tidal shelf. This means that in the sequences across Sheet 64W the Islay Subgroup contains only one formation (Table 3).

The glacigenic horizon is an important marker horizon along the strike of the Argyll Group, distinguishing the Islay Subgroup quartzites from those in the Ballachulish and Lochaber subgroups. In this district critical evidence for the glacial origin of the boulder beds is lacking due to poor exposure and the degree of tectonism. However, because the boulder beds are interbedded in waterlain deposits and not associated with a glaciated ‘basal pavement’ (Spencer, 1971), they were probably deposited under water. Because the glacigenic horizon (boulder bed or tillite) is widespread in the Islay Subgroup and is associated with dolomitic beds (cap carbonates) it is considered to be the product of a widespread Neoproterozoic glaciation. This was formerly considered to have occurred during the latest Precambrian to Cambrian (Spencer, 1971). Later geochronological data indicated that this could be correlated with the Varangerian/Marinoan glaciation which lasted from about 590 to 564 Ma but since the Tayvallich Volcanic rocks are dated at about 600 Ma, this correlation is unlikely (Prave, 1999). Prave (1999) argued that the glaciation at the base of the Argyll Group (Port Askaig Tillite) is the Sturtian glacial episode at around 750–700 Ma and that the Varangerian episode is represented by the Loch na Cille Boulder Bed in the Southern Highland Group.). U-Pb Shrimp ages from Idaho (Fanning and Link, 2004) indicate that the Sturtian glacial epoch may have lasted until 670 Ma. The δ¹³C of the carbonates associated with the Port Askaig tillites were correlated with the Sturtian glaciation elsewhere (Brasier and Shields, 2000). No equivalent carbonates are recorded in this district.

The subsequent correlation of the glaciation at the base of the Argyll Group with the Marinoan event c. 635 Ma (Leslie at al. 2008) was based on correlations with strata in East Greenland and Northern Namibia (Gaucher et al. 2005). However, new isotopic evidence from carbonate beds associated with glacigenic strata indicates that the Argyll Group covered both the Sturtian and Marinoan events. The isotopic evidence for Marinoan-equivalent events within the Easdale Subgroup comes from the Stralinchy–Reelan–Cranford sequence in Donegal (McCay et al., 2006) and the Whiteness Limestone in Shetland (Prave et al., 2009).

Schiehallion Quartzite Formation. This formation forms the bulk of the Islay Subgroup in this district and contains glacigenic boulder beds near its base which are considered to be the equivalent of the Schiehallion Boulder Bed Formation (Treagus, 2000) in the Schiehallion area. Most of the quartzite formation consists of white to yellow-brown-weathering fine- to medium-grained metaquartzite. The rock is commonly flaggy, particularly near shear-zones and slides. The boulder beds contain pebbles to boulders of granite and quartzite in a finer grained matrix. There are also thin interbeds of pale grey metacarbonate and calcsilicate rock locally near the base of the formation. In this district the quartzite occurs in a narrow NE-trending outcrop on the south-east flank of Glen Tilt (Figure 3) and the most continuous section through the formation occurs in Allt Coire Fhiann [NN 930 730] although this sequence is tightly folded and no boulder beds were recorded.

In Allt Torcaidh [NN 9322 7374] a representative of the Schiehallion Boulder Bed appears to be repeated by the folding within the quartzite. The boulder bed contains rounded pebble-sized clasts of pink granite and quartzite set in a psammitic to semipelitic matrix. The matrix is up to 70% quartz with scattered grains of plagioclase and K-feldspar and laths of biotite and muscovite. The strata on either side of the quartzite are assigned to the Blair Atholl Subgroup. Hence the formation here is interpreted to lie in a sliced isoclinal synform closing to the north- east (Stephenson, 1995), affected by slides within and structurally below the quartzite.

Creag Leacach Quartzite Formation. In the Gleann Fearnach area [NO 009 740], cream quartzite and micaceous psammite is assigned to the Bad an Loin Quartzite Member of this formation (Crane et al., 2002). The member acts as a marker between the Blair Atholl and Easdale subgroups and forms the upper immature facies of the formation (Crane et al., 2002). It is highly attenuated around the head of Gleann Fearnach (Figure 3) and is no more than a few tens of metres thick. As a consequence, the full Islay Subgroup succession is not seen, and no associated boulder beds are known, in the south-east of Sheet 64E (Ben Macdui). The rock generally weathers white, but has a buff or beige colour on a fresh surface. The member is thinly bedded with little internal structure although rare pebbly horizons were recorded along with other details by Crane et al., (2002).

Easdale Subgroup

This subgroup contains several laterally variable and impersistent formations due to increasing instability in a deepening basin as continued extension led to rifting and volcanic activity. A rapid shift in seawater ⁸⁷Sr/⁸⁶Sr in Dalradian limestones between the Islay and Easdale subgroups recorded by Thomas et al. (2004) is consistent with a significant change in basin dynamics. Currently there is no correlation with the Marinoan-equivalent glacial and associated cap carbonate sequence recognised within this subgroup in Donegal (McCay et al., 2006).

In the Gleann Fearnach area (south-east corner of the Ben Macdui sheet), the Islay Subgroup is succeeded, through a thin transitional passage, by the Ben Eagach Schist Formation followed by the Ben Lawers Schist Formation (Table 2).

Ben Eagach Schist Formation. This formation is stratigraphically the lowest formation in this subgroup. It is a schistose pelitic unit that is typically black to dark grey due to disseminated graphite and sulphides. The schist is relatively soft and weathers to a rusty brown colour due to the sulphide content, but no significant mineralisation has been noted that could compare with the baryte deposit in the same formation near Aberfeldy. Thin fine-grained, micaceous psammites and semipelites are interbedded and these fine-grained and graphitic lithologies are taken to indicate a renewed period of rapid basin deepening. Because the schists are relatively incompetent, they act as a locus for deformation with significant thickening in fold hinges and attenuation on limbs e.g. at [NO 009 739]. Even allowing for the deformation the unit is relatively thin in west Gleann Fearnach (Figure 3) and hardly more than 100 m thick. The formation is cut out to the south by the Creag Uisge Slide.

Within the schist, quartz comprises about 50% of the rock and plagioclase, less than 10%, in a granoblastic matrix containing graphite and lepidoblastic muscovite and biotite. Garnet and retrogressive chlorite occurs locally. Pyrite is the dominant sulphide; pyrrhotite has been recorded and some magnetite (Crane et al., 2002). The predominantly pelitic deposition together with carbonaceous material suggests a euxinic environment with a restricted clastic input.

Ben Lawers Schist Formation. This formation consists of mainly schistose carbonate-bearing to calcsilicate rocks with thin beds of psammite. Its maximum thickness is about 700 m (Crane et al., 2002) but its full thickness is not seen on the Ben Macdui sheet. It extends south-east into the Glen Shee district where it is succeeded lithostratigraphically by the Farragon Volcanic Formation. Its main outcrop in this district is in the east of Upper Gleann Fearnach [NO 02 73] but it is poorly exposed on grassy slopes. The calcareous schists tend to be pitted where the calcite grains have been dissolved. Local interbeds of cream-white metalimestone occur up to 15 m thick.

The rocks are commonly greenish due to their content of chlorite and amphibole. The amphiboles are typically developed as grabenschiefer textures on the schistose surfaces and amphibolite-rich rocks grade into schistose amphibolites. At deposition, the background siliceous detritus may have mixed with a basic volcanic source. In the upper part of Gleann Fearnach [NO 01 73] a distinctive fine-grained hornblende schist contains thin psammitic beds. This unit is considered to have a volcanic protolith and has been referred to informally as the ‘Laoigh metabasites’ (Crane et al., 2002). On Creag Beag [NO 0113 7368], these basic meta- igneous rocks contain a cream metacarbonate layer, similar to that seen elsewhere within the calcareous schists. Geochemical analysis of the hornblende schists has shown that they are comparable to the overlying Farragon Volcanic Formation in the Glen Shee area (Goodman and Winchester, 1993) and they probably represent an early stage in the volcanic activity that culminated in the more widespread eruption of the Farragon ‘Beds’ from Glen Shee to the Loch Tay area. Stable isotope studies (Scott et al., 1991) confirmed the importance of a magmatic and volcanoclastic input into the Ben Lawers Schist Formation.

Thin section details are given by Crane et al. (2002). Typically fine-grained quartz and plagioclase mosaics are interspersed with laths of muscovite, green-brown biotite and chlorite as well as local aggregates of calcite. Large amphiboles are poikiloblastic, epidote occurs in small grains and garnet is rare.

1.4 Igneous intrusions

1.4.1 Neoproterozoic to Ordovician: pre- and syntectonic intrusions

Metamafic rocks

Amphibolite bodies (with relict ophitic texture in places) probably had intrusive basalt or microgabbro protoliths produced during rifting of the extensive Dalradian basin at about 600–590 Ma (as they lie below the Tayvallich Subgroup). As far as is known, the amphibolites on Sheet 64 all pre-date D2 and therefore belong to the ‘older suite’ of metamafic rocks described by Crane et al. (2002).

A minor plug-like body of garnet amphibolite is recorded from the Glen Banchor Subgroup [NN 653 974] and within the Torr na Truim Semipelite Formation nearby, two small lenses of metagabbroic amphibolite at [NN 693 995] and [NN 6695 9787] occur. The latter locality is associated with the Spirean Mor Granite Sheet-complex and is too small to be shown at the 1:50k scale. It is not certain if the amphibolites in the Glen Banchor Subgroup are the same age (or suite) as those in the Grampian Group as they have slightly different mineral compositions. The amphibolites within the Glen Banchor Subgroup of the Dalwhinnie district are probably older since there is evidence that they experienced earlier eclogite-facies conditions (Baker, 1986).

Several concordant to semi-concordant metamafic sheets and lenses occur within the Blair Atholl to Easdale subgroups, being particularly common in the Tulaichean Schist Formation e.g. around Meall Reamhar, [NN 995 759]. These sheets are generally a few metres to tens of metres thick and can be traced laterally for a few hundred metres. They are clearly porphyritic in places and are interpreted as pre-metamorphic basic sills and dykes. The amphibolites range from fine- to coarse-grained and commonly contain biotite and garnet. In places the bodies are discordant to bedding, but tend to have a foliation at their margins parallel to S2 in the country rocks. In the thick amphibolite outcrop west of Cnapan Liath at [NO 003 763] the foliation appears to have been folded prior to being folded round the Meall Reamhar Synform (see below).

Clachghlas and Fealar metagranites and associated granite intrusions

These foliated pinkish coarse-grained biotite-granites are similar in character to the larger Ben Vuirich granite (Pitlochry Sheet 55E) and may have a similar Neoproterozoic age, about 590 +/− 2 Ma (Rogers et al., 1989). The Ben Vuirich Granite is described as exhibiting a mildly A-type character and is considered to be a pre-orogenic rift-related intrusion (Tanner et al., 2006). The foliated granites and mafic igneous rocks about 600 Ma in age are now formalised as the Vuirich Suite (c.f. Tanner et al., 2006). No geochemical data are known from the foliated granites in the Newtonmore–Ben Macdui district, but a range of bulk compositions is likely within the Vuirich Suite as the small metagranites in this district appear to lack the metamorphic garnet present in the Ben Vuirich Granite (although they lie well within the Barrovian garnet zone).

On the south-eastern side of Glen Tilt a lenticular body of pink and grey spotted coarse-grained biotite-granite, known as the Clachghlas Granite, extends south-west from opposite Forest Lodge [NN 933 740] for about 2 km on to Sheet 55E. The margins and foliation of the granite dip at 25° to 54° to the south-east. This body consists of a series of granite sheets, up to 250 m thick in total, intruded concordantly into metasedimentary rocks. The body includes screens of the latter which are metamorphosed and/or metasomatised so that the junctions are indistinct and commonly biotite-rich. The granite lies close to or at the lower margin of the Schiehallion Quartzite Formation in Glen Tilt which has been subjected to strong shearing, and the latter may have affected the body or controlled its intrusion (Stephenson, 1995).

Farther north-east, a smaller foliated granite is exposed on the south-east slopes above Allt Garbh Buidhe and extends south-east to Tulach Breac [NN 993 801]. This fractured and shattered red granite/microgranite has a foliation dipping 60°–65° to the north-east.

Highly sheared granite is also exposed in Allt Feith Lair [NO 016 805], about 800 m north-east of Fealar Lodge. It is a fine- to medium-grained, grey microcline-biotite granite with a strong foliation wrapping feldspar augen and a stronger stretching lineation plunging east-south-east. The microclines are well rounded and set in a fine-grained groundmass of quartz and other minerals, probably as a result of shearing.

Lying to the north-west of the Loch Tay Fault, the foliated granitic body on the western side of Meall Dubh-chlais [NN 920 795] lies within, and contains xenoliths of, the Gaick Psammite Formation. It contains thin dykes of foliated aplite and is associated with a foliated dioritic dyke. It appears to be older than the Glen Tilt Pluton but it is uncertain if the body is of similar age to the Ben Vuirich Granite or is an early Caledonian intrusion (see below).

1.4.2 Caledonian Igneous Supersuite

1.4.2.1 Late-Tectonic (Ordovician to Silurian) igneous rocks (Late D3)

These rocks include the veins and larger intrusive bodies related to the Strathspey vein system i.e. the Spirean Mor Granite Sheet-complex and scattered small bodies of granite, pegmatitic granite and aplitic microgranite.

Pegmatitic granites

Numerous but scattered coarse-grained to pegmatitic veins occur within the metasedimentary rocks of the district. Most individual pegmatitic veins have been omitted from the 1:50k maps for clarity. The limit of pegmatitic/granitic veins as shown in the north-western area of the Newtonmore sheet includes veins within the Glen Banchor Subgroup as well as those intruding the adjacent Grampian Group. A few foliated pegmatitic veins, with radiometric ages of around 800 Ma, have been identified within the Glen Banchor Subgroup of this district (Piasecki and van Breemen, 1983; and cf. the Tomatin district) but the undeformed veins are considered to be Ordovician in age. Many undeformed pegmatitic veins in the Glen Banchor area are probably related to the Spirean Mor Granite Sheet-complex (see below).

The pegmatitic veins are predominantly composed of pale or pinkish potash feldspar, mainly microcline, with quartz, some plagioclase, and commonly large plates of muscovite. Pegmatitic rock locally grades into coarse granite and in places contains garnet and biotite. Associated aplitic microgranite is also garnet-bearing. The pegmatitic rocks occur in veins or lenses up to 2 m thick, but are generally 0.1–1 m thick, roughly concordant with the main foliation. However, most pegmatitic rocks in the district are not foliated and are considered to be later than the foliation (i.e. post-D2 in age). Some pegmatitic rocks are foliated locally, but this could reflect D3 or late local deformation as some Silurian dykes are also foliated.

Additional areas of complex granitic/pegmatitic veining are delimited on the map in the Loch Cuaich area and on Meall an Dubh-chadha [NN 790 908]. Scattered pegmatitic granites within the Gaick Psammite Formation are shown on the map to the north and west of Carn na Cairn [NN 675 820] as well as farther to the east in Allt Gharbh Ghaig [NN 796 817]. However, the pegmatitic granite suite appears to be lacking from the eastern margin of the Newtonmore Sheet.

Pegmatitic rocks also have a limited distribution on the Ben Macdui Sheet where pegmatitic granites, which generally have an easterly trend, occur around the Chest of Dee [NO 013 886]. The veins are between 0.4 and 2 m thick and can be traced for up to 200 m along their length.

They may be related to an early phase of the Cairngorm Pluton but are described by Barrow et al. (1913 p.59) as ‘regionally metamorphosed, foliated muscovite-pegmatites’ and may be related to the foliated granites (see above).

Farther south on the Ben Macdui sheet, veins of sheared pegmatitic rock (σπ) are exposed within the Gaick Psammite Formation in the Bynack Burn area, for example at [NN 995 848]. They are rarely over 0.5 m thick and may be Neoproterozoic in age; related to the foliated granites (see above).

Spirean Mor Granite Sheet-complex

This complex of porphyritic coarse granite and pegmatitic sheets includes numerous screens and xenoliths of country rock. The limits of predominantly granitic sheet-complex are shown on the Newtonmore Sheet (Figure 2) but the complex continues westwards onto the Dalwhinnie Sheet 63E (British Geological Survey, 2000). The sheet-complex is garnet- and muscovite-bearing and locally foliated. Passive intrusion of the complex is indicated by the preservation of the rock-type and strike of the xenoliths concordant with the surrounding country rocks. The trace of a ductile shear-zone between the Glen Banchor Subgroup and the Grampian Group can also be followed through the complex on account of the numerous xenoliths of sheared rock. The complex is considered to be Ordovician in age (British Geological Survey, 2000) as post-tectonic (Silurian) dyke intrusions of microgranodiorite cut the complex.

1.4.2.2 Post-Tectonic (Late Silurian to Mid Devonian) igneous rocks

The major plutons in the district (the ‘Newer Granites’ of Read, 1961) are the products of widespread uplift and granitic plutonic activity towards the end of the Caledonian Orogeny. Intrusive activity occurred towards the end of the Silurian and into the Early Devonian (about 427–395 Ma), contemporaneous with the final oblique closure of the Iapetus Ocean (Hutton, 1987) in which sinistral transpression was involved (Soper, 1986). Formerly the Glen Tilt granite was classified with the late-orogenic Caledonian granites (main phase) and the Cairngorm granite with post-orogenic granites (Watson, 1984). The Caledonian plutons are essentially calc- alkaline in character. Using petrochemical and isotopic criteria, Stephens and Halliday (1984) divided the post-tectonic granites of the Grampian Highlands into Argyll, South of Scotland and Cairngorm suites. These three suites are presumed to reflect the different nature of the lower crust under the areas occupied by the suites. The Cairngorm granite is probably interconnected at depth with the nearby Glen Cairn, Lochnagar, Ballater and Mount Battock granites (Rollin, 1984).

The Cairngorm Suite, which intrudes the Aberdeenshire-Buchan area, includes the Cairngorm Pluton and was considered to extend eastwards on a structural lineament, the Deeside Lineament (Fettes et al., 1986). Trewin and Rollin (2002) found no geophysical evidence for the Deeside Lineament, but they considered an ESE East Grampian Lineament to be the main control on the Cairngorm Suite of granites. The suite consists of evolved, largely I-type, high heat-producing granitic plutons (Webb and Brown, 1984). The Cairngorm Pluton is dated at around 408–404 Ma (see below) and as the suite clearly post dates the Iapetus Suture, Harrison (1987) concluded it could not be subduction related. Halliday and Stephens (1984) argued for a predominantly lower crustal origin on the basis of geochemical and isotopic evidence. Evidence for an underlying ‘Knoydartian’ granitic protolith (c. 845 Ma) below this suite was produced by Oliver et al. (2000). Studies of zircons from I-type granites, such as Lochnagar, in the Grampian Highlands (Appleby et al., 2006; 2007) showed contrasting whole-rock isotope and geochemical characteristics pointing to sources of significantly different age and/or composition compared to the Argyll Suite (Etive Pluton). The studies also indicated that formation of these 430–400 Ma Caledonian granites is dominated by crustal recycling rather than crustal growth.

The Glen Tilt Pluton is part of the South of Scotland Suite (Stephens and Halliday, 1984), which contains more granodioritic and dioritic intrusions than the Cairngorm Suite, with pyroxene-mica diorite and appinitic components. Stephenson and Gould (1995 included it more specifically in their South Grampians Suite and this has been defined formally in recent BGS publications as the South Grampian Subsuite (Gillespie et al. 2011). Recent work on U-Pb zircon dating (Oliver et al., 2008) of the Glen Tilt granite has produced one of the youngest ages among the Scottish granitoids at 390±5 Ma. In fact, they attribute the Mid Devonian intrusion to a far-field effect of the Acadian Event.

Cairngorm Suite

Cairngorm Pluton

The Cairngorm Pluton is the largest exposed component body of a distinct Cairngorm Suite forming the inferred East Grampian Batholith (Plant et al., 1980) and covers a total area of 365 km2. Over 140 km2 of the south-west of the pluton is exposed on the Ben Macdui Sheet 64E (Figure 4) which includes three of the six component granitic phases recognised by Harrison (1986; 1987a, b). The exposed phases are shown on the 1:50 000 map and are mainly textural varieties of biotite monzogranite with approximately equal proportions of quartz, plagioclase (oligoclase) and K-feldspar. The phenocrysts of K-feldspar are commonly large twinned orthoclase and quartz has a brown colour. Biotite is the only mafic silicate present, although it is locally altered to secondary muscovite. Common accessory minerals are apatite, Fe-Ti oxides, zircon and monazite (Harrison, 1988). Late hydrothermal alteration has caused widespread reddening of feldspars, due to exsolution of hematite from plagioclase. The presence of numerous aplitic and pegmatitic rocks with vuggy cavities suggests emplacement at a relatively high structural level, less than 12 km (Harrison, 1986); possibly 5–8 km below the surface (Harrison and Hutchison, 1987). Harry (1965) concluded that the pluton had a stock-like form and distinguished the Porphyritic or Carn Ban Mor phase in the western lobe of the pluton (Figure 4). The part of the pluton, including phases 2, 4 and 5, lying on the Aviemore Sheet (74E) has been described by Highton (1999). Phases 1 (Glen Avon Granite) and 3 (Beinn Bhreac Granite) are exposed in the eastern lobe of the pluton in the adjacent Braemar and Glenlivet districts (Figure 4).

Harrison (1987) found the Cairngorm Granite almost structureless internally, its external contacts vertical, discordant and unchilled, and large country rock xenoliths are rare. Hornfelsing is absent or localised and the foliation in the Grampian Group country rocks is undisturbed. Harrison therefore concluded that it had reached its present level of exposure by stoping large blocks of country rock and found no evidence of diapiric emplacement. A whole-rock Rb/Sr age of emplacement is recorded as 408±3 Ma (Pankhurst and Sutherland, 1982). A U-Pb zircon study (Oliver et al., 2008) dated the Main Phase on the Aviemore Sheet at [NH 986 072] at 404±18 Ma. The pluton comprises I-type granites with an ⁸⁷Sr/⁸⁶Sr initial ratio of about 0.706.

The Cairngorm granite has a high SiO₂ content (72–77%) and is moderately peraluminous (Harrison, 1988). The chemistry of all the phases is typical of minimum melt granites with a low MgO, CaO and P₂O₅ content. No systematic differences in chemistry between the porphyritic and non-porphyritic types were found and compositions plot close to the thermal minimum in the granite system (Harrison, 1988). Chemical analyses of the granite, including some samples containing small Mn-rich garnets, indicate that the pluton is transitional between I-type and A- type granites since it is very restricted in its major element composition, enriched in incompatible elements such as Y, Nb, Th, U, Sn, Be, and F, yet is part of a broadly calc-alkaline suite (Harrison, 1988). The garnets occur locally near the margins of the pluton and are all considered to be the products of magmatic crystallisation from a Mn-enriched, volatile granite ponded against the walls of the body (Harrison, 1988). The Cairngorm Pluton (Brown et al., 1981) is relatively depleted in Ba and Sr and enriched in radio-active elements such as U and Rb, also tin and beryllium. They are considered to be primary constituents of the intrusion and the incompatible behaviour of REEs (e.g. high Y content) in the Cairngorm intrusion coupled with a strong negative europium anomaly suggests cumulate feldspar at depth (Brown et al., 1981).

Exposures vary from jointed surfaces of solid rock to masses of slightly displaced blocks and a deeply weathered quartz and feldspar sand. The best exposures of the Main Phase (Phase 2) are in the corrie walls around Ben Macdui, Cairn Toul, Braeriach and Derry Cairngorm. Farther west exposures are common around Loch Einich. Areas of tors and sheet jointing in Glen Geusachan (Glasser, 1997) occur in the coarse-grained porphyritic subphase of the Main Granite.

Lower Devonian conglomeratic outliers overlie adjacent granitic intrusions and it is likely that parts of the Cairngorm granite were first exposed as high mountains around that time (Glasser, 1997) and that post-Devonian depths of erosion have been modest, since the Cairngorm granite retains near-surface (< 1.5 km) hydrothermal effects (Hall, 1991). The Cairngorm Mountains have formed the main Grampian watershed since the Early Devonian (Trewin and Thirlwall, 2002), suggesting that erosion over the mountains has been limited since that time. The current elevation of the Cairngorm massif is a result of Palaeogene uplift and subsequent minor phases of tectonic and isostatic vertical movement (Hall, 1991). Pre-glacial landform elements of the Cairngorms have been discussed by Gordon (1993).

Main Phase Granite (Phase 2). This is the main component of the pluton at outcrop (Figure 4) and comprises a largely coarse-grained biotite monzogranite with three textural varieties (Harrison 1986). The predominant variety is pink to red non-porphyritic medium to coarse biotite granite (G^G_c2a) with a grainsize between 4 and 7 mm. This variety forms the main exposures, for example, south-east of Ben Macdui [NN 998 980] (Plate 13) and around Cairn Toul [NN 961 972].

North-east of Ben Macdui [NJ 022 002] there is a coarser grained (7–12 mm) non-porphyritic granite (G^C_2a) in which pegmatitic patches are common. Porphyritic granite (G^C_2b), with megacrysts of K-feldspar 1–3 cm long, lies mainly in a belt south of Ben Macdui, around the Devil’s Point at [NN 976 951]. The contacts between the two facies are gradational over several hundreds of metres (Harrison, 1987). The Main Granite intrudes the earlier white porphyritic Glen Avon Granite (Phase 1) and is intruded by the finer grained leucocratic Beinn Bhreac Granite (Phase 3) in the Glenlivet district (British Geological Survey 1996).

Porphyritic Aplitic Microgranite (Phase 4). This grey-pink, medium granite (αF_C4) is weakly porphyritic and forms relatively small lenses or sheets within, or at the margin of the Main Phase Granite. It contains oligoclase and biotite with interstitial microcline and quartz. On its south- west margin the sheets at [NN 923 970] and [NN 932 930] are peripheral to the pluton. Here the porphyritic aplogranite does not appear to grade into the Main Granite and may indicate a localised concentration of volatiles at the contact (Harrison, 1987). Another belt within the Main Phase Granite has been traced across Gleann Einich [NN 915 990].

Carn Ban Mor Granite (Phase 5). In the western lobe of the pluton [NN 890 990], this porphyritic microgranite (FG_C5) lacks aplitic and pegmatitic patches and was mapped as a separate late phase (Harrison 1986; 1987). Its contact with the Main Phase Granite (Figure 4) is sharp and subvertical and it is the last major intrusive phase in the pluton. Although there is no evidence of a chilled margin, this phase becomes less porphyritic within a few metres of the contact with the Main Phase Granite.

Aplitic microgranite dykes αF e.g. at [NN 910 906] are associated with the Cairngorm Suite and dykes filled with hydrothermally altered intrusion breccia and accompanying quartz veins appear to be late-stage components.

Hydrothermally altered intrusion breccias. These minor late phase intrusions (htX) generally form dyke-like bodies trending north-east to north-north-east and are associated with similarly trending, late (?Siluro-Devonian) quartz veins in the vicinity of the Cairngorm Pluton. The hydrothermal alteration generally results in kaolinisation of feldspars, chloritisation of biotite and oxidation of iron oxides to produce hematite.

Contact metamorphic aureole. The metamorphic aureole around the Cairngorm Pluton in this district is difficult to determine as it lies within the psammitic lithologies of the Grampian Group. The aureole extends for at least 1 km south of the pluton on to Cairn Geldie [NN 995 885] in Gaick Psammite Formation where assemblages of biotite-plagioclase-quartz-K-feldspar occur. These minerals have been recrystallised into a harder fine-grained hornfels. The foliation is barely discernable in this zone but way-up structures have locally been preserved e.g. at [NO 007 897]. To the north of the pluton in the Aviemore district (Highton, 1999), semipelitic interbeds contain biotite+plagioclase+quartz+K-feldspar assemblages, including cordierite and andalusite. No new garnet or sillimanite has been recorded from the aureole of the Cairngorm Pluton. This assemblage is consistent with the P-T estimate of 650°C and 4.6 kb for another semipelitic assemblage recorded north of the pluton (Wells, 1979). However, P-T estimates recorded by Harrison (1988) for calcareous and pelitic assemblages in the Cairngorm aureole lie in the range 550±30°C and 1.2–2.0 kbar.

South of Scotland Suite (South Grampian Subsuite)

Glen Derry Diorite H_C

A small portion of the Glen Derry Diorite (Figure 1) lies on the north-eastern margin of the Ben Macdui sheet [NO 037 997]. The bulk of the intrusion lies on Sheet 65W (Braemar). The coarse diorite consists mainly of labradorite, augite and hornblende (mostly secondary). The diorite intrudes the Gaick Psammite Formation but the western contact of the body against the main phase of the Cairngorm granite includes a zone of intermediate composition as a result of fusion by the granite (Barrow et al., 1912; 1913). The fused rock contains biotite, interstitial quartz and a less calcic plagioclase than the hornblende-bearing diorite. The diorite is one of a number of small dominantly intermediate plutonic intrusions including the Glen Doll Diorite (Smith et al., 2002) considered part of the South of Scotland Granitic Suite. It is considered to be Silurian in age and certainly older than the Cairngorm Pluton dated at 404–408 Ma.

Glen Tilt Pluton

The Glen Tilt Pluton lies just to the north-west of the Loch Tay Fault (Figure 1) and covers an area of nearly 77 km². It intrudes Grampian and Appin group metasedimentary rocks (Plate 14) juxtaposed by the Boundary Slide. The pluton comprises a main intrusion of pink granite together with marginal granodioritic to dioritic rocks on its south-east side (Deer, 1938; 1950; 1953; Mahmood, 1986; Beddoe-Stephens 1993; 1994; 1997; 1999). South-east of the main Beinn Dearg granite (Figure 1), a smaller satellite body is centred on Sron a’ Chro. This satellite and an extensive area of the south-eastern part of the main pluton is granodioritic (sensu stricto). The granodioritic rocks typically have 64–72% SiO₂; the more basic variants representing hybridisation between granodiorite and diorite magma. The contacts between the main phases are gradational and show evidence of hybridisation. The Beinn Dearg granite is pink and coarse- grained with little biotite. High-silica granite varieties become more abundant towards the centre of the Beinn Dearg intrusion.

The diorite is mainly a massive, coarse-grained non-porphyritic intrusion containing abundant hornblende and variable proportions of biotite, plagioclase, quartz, K-feldspar, iron oxides and titanite. The diorite includes varieties of quartz-diorite, some of which contain chlorite as an alteration product of the mafic minerals (Mahmood, 1986).

Appinitic variants, containing large zoned hornblendes preserving clinopyroxene cores, occur locally grading into the main body (Beddoe-Stephens, 1993). Compositionally the diorite ranges from 48–58% SiO₂ and the appinitic lithologies are significantly richer in MgO, Ni and Cr. Inhomogeneous crystallisation led to initial localised accumulation of clinopyroxene to form the appinitic rocks. A later increase in the water content of the melt caused the alteration of the pyroxene to hornblende (Beddoe-Stephens, 1997). The non-appinitic diorites record variable cumulus enrichment of plagioclase or Fe-Ti oxide with movement and variable entrapment of residual intergranular melt in the form of quartz-K-feldspar crystallisation. In more-evolved diorites, the biotite content dominates over hornblende which it replaces. Mineral chemistry and analyses were given by Mahmood (1986) and Beddoe-Stephens (1999). Consideration of mineral chemistry and experimental phase relations led Beddoe-Stephens to conclude that the diorite crystallised from hydrous basic magma at 2–4 kbar and over the temperature range 1000–700°C. The pressure estimates are consistent with phase assemblages developed in the contact metamorphosed pelitic rocks. Mahmood (1986) noted that plagioclase and clinopyroxene were early crystallising phases and that biotite, alkali feldspar and quartz were late interstitial phases. The quartz-diorites, granodiorites and biotite granites show trends of enrichments in Th, Zr, K and Rb and depletion in Nb, P, Ti and La. A parental diorite composition appears to be incompatible with fractionation to quartz-diorite (Mahmood, 1986) and the quartz-diorite could not be modelled to form the biotite granite. She concluded that separate pulses of magma formed the Glen Tilt Pluton. The quartz-diorites and granodiorites could be linked by fractionation of a magma derived by melting of continental crust, whereas the earlier dioritic parental magma may have been derived from the upper mantle, as indicated by the high Ni and Cr values in the diorites and appinitic diorites (Mahmood, 1986).

The earlier workers (e.g. Deer 1938; Mahmood, 1986) stated that the diorite was intruded earlier than the granite as supported by the evidence of granitic veins and feldspar porphyroblasts within the diorite. Subsequently Beddoe-Stephens (1999) concluded that biotite granodiorite/granite was intruded before the diorite but the intrusions were close enough in time that local melt remobilisation and back-veining accompanied the diorite emplacement. A comagmatic suite of microdiorite porphyry dykes intrudes the granitic rocks and more rarely the diorite in the pluton. They contain zoned plagioclase microphenocrysts 1–2 mm long in a fine-grained matrix including primary brown hornblende and biotite and are locally quartz-phyric. These slightly fractionated melts were expelled during crystallisation of the diorite (Beddoe-Stephens, 1999).

The complex shape of the diorite is partly controlled by the late NW-trending fold of the junction between the Grampian Group and the Glen Banvie Formation. On the south-east side of the diorite, major movements on the Loch Tay Fault post-date the main intrusions of the pluton as they are truncated and do not appear on the south-east side of the fault (Stephenson, 1999). Despite later brittle movements on the Loch Tay Fault, which brecciate some minor microgranitic and microdioritic intrusions in the fault zone, this fault or a precursor, appears to have controlled the southeasterly extent of the pluton. A similar conclusion was arrived at by Oliver et al. (2008) in their study which dated the Sron a’ Chro body at 390±5 Ma using ion microprobe U/Pb zircon methods. They concluded that these I-type granites were intruded along the active, sinistrally transpressive Loch Tay Fault as an effect of far-field Acadian (Mid- Devonian) events. Oliver et al. (2008) also recorded local east-west-striking subvertical foliation and parallel ellipsoidal enclaves of (unfoliated) diorite and (foliated) psammite within the granite as evidence of deformation related to these events.

Later small leucogranite bodies, such as that on the south-east side of Conlach Mor [NN 932 766] intrude both the granite and the diorite. They are more evolved and contain muscovite (+/− biotite) and microcline. The body near Conlach Mor is a fine- to medium-grained, elongate stock-like intrusion (Beddoe-Stephens, 1993). The muscovite occurs as squat crystals up to 2.5 mm long, which appear to be primary (magmatitic in origin), rather than a replacement phase. The decrease in biotite content corresponds with an increase in SiO₂ and there is a silica gap between evolved granite (>75% SiO₂) and granite/granodiorite (<71–72% SiO₂). The commonly associated Siluro-Devonian dykes of quartz porphyry, felsite and lamprophyre do not appear to have intruded the Glen Tilt Pluton, but in the west minor microgranite and granodiorite, micromonzodiorite and quartz-diorite dykes occur both within and outside the Beinn Dhearg granite. The Sron a’ Chro intrusion is cut by microdiorite dykes typical of the late-stage activity within the pluton.

Contact metamorphic aureole. Because of the unreactive nature of the lithologies in the bulk of the Gaick Psammite Formation on the north-west side, and truncation by the Loch Tay Fault on the south-east side of the pluton, its metamorphic aureole has not been mapped. Barrow (1893; 1904) found evidence for an extensive ‘sillimanite aureole’ extending between the Glen Tilt Pluton and the Chest of Dee, based on the presence of small sillimanite needles in the psammites above Loch Tilt (Barrow et al., 1913 p.37).

Pelitic schists of the Glen Banvie Formation occur within enclaves of country rock north-west of the Loch Tay Fault and include contact metamorphic aluminosilicates, cordierite, spinel and corundum while associated calcsilicate rocks include garnet (grossular-andradite?)-diopsidic pyroxene skarns and tremolite/actinolite-diopside rock (Beddoe-Stephens, 1997). Sillimanite- bearing assemblages (S95369) occur in Appin Group strata 50 m south-east of the Loch Tay Fault (Beddoe-Stephens, 1997) indicating that the intrusion affected these rocks and limiting the displacement on the fault.

An early attempt to calculate the contact temperatures and pressures of the aureole of the Glen Tilt Pluton was made by Wells and Richardson (1979). Based on the assemblages cordierite- biotite-sillimanite-K-feldspar-plagioclase-quartz and cordierite-biotite-hypersthene-anthophyllite -plagioclase-K-feldspar-quartz, they calculated that the intrusion induced temperatures of 770°±40°C at 5.5 ±1.2 kb (total pressure with PH₂O close to Psolid ) within the aureole.

As a result of the mapping by the Geological Survey, around the Glen Tilt Pluton near Beinn Mheadhonach [NN 880 756] contact metamorphic mineral assemblages were recorded in local semipelitic beds within the Gaick Psammite Formation. These include garnet-cordierite (biotite- quartz-plagioclase-K-feldspar) rocks with a granoblastic texture (Beddoe-Stephens, 1997). The diagnostic assemblage of cordierite, sillimanite (locally after andalusite) and K-feldspar (S96542), and also minor amounts of spinel and corundum were recorded (Beddoe-Stephens, 1999) from pelites and semipelites in the Glen Banvie Formation. After consideration of mineral assemblages and inferred reactions in the contact rocks, Beddoe-Stephens (1999) concluded that a best estimate of pressure was in the range 2–3 kbar (consistent with other evidence that the pluton crystallised at 2–4 kbar under hydrous conditions). The evidence for partial melting is limited to small areas of granophyric quartz-feldspar intergrowth and these features are compatible with a relatively low pressure of contact metamorphism and peak temperature around 650–700°C. Locally (e.g. S95384) muscovite has been consumed to form aluminosilicate or cordierite assemblages. In places andalusite is partially replaced by sillimanite, probably as temperature increased. The locally observed breakdown of sillimanite (with biotite) could have occurred at higher PT to produce the cordierite+K-feldspar+/−corundum assemblages (Beddoe-Stephens, 1997).

1.4.2.3 Siluro-Devonian Calc-Alkaline Minor Intrusion Suite

The numerous minor intrusions of Caledonian (Siluro-Devonian) age include intrusion breccia, and silicic and intermediate to basic minor intrusions. Some of them can be related to the plutonic suites in the area (Beddoe-Stephens, 1997).

Intrusion breccia

Intrusion breccia forms an oval body intruding the Gaick Psammite 4 km south of the Cairngorm Pluton at [NN 996 840] and may be related to an explosive late stage of the pluton’s emplacement. The rock is a densely packed breccia with psammite fragments in a sparse fine- grained microgranitic matrix.

Silicic minor intrusions

Several minor granite, biotite granite, granodiorite, biotite granodiorite and granitic rock bodies are present although many cannot be directly related to the major plutonic suites. However, they all appear to be post-orogenic and probably late-Silurian to Mid-Devonian in age.

There is a distinct swarm of microgranitic or felsite (fine- to medium-grained felsic rock, unclassed) rocks, which are locally quartz- and/or feldspar-phyric. This swarm intrudes the Main Phase Cairngorm granite so is relatively late (i.e. post 404±18 Ma). The sheets and dykes also intrude some of the pre-existing shear-zones and north-north-east-trending faults, as well as along minor mainly east-north-east-trending faults, e.g. on Meall na Spionaig [NO 005 774]. On the south-east side of Glen Tilt they commonly occur in groups of three or four sheets, each up to 3 m thick. The rock typically has a pale grey-green, very fine-grained groundmass containing prominent white feldspar and less common clear quartz phenocrysts. A large sheet of quartz- feldspar porphyry with a granodioritic groundmass trends north-north-easterly from Gleann Mor, across Carn Dearg [NO 022 799] and on to Sheet 65W (Braemar). This rock consists of abundant euhedral phenocrysts of plagioclase feldspar and rounded quartz up to 4 mm in diameter. Additionally, numerous smaller biotite and a few amphibole phenocrysts are set in a fine- to medium-grained, granular, quartzofeldspathic matrix (Stephenson, 1990). It was emplaced into a major dislocation that significantly affects the Dalradian outcrop (see below).

Porphyritic microgranitic bodies, commonly in the form of sills, are relatively abundant intrusions in the Gaick Psammite Formation around the Gaick Forest e.g. at [NN 739 849]. Microgranodiorite is less common and locally granophyric /porphyritic.

Intermediate to basic minor intrusions

These are on the whole less common volumetrically than silicic minor intrusions. They include:- microdiorite, porphyritic microdiorite, quartz-microdiorite, micromonzodiorite, diorite and quartz-diorite, occurring mainly in the form of dykes, sills or plugs. Small dioritic bodies with a wide variety of grain-size, texture and colour index occur in the Loch Tay Fault-zone and probably relate to the Glen Tilt Pluton. A fine grained grey-green microdioritic rock is exposed in Allt na h-Easg’ Leathain [NN 9972 8165]. Locally it has a texture like a microdiorite but is pale and siliceous and may be more felsitic (Stephenson, 1995).

Minor appinitic diorite occurs in very small plugs north-east of Creag Dhubh [NN 6926 9915 and 6936 9895]. The rock is coarse grained, mafic rich and contains titanian amphibole, biotite and clinopyroxene set in a matrix of plagioclase, K-feldspar and quartz.

Lamprophyric dykes include spessartites, which are the hornblende-plagioclase-rich variety. They are fine to medium grained with locally titanian amphibole macrophenocrysts.

1.4.2.4 Late (?Siluro-Devonian) quartz veins and sheets, and late-Carboniferous dykes

A swarm of quartz veins and sheets of probable Siluro-Devonian age cuts the Gaick Psammite Formation between A’ Bhuidheanach Bheag [NN 660 775] and Carn na Caim [NN 670 820] in the south-west of Sheet 64W. The veins commonly trend north-east or NNE and may relate to the brittle faulting of similar trend in the area.

Minor undeformed doleritic dykes of probable Late Carboniferous age (about 300 Ma) crop out at [NO 090 748] and [NO 090 749] within the Gleann Beag Schist Formation in the core of the Meall Reamhar Synform.

1.5 Structure and metamorphism

1.5.1 Ductile deformation

The district is predominantly affected by the Grampian orogenic event, which occurred in the mid Ordovician at about 470 Ma. Any earlier Precambrian tectonothermal metamorphism (see below) is considered to be limited to the Glen Banchor Subgroup (cf. Dempster et al., 2002). The Grampian Event deformed the Dalradian Supergroup into a complex regional fold pattern of tight to isoclinal folds with amplitudes of up to tens of kilometres. In the ‘root zone–mushroom model’ of Thomas (1979) the Grampian Group was folded into the Atholl Nappe and the younger part of the Dalradian Supergroup into the Tay Nappe during D1. The early folds, mainly developed during intense D1 and D2 deformation (Lindsay et al., 1989), and their associated axial planar cleavage were considered to ‘fan’ across the region from upright structures in the north-west to south-east facing fold-nappes in the south-east where they pass into the overturned limb of the Tay Nappe (see Stephenson and Gould, 1995). There has been a debate as to whether the upright folds were part of a root zone to the recumbent folds or later refolding (Thomas, 1979; Bradbury et al., 1979; Treagus, 1987; Krabbendam et al., 1997). The recent British Geological Survey mapping (Leslie et al., 2006) has identified large-scale recumbent F2 folds, which face consistently south or SSE. The Gaick area is essentially a flat belt which gradually steepens to face downwards to the south below the Appin Group rocks and the Boundary Slide structure.

1.5.1.1 Deformation in the Glen Banchor Subgroup

The main phase of deformation to affect these rocks is characterised by large-scale, gently inclined to recumbent folds with axes trending east-west. These were affected by widespread north- to north-north-westerly-directed shearing along major high-strain zones such as the Blargie-Glen Banchor shear zone (Phillips et al., 1999). The high-strain zones include the Grampian Shear-zone (Piasecki, 1980).

The S2 fabric seen in the Glen Banchor pelites is a composite, coarse schistose to gneissose foliation. At a later stage in the D2 phase, much of the deformation became focussed along the major ductile shear-zones characterised by schistose to blastomylonitic rocks. Some of the shear- zones in the Glen Banchor basement, including one on An Stac [NH 680 003], were studied by Temperley (1991), Hyslop and Piasecki (1999), and the latter authors concluded that transport during the Knoydartian was approximately to the north-north-east. Sheared pegmatitic veins and pods are included in these zones and though it is debatable whether these are syn-tectonic, they have yielded Rb-Sr muscovite ages of 750–700 Ma (Piasecki and van Breemen, 1979; Piasecki and van Breemen, 1983). Within the Grampian Shear-zone farther north, neocrystalline monazite from both the associated pegmatitic rocks and the blastomylonite have yielded U-Pb isotope ages of about 806 Ma (Noble et al., 1996) and a U-Pb zircon age of 840+/−11 Ma was obtained from leucosome in a migmatitic psammite near Slochd Summit (Highton et al., 1999). This forms the evidence that at least part of the metasedimentary sequence south-east of the Great Glen Fault experienced a Neoproterozoic tectonometamorphic event (Highton et al., 1999). The fabric in the shear-zones is a composite S0-S2 fabric and is broadly coplanar with S2 outside the shear-zones.

The D2 structures are reworked by later upright north-east-trending F3 folds, which control the outcrop pattern of the main lithostratigraphical units. A large-scale upright antiform, cored by interlayered psammite with subordinate semipelite (QGB), has an axial trace trending north-east to the north of An Stac (Figure 2). About 2 km to the west, a complementary upright synform plunges to the north-north-east.

The later stages of this D3 phase were contemporaneous with the emplacement of pegmatitic and granitic intrusions at about 450 Ma (van Breemen and Piasecki, 1983). Since this would appear to be the same D3 as that which affects the Grampian Group, it begs the question as to whether the D1/D2 that affects the Dalradian rocks has also affected or overprinted the early deformation recognised in the Glen Banchor Subgroup. Lindsay et al. (1989) concluded that none of the migmatised rocks in the Glen Banchor Subgroup carried earlier (pre-Grampian Event) deformation or metamorphic fabrics but only the D1–D3 recognised in the Grampian Group and they could not confirm the existence of the Grampian Slide.

1.5.1.2 Ductile deformation North-West of the Loch Tay Fault

The nature of the ductile fabrics and fold architecture developed in much of the Grampian Group north-west of the Loch Tay Fault is relatively simple, in part due to the dominantly psammitic nature of the succession. Three phases, D1–D3, have been recognised. Planar S fabrics (Plate 15) dominate; with the exception of intersection lineations (S2 on S0 or vice versa) and/or F2 fold hinges, there are no widely developed L fabrics, e.g. conspicuous mineral or rodding lineations, apparent on S0 or S2.

The degree of co-axial flattening strain may have been considerable, the best preserved cross- bedding occurs in hinge-zones, elsewhere on folds limbs original bedding features have not been readily identified. Only hints of convergence of compositional laminae are seen in places. Regional facing is typically gently down to the south. The D2 structure only appears significantly modified (by D3 folding and fabrics) in the north-west part of Sheet 64W towards the Glen Banchor ‘high’, and suggests that buttressing against this basement feature (Robertson and Smith, 1999) is a significant factor south-east of Glen Truim.

D1 Deformation

The earliest phase, D1 has only been recognised in a few well-exposed sections such as on the A9 road cutting (Thomas, 1988) east of Crubenmore Lodge [NN 678 916]. Here isoclinal minor F1 folds with weak axial planar fabrics are refolded by the main recumbent F2 folds (Figure 5), (Figure 6) carrying the main regional foliation, S2. One recumbent refolded metre-scale F1 fold has a horizontal hinge trending N103° but interference patterns show that the F1 folds have curvilinear axes. No large scale or regional F1 folds or shear-zones have been identified in the Gaick area. In some semipelitic lithologies, the S1 fabric can be seen crenulated by S2, particularly in F2 fold hinges and it appears to be tectonic in nature (Leslie et al., 2006). Elsewhere the ‘bedding parallel’ fabric is defined by a preferred parallel orientation of small prismatic biotite crystals, which is rarely oblique to S0.

D2 Deformation

In the north-west of the district, in Glen Truim, both south-east- and north-west-verging folds are present and the overall geometry of the minor structures is consistent with the steep common limb of a F2 syncline-anticline pair, now displaced by faulting (see the cross-section on the Newtonmore sheet).

In the Gaick area, D2 produced the dominant regional planar foliation accompanying kilometre- scale recumbent folds and minor parasitic folds. In psammites S2 is the main penetrative foliation, defined by a stubby biotite alignment. In more pelitic intercalations S2 is a tight crenulation cleavage (Leslie et al., 2006). Systematic observation of the transection of the biotite foliation with bedding (S0) is commonly possible and together with identification of discrete hinge-zones (e.g. Allt Bhran [NN 765 903]), has been used to constrain a stack of kilometre-scale tight to isoclinal recumbent F2 folds (Figure 5), (Figure 6) and the cross-section on the 1:50k Bedrock geology Sheet 64W Newtonmore). A section through the Creag an Lochan Duin recumbent F2 syncline is exposed above Loch an Duin (Plate 16).

F2 fold traces extend broadly north-south across the Gaick plateau and represent typically gently east-dipping fold axial surfaces with gently east-plunging fold axes. The regional F2 hinge-zones are marked by stacks of close to tight F2 folds each with wavelengths of 0.5 m or less and commonly with a good axial planar fabric. Such hinge-zones can be several tens of metres thick and can have quite rounded profiles in a subvertical fold envelope as on Creag an Loch [NN 7292 8030] and along Allt Bhran [NN 770 897], which both lie on the trace of the Gaick Syncline (Figure 5). The limbs of the regional F2 folds are characterised by zones of moderately high strain, with very straight laminations in psammite, subparallel to S0.

The best examples of stratigraphical younging evidence and hence facing are seen along Allt Bhran [NN 772 896–764 902]. This section contains many F2 folds in mm-scale laminated psammites, clearly preserving cross-bedding and indicating southerly facing (Plate 17).

Southerly facing is also demonstrated in right-way-up graded (turbiditic) psammites at Feshie Bridge [NH 852 043], Creag Dhubh [NN 824 996] and Creag an Sroine [NN 838 970]. However, in an exceptional high-strain zone (depicted in the south-east of (Figure 5) with sheared rock symbols) S0, S1 and S2 are absolutely parallel. This zone is up to 1 km thick and is apparently a north-eastward continuation of the Dalnacardoch Banded Zone as defined in the Schiehallion area (Treagus, 2000) and section B in (Figure 6). The boundaries of the zone are transitional and it appears to die out north-eastwards on the Newtonmore Sheet. To the south-west in the Schiehallion district this increasingly flaggy zone is considered the product of strong deformation associated with the Boundary Slide and the tight Errochty Synform (Treagus, 2000). This deformation effectively prevents the subdivision of the Grampian Group into formations (British Geological Survey, 2000b). The Dalnacardoch Banded Zone had formerly been considered as a likely product of D3 sliding creating a composite S1, S2 and S3 (Thomas, 1980). The lack of a conspicuous linear fabric in this high-strain zone suggests intense flattening rather than highly non-coaxial strain.

South-east of the Dalnacardoch Banded Zone, south-facing F2 folds can again be traced due to systematic changes in vergence (Leslie et al., 2006). These are part of the Meall Reamhar/Clunes system of F2 folds recognized by Treagus (2000). The extension of the Coire Bhran Anticline (Clunes Syncline on (Figure 6)), the Clunes Anticline, and the Errochty Syncline have been traced (Figure 6). Although the trace of the Coire Bhran Anticline is aligned with the Bohespic Antiform (part of the post D2-preD3 Errochty phase of Treagus, 2000) on the current edition of the Schiehallion sheet (British Geological Survey, 2000b), this is unlikely to mark a single continuous fold trace. The relationships of the Bohespic Antiform were re-examined in critical sections of the Schiehallion district (Leslie et al., 2006) and it was concluded that the fold is a composite structure consistent with a F2 antiformal syncline closure that is overprinted by co- axial crenulations folding and co-planar cleavage development.

D3 Deformation

Evidence for D3 deformation is found in the north-western part of the Newtonmore Sheet as the Glen Banchor ‘high’ is approached. Open to close folds with overturned axial planes assigned to D3 are common in the Glen Truim area but vary in abundance and tightness. The major F3 synform passing through Creag Dhubh [NN 677 975] can be traced south-west for over 3 km and a complementary anticline may be located in the poorly exposed ground east of the Glen Truim Fault [NN 687 944]. On the south-east limb of the Creag Dhubh synformal structure, numerous minor F3 folds with an associated crenulation cleavage are seen in interbedded psammite and pelite lithologies. The axes generally trend north-east–south-west (in contrast to the south- easterly orientation of F3 ‘crossfolds’ in the Glen Tilt and Gleann Fearnach areas). In the north of this area the axes of minor F3 folds plunge ≤ 20° to the north-east, but to the south they plunge ≤ 30° to the south or south-west. The folds generally have steep long limbs and shallowly dipping short limbs; axial surfaces dip moderately to steeply south-east in contrast to the gently ESE-inclined S0/S1 in the Gaick region. In semipelitic bands the axial planar, commonly widely- spaced, crenulation cleavage dips about 60° to 80° to the south-east. The crenulation cleavage is well seen in the Creag na Sanais semipelite east of Meall Ruigh nam Biorag. However, locally there appears to be fanning of the crenulation cleavage to dip up to 40°NW. Near the summit of Am Binnein a large scale F3 culmination appears to be upright and plunging south-west.

In the south-east of the Ben Macdui area (Figure 7), north-west-trending axial traces of folds of the S2 fabric indicate the later development of ‘F3 crossfolds’. These later structures have a similar relationship to the local earlier fabrics as that of the Trinafour phase folds in the Schiehallion area (Treagus, 2000).

The open north-west-trending synform (Conlach Mhor Synform) within the Glen Banvie Formation to the north-west of the Loch Tay Fault appears to fold the Boundary Slide (see below) and has its axial trace passing south of Conlach Mhor. A complementary dome-like antiform (An Sligearnach Antiform) has a NW-trending trace lying about 2.5 km north and passing through An Sligearnach [NN 952 782]. Close to the Loch Tay Fault, in the north-western bank of the River Tilt between [NN 935 743] and [NN 938 745], minor upright close to tight folds of Glen Banvie Formation strata plunge at 10–20° to the north. The axes, some with related mullions, are fairly consistent in orientation but might reflect their proximity to the fault. The major fold axes probably plunge to the south. However, in close proximity to the Glen Tilt Pluton the foliation in the country rock is variable suggesting intrusion-related deformation.

Boundary Slide north-west of the Loch Tay Fault

On the north-west side of the Loch Tay Fault, the Boundary Slide (Figure 7) is considered to be a D2 high-strain zone separating the Glen Banvie Formation from the Gaick Psammite. It is largely obscured by the intrusion of the Glen Tilt Pluton but Beddoe-Stephens (1997) found slight evidence of higher degrees of strain-related recrystallisation in the flaggy rocks at Allt Mheann [NN 973 781] and Allt a’ Chrochaidh [NN 956 768].

1.5.1.3 Ductile deformation South-East of the Loch Tay Fault

Early Folds (F1/F2)

Early folds are tight to isoclinal and, west of Glen Loch, axes trending NE-SW are common (due to refolding?) whereas east of Glen Loch, axes trend nearly E-W. In common with other regional interpretations (e.g. Treagus, 2000), isoclinal F1 folds in the Glen Tilt area are interpreted as being refolded by near-coaxial, tight to isoclinal F2 folds. Therefore F1 folds either face upwards to the north-west or downwards to the south-east.

The major F1 fold in the south of the Ben Macdui 64E sheet is the Beinn a’ Ghlo Anticline which has a north-north-east-trending axial trace (Figure 7) and lies above the Carn an Righ Slide (see below). Older rocks of the Glen Clunie Graphitic Schist are exposed in its core where the axial plane dips steeply to the east-south-east. The fold axis appears to be curvilinear and the fold is upward facing according to Bradbury et al. (1979).

In the Bedrock cross-section 1 on the accompanying 1:50k edition of Sheet 64E the Beinn a’Ghlo Anticline is interpreted as an F1 fold lying on the short limb of a larger north-west- verging F2 fold whereas Bradbury et al. (1979) considered it to be F3.

One major fold which has been clearly assigned to F1 is the downward south-east-facing anticline in the Beinn a’ Ghlo Transition exposed in northern Glen Loch between the Coire Breac and Fealar faults [NN 988 777] near Creag Dhearg (Stephenson, 1991). It is downward facing due to refolding by D2 and is also refolded by smaller-scale upright F4 folds. The core of this anticline, occupied by the Glen Clunie Graphitic Schist Formation, is exposed west of Creag Dhearg, on the immediate south-east side of the Coire Breac Fault around [NN 988 779].

Throughout the area south-east of the Loch Tay Fault minor tight to isoclinal folds with axes trending approximately NE-SW are assumed to be early (F1 or F2). Both north-easterly and south-westerly plunging folds are observed but the majority plunge north-east at gentle angles (< 20°) and most have north-westerly vergence. Good examples are exposed in Allt Fheannach, just below the junction with Allt Coire a’ Chaisteil [NN 967 747], (Plate 18). A range of folds are well seen in metalimestones below the Glen Tilt Thrust where they have attenuated limbs and resemble the shear folds attributed to D2 elsewhere (Stephenson, 1995).

North-east of Glen Loch, a stack of recumbent F2 folds facing north, has been identified within the Ballachulish and Blair Atholl subgroup strata below the Carn an Righ Slide (see cross- section 2 on 1:50k Bedrock Sheet 64E). One downward-facing closure is marked by a limestone within the Glen Loch Phyllite and Limestone Formation at [NO 007 786]. To the north [NO 007 797] the axial trace of a northward-facing F2 recumbent fold is repeated about the open F3 Meall Chrombaig Synform. To the north of the loch, below the slide on Sron a Bhoididh at [NO 001 830], An Socach Quartzite is folded by a tight F2 fold repeated about a NW-SE trending F3 fold cored by Glen Loch Phyllite and Limestone Formation. South of the Boundary Slide, near Buachaille Breige, at [NO 022 852] a downward-facing F2 antiform is cored by Glen Clunie Graphitic Schist Formation.

F3 folds

The major fold of this ‘D3 crossfold’ phase is the Meall Reamhar Synform (Figure 7). This broad synform (see cross-section 2 on 1:50k Bedrock Sheet 64E) has a south-easterly trending trace [NO 013 740]. It is an upright to inclined fold with an axial plane that dips steeply to the north-east in places and subsidiary minor folds that plunge 25° to 40° ESE. The S2 fabric and Glen Loch Slide are folded around this syncline (Stephenson, 1995). It has a complex D1 to D3 history according to Crane et al. (2002) as it refolds the F1 Gleann Fearnach Syncline. The synform has an open closure at the head of Gleann Fearnach and the Ben Lawers Schist Formation crops out in the fold core. On the south-west limb of the synform, minor F3 folds consistently verge to the north-west across the earlier Beinn a’ Ghlo Anticline.

The Meall Chrombaig Synform (Figure 7) is another large-scale open F3 fold centred on the ridge of An Socach Quartzite Formation at Meall Chrombaig [NO 008 807] where exposure is poor but the fold appears to have an easterly trending fold trace. Farther north a large F3 fold with a roughly E-W axial trace at [NO 019 850] occurs south of Buachaille Breige.

Elsewhere in the area many of the F3 intermediate and minor scale folds are close to tight, or locally isoclinal, with axes that plunge east to south-east. A large cylindrical F3 fold in quartzite [NN 9516 7509] appears to deform the Carn Torcaidh Slide.

East of Glen Loch, D3 affects the rocks in the Gleann Mor section below the Carn an Righ Slide, where a prominent late, spaced or crenulation cleavage in pelites has a regular dip of 40° to 60° to the south-east. This is axial planar to NW-verging folds. Close F3 folds exposed in the Sron nan Dias Pelite and Limestone Formation in Crom Allt [NO 026 788] plunge 42° NE, verge north-west and have an axial planar cleavage dipping to the south-east.

Late folds (F4)

NW- to NNW-verging F4 monoclines and close folds with a wavelength of up to a few tens of metres are prominent locally, and are particularly well exposed in the Beinn a’ Ghlo Transition Formation in the An Lochain stream section [NN 984 776] (Stephenson, 1991). Here, between the Carn an Righ Slide and the Glen Tilt Thrust, is a NE-SW-trending synform of F4 age with associated near-horizontal minor fold hinges. The F4 fold is complicated by the effect of the Coire Breac Fault which lies close to or coincident with the F4 hinge for most of its length (Stephenson, 1995).

An associated spaced or crenulation cleavage, dipping steeply to the south-east or SSE, is widespread throughout the area in suitable lithologies. This deformation is considered to relate to the D4 Highland Border Downbend (Crane et al., 2002), which mainly affects the Tay Nappe farther south-east.

Shear-zones/Slides/Thrusts south-east of the Loch Tay Fault

The differing competencies of the lithological units within the Appin Group, and that between the Grampian Group and the Appin Group as a whole, have caused several thrusts and slides to develop in the area south-east of the Gaick Psammite Formation (Figure 7). The more major ductile breaks in the succession divide the pile into fold packages. Most of the ductile dislocations are considered to relate to the D2 deformation phase.

The Boundary Slide is the term given to the D2 high-strain zone between the more competent Gaick Psammite and the overlying heterogeneous Appin Group succession in the Glen Tilt area (Figure 7). The Boundary Slide crops out on the south-east side of the Loch Tay Fault north of Buachaille Breige [NO 022 854] and continues striking east onto Sheet 65W (Braemar). Farther south it is inferred to lie below the current level of erosion. North of Buachaille Breige the slide is a steep discordant structure (see Cross-section 2 on the 1: 50 000 map) which appears to have been reactivated in D3. Just to the south of the slide here, the Lochaber Subgroup is excised and Ballachulish Subgroup strata are tentatively interpreted as being disposed in a downwards-facing F1/F2 fold refolded by D3.

In Glen Tilt south-west of Creag an Duibh [NN 983 788] a dislocation, possibly indicating the Boundary Slide, trends N046°, dips 36°SE and separates metalimestones above from shattered green-grey banded psammites below (Stephenson, 1991; 1995). The sinistral offset of the slide either side of the Loch Tay Fault system as observed in the Glen Tilt area is therefore estimated to be about 6 km (see discussion on the Loch Tay Fault).

On the south-east side of Glen Tilt, the Carn Torcaidh Slide superimposes the An Socach Quartzite above tightly folded metalimestones of the Blair Atholl Dark Limestone. It extends for about 2.5 km from just east of the Allt a’ Choire Bhuide Mhoir at [NN 9515 7492], where it is over-ridden by the Glen Tilt Thrust, to [NN 940 732] where it is over-ridden by the Carn an Righ Slide. Where the dislocation follows Allt air Chul, a strong planar fabric occurs in the quartzite at [NN 9435 7417] and the underlying metalimestone is very reddened with hematite (Stephenson, 1995). At a waterfall [NN 9494 7501] in Allt air Chul, a tight fold in the quartzite over-riding schist suggests movement of the quartzite to the NW. The Carn Torcaidh Slide appears to be an early structure as it has been folded around a SE-plunging tight F3 fold.

The Glen Tilt Thrust lies 100–300 m to the south-east of the Carn Torcaidh Slide, dipping 35° to 50°SE. This dislocation thrusts a right-way-up succession of lower Ballachulish Subgroup rocks over a tightly folded package of upper Ballachulish Subgroup and Blair Atholl Subgroup rocks, all dipping regionally to the south-east. The structure is most noticeable where it superimposes the orange-brown-weathering Beinn a’Ghlo Transition Formation above grey Glen Loch Phyllite and Limestone rocks. It crops out for about 7 km from Allt Ruigh na Cuile in the north-east at [NN 9866 7873] to Allt Coire Riabhaich in the south-west [NN 942 734]. At its north-eastern end it is cut off by late brittle faults and at its south-western end it is over-ridden by the Carn an Righ/Glen Fender Slide. Details of the thrust and minor parallel dislocations were given by Stephenson (1995).

The Carn an Righ Slide is a major dislocation (Figure 7) which along most of its strike length juxtaposes An Socach Quartzite, much of which lies on an inverted fold limb, over a range of older and younger rocks. It crops out on the northern side of Carn an Righ [NO 026 782]. The slide has been traced for about 24 km south-west into the Glen Fender Slide on Sheet 55E. North of the Beinn a’ Ghlo range, it passes to the north-west of Meall a’ Mhuririch at [NN 970 755], then can be traced north of Meall na Spionaig, across the north of Carn an Righ and on to Sheet 65W. Locally it is associated with a wide zone of shearing and stretching lineation developed in the strong planar fabric. The position of the slide is commonly marked by later brittle deformation, which caused growth of coarse-grained muscovite and pyrite.

The Carn an Righ Slide is associated with two subsidiary slides; one on the south side of the Carn an Righ summit and the other, farther south, repeats the An Socach Quartzite and the Glen Loch Phyllite and Limestone formations on a southward younging fold limb. Details of the slide, related dislocations and folds were given by Stephenson (1995) and Crane et al. (2002). About 500 m north-west of the main slide, the slides mapped encircling Meall Gharran at [NN 980 770] and crossing Creag Dhearg at [NN 989 779] may be related to, or down-faulted sections of the Carn an Righ Slide (Stephenson, 1995).

The Glen Loch Slide is folded around the F3 Meall Reamhar Synform to the east of Glen Loch. It juxtaposes the Tulaichean Schist above the Glen Loch Phyllite and Limestone Formation and, farther south, the Sron nan Dias Pelite and Limestone Formation as it cuts up slightly through the underlying succession (see Cross-section 2 on the 1:50K Bedrock Sheet 64E). It is exposed in Allt Glen Loch near [NO 001 718], where metalimestones and pelites are overlain by very flaggy grey psammite/quartzite (Stephenson, 1995). Adjacent to the slide the rocks carry a persistent stretching and/or intersection lineation plunging ESE at about 40°.

Extended south onto Sheet 55E (Pitlochry) this slide equates to the Killiecrankie Slide (Bradbury et al., 1976), but the section north of the Ben Vuirich Granite was renamed the Glen Loch Slide (Crane et al., 2002) as the slide has been interpreted to act within the Blair Atholl Subgroup (and not between the latter and the Killiecrankie Schist of the Easdale Subgroup; see section on the Tulaichean Schist Formation)

The Carn Dallaig Slide zone is marked by the attenuation of the Glen Lochsie Calcareous Schist Member between the Tulaichean Schist and Ben Lawers Schist formations in upper Gleann Fearnach [NO 02 73]. This D2 slide extends on to Sheet 65W/56W (Crane et al., 2002) and links with the Glen Lochsie and other slides at higher structural levels. At lower structural levels, a high-strain zone, marked by attenuated Blair Atholl to Easdale Subgroup formations in west Gleann Fearnach [NO 00 74] links with the Carn Dallaig Slide. The slide and the S2 fabric in its vicinity have a north-westerly strike and dip steeply to the north-east.

1.5.2 Regional metamorphism

The regional metamorphic zones were first described in the Grampian Highlands in terms of zones defined by a set of index minerals developed in pelitic rocks (Barrow, 1893; 1912). These prograde Barrovian Zones (chlorite-biotite-garnet-staurolite-kyanite-sillimanite) were slightly modified by Tilley (1925) and extended across the Scottish Highlands. The Barrovian metamorphic facies series proved to be distinct from the Buchan facies series in the north-east Grampian Highlands as the difference between an intermediate to high pressure series and a low pressure series was established (Fettes et al., 1976). The metamorphic zones were extended into areas to the north-west which generally lacked pelites by Winchester (1974) working mainly on comparisons with calcsilicate assemblages. According to the widespread definitions of metamorphic facies the Newtonmore and Ben Macdui sheets lie within the medium pressure lower amphibolite metamorphic facies (Harte, 1988) developed during a single, but polyphasal, Grampian Event.

Wells (1979) used calcsilicate assemblages (commonly hornblende-plagioclase-garnet-epidote- quartz-titanite) within the Grampian Group to calculate P-T which varied from 560°C and 7 kbar near Loch Laggan to 650°C and 9.5 kbar in the centre of the Spey Valley. The uncertainties in the P-T estimates correspond to a minimum pressure for the central Spey Valley near Kingussie of 8 kbar at 600–650°C. The pressure conditions correspond to 30–35 km of tectonic cover, increasing to 35–40 km in the direction of Glen Tilt, at metamorphic temperature maxima. These P-T estimates place the Grampian Group of the district, including Glen Tromie, within the kyanite zone (Wells, 1979).

However, the P-T conditions estimated over the district are not likely to be coeval and the Glen Banchor Subgroup might have been metamorphosed prior to the Grampian Event so that the facies pattern there is likely to be composite and polymetamorphic due to the Grampian overprint. Semipelites within the Glen Banchor Subgroup have assemblages:- quartz, biotite, plagioclase, muscovite +/− garnet, kyanite, K-feldspar and rare staurolite. In the Glen Banchor Subgroup within this district two garnet-bearing semipelite samples at [NH 700 008] and [NN 668 983] have been calculated to have mean P-T estimates of 638° and 4.6–5.4 kbar and 575° and 4.0–4.3 kbar (Phillips et al., 1999).

In the coarse schistose to gneissose Glen Banchor lithologies the S1 foliation is defined by lenticular, anastomosing mica folia wrapping quartz-plagioclase lenticles; K-feldspar is rare. In these rocks the local development of migmatites suggests that incipient anatexsis occurred (at high pressures indicated by kyanite assemblages).

Within the ductile late-D2 shear-zones, the assemblage quartz-biotite-muscovite-garnet- plagioclase (An_20–33) +/−fibrolite and rare prismatic sillimanite is found (Phillips et al., 1999). The fibrolite appears to form from the breakdown of biotite, not necessarily from breakdown of kyanite. The fibrolite foliae are deformed by tight crenulations and S-C-like fabrics and so appear to be contemporaneous with the late-D2 shearing. Fibrolite development may be strain induced (Vernon, 1987), but the P-T estimates for the Glen Banchor Subgroup within the district lie within the sillimanite stability field (Phillips et al., 1999) which followed a decompression event (i.e. after the eclogite facies). The same sillimanite-grade conditions however, affected to varying degrees the Glen Banchor succession, the Grampian Group, and the Appin Group in the Blargie-Glen Banchor area (Phillips et al., 1999 (Figure 4)). Phillips et al. (1999) concluded that there is no evidence for more than one orogenic event in the area, and according to available geochronological data for the Central Grampian Highlands, this could be entirely Precambrian (840–800Ma).

Across much of the Grampian Group there are few lithologies which give the true regional metamorphic index minerals. This makes any distinction between the metamorphism of the Glen Banchor Subgroup and the Dalradian Supergroup difficult. However, it is known that during the Grampian Event, peak metamorphic conditions were attained broadly synchronous with the main deformation phase (D2) (Phillips et al. 1994; 1999). Petrological textural studies show that most of the mineral growth was post D2. Biotite, and local garnet, assemblages are recorded from these rocks. Phillips et al. (1999) reported a coherent metamorphic history in all the stratigraphical groups with development of biotite during D1 and kyanite early in D2. The general conditions for this kyanite growth as calculated by Phillips et al. (1999) are 7–8 kbar and 500–600°C.

During the later part of D2, significant decompression resulted from movements on the shear- zones in the north-west of the Newtonmore area and consequently P-T conditions of 5–6 kbar and 585–695°C prevailed (Phillips et al. 1999). The rocks were therefore moved out of the stability field of kyanite into that of sillimanite. However, in the south of the district no similar pressure decrease occurred and the rock remained in the kyanite field.

Within the Gaick Psammite Formation north of the Glen Tilt Pluton, assemblages in psammitic and semipelitic schists include quartz-plagioclase-biotite+/−garnet+/−K-feldspar (+/−chlorite). Garnet is a minor phase found in some lithologies, occurring as small inclusion-free anhedral porphyroblasts in the quartz-feldspar-biotite matrix. The biotite is usually green-brown, weakly to moderately aligned parallel to compositional layering or the laminar fabric of the rock (Beddoe-Stephens, 1997). One psammitic schist (S99219) contains the assemblage quartz- plagioclase-biotite–hornblende-?K-feldspar and appears to be a calcsilicate assemblage. These rocks appear to be essentially muscovite-free; except in one case (S95398) where ragged laths are probably retrogressively replacing biotite and K-feldspar. Chlorite has locally replaced biotite. Small areas of granophyric quartz-feldspar intergrowth are evident suggesting incipient melting in suitable lithologies (Beddoe-Stephens, 1997). Cordierite-bearing assemblages are considered to lie within the metamorphic aureole of the Glen Tilt Pluton.

The Glen Banvie Formation contains a wider variety of lithologies, including amphibolites with assemblages of quartz-plagioclase-K-feldspar-hornblende+/−biotite; metacarbonate rocks typically with calcite+/−plagioclase+/− secondary chlorite; calcsilicate rocks with quartz- muscovite-chlorite-actinolite/tremolite+/−biotite+/−plagioclase; quartzites containing quartz- plagioclase+/−muscovite+/−biotite+/−chlorite as well as pelites and semipelites containing quartz- plagioclase-muscovite-biotite-garnet+/−calcite. These assemblages have commonly been overprinted in the contact metamorphic aureole of the Glen Tilt Pluton and as a result these pelites contain cordierite and /or sillimanite/andalusite. The overprinted calcsilicate rocks contain diopside together with the above assemblages (Beddoe-Stephens, 1997). Prior to the overprinting, all these assemblages are presumed to have formed within the medium pressure kyanite zone. Pelitic assemblages from the formation containing kyanite were recorded in the adjacent Pitlochry district to the south (Smith, 1980). The chlorite and some white mica are products of retrogression.

The Dalradian Supergroup to the south-east of the Loch Tay Fault in this district is indicated to be within the kyanite zone (Baker, 1985). This agrees with the Appin Group assemblage quartz- plagioclase-garnet-biotite recorded in the area (Beddoe-Stephens, 1997) and the records of garnet (Stephenson, 1995) and kyanite (Pantin, 1961) found within the Glen Clunie Graphitic Schist Formation.

Within the Appin Group in Glen Tilt at [NN 928 737] and [NN 937 736] two pelitic samples indicated temperatures of 630–635°C from garnet-biotite exchange thermometer data (Baker, 1985). This is at the upper end of the range calculated earlier by Wells and Richardson (1979). However, regional metamorphic pressures calculated by Baker (1985) tend to be lower (6–10 kbar) than those calculated for the eastern Dalradian (9–12 kbar) by Wells and Richardson (1979). Also within the kyanite zone east of Glen Tilt, Chinner (1980) traced an isograd Ky50, based on the coexistence with kyanite and staurolite of biotite M/FM=50, which delineates a recumbent thermal anticline. However, the wide crop of kyanite zone regional metamorphism across the district suggests that the metamorphic gradient is low and hardly affected by post-peak of metamorphism (Grampian deformation).

The kyanite zone extends to the south-east corner of the Ben Macdui district (Crane et al., 2002), although pelitic assemblages containing kyanite are rare due to lack of rocks with suitable composition. In this area, at this grade, semipelites (and psammites) in the Tulaichean Schist Formation contain the assemblage quartz-biotite-muscovite-plagioclase-K-feldspar-garnet. The garnets are wrapped by the S2 foliation and grew syn to late D2 based on inclusion trail evidence, and before the development of S3 crenulation cleavage (Crane et al., 2002).

The basic meta-igneous rocks (pre-D2), within the Ben Lawers Schist Formation contain assemblages: hornblende-plagioclase-quartz-epidote+/−garnet+/−calcite. These indicate that a lower grade epidote-bearing zone can be tentatively drawn in the south-east corner of the district (Crane et al., 2002; figure 22), since farther north-west, in Gleann Mor, amphibolites contain garnet but no epidote. The typical assemblage in the latter amphibolites is: hornblende- plagioclase-quartz-garnet with accessory opaque minerals and titanite; biotite is probably retrogressive. As epidote is trapped in syn-D2 garnet inclusion trails but absent outside the porphyroblasts, the garnet amphibolites probably indicate a north-westwards increase in metamorphic grade from the epidote-garnet amphibolites.

Within the Appin and Argyll groups there are also numerous metacarbonate and calcsilicate rock assemblages and potentially any systematic change in their metamorphic assemblages should be recognisable across the area from Glen Tilt to Gleann Fearnach. However, the variation in composition, with impurities such as feldspar, phlogopite and epidote group minerals, together with the dependence on fluid composition, make it difficult to distinguish any zonation (cf. Crane et al., 2002). This means, for instance, the range of metacarbonate rocks to calcareous schists in the Glen Loch Phyllite and Limestone typically contain assemblages: calcite+/− quartz+/−muscovite+/−biotite or phlogopite+/−plagioclase+/−K-feldspar+/−garnet. Calcareous assemblages in the Blair Atholl Subgroup are similar but locally with abundant graphite.

The calcareous schists in the Ben Lawers Schist, Gleann Beag and Tulaichean Schist formations typically contain assemblages: quartz-muscovite-biotite-chlorite-amphibole-feldspar-epidote and the amphibole is commonly hornblende and/or tremolite in large porphyroblasts arranged in a random garbenschiefer texture on the schistosity surface (Crane et al., 2002).

Calcsilicate rock layers, for example, forming subordinate poorly foliated layers in the Tulaichean Schist Formation and the Glen Lochsie Calcareous Schist Member typically contain assemblages: quartz-plagioclase-biotite-epidote/zoisite/clinozoisite-garnet-amphibole. The amphibole may be tremolite/actinolite or hornblende.

1.5.2.1 Age of metamorphism

The exact age of the initial progressive tectonothermal event affecting the Central Grampian Highlands is uncertain. Within the equivalents to the Glen Banchor Subgroup, ages from partial melt migmatitic psammites (Highton et al., 1999) give a potential age constraint on the Central Grampian Highland D1-D2 of 840 Ma, while the shear-zone fabrics suggest that late-D2 decompression occurred about 800 Ma (Noble et al., 1996). If this D2 is the same as that in the overlying Dalradian Supergroup, it places much of the tectonic history of the district in the Precambrian, as opposed to the Ordovician as it would be if the early deformation was due to the Grampian Event. This interpretation requires a major unconformity or tectonic discontinuity at a higher level in the Dalradian Supergroup for which there is no current support. While it may be argued that the older ages were obtained from tectonically emplaced older basement, Phillips et al. (1999) noted that this does not explain the c. 800 Ma U-Pb monazite ages obtained from the blastomylonitic schists derived from late-D2 shear-zones associated with muscovite and fibrolite growth. The timing of the D3 phase is constrained by granites, such as the Strathspey Granite and associated pegmatitic rocks which give U-Pb monazite ages of c. 447 Ma (Noble in Phillips et al., 1999).

Metamorphic garnet from the Southern Highland Group in the Pitlochry area has been dated using Sm-Nd isotopes (Oliver et al., 2000) at between 476.6+/− 2.5 Ma and 472+/−2 Ma. These are the times at which the garnet stopped growing, as the rocks were metamorphosed below the c. 700°C Nd diffusion blocking temperature. Dalradian metamorphic K-Ar and Rb-Sr muscovite and biotite ages tend to be younger, as they are dependent on cooling/blocking temperatures, but they are generally the same age, within error, or younger than the age (467+/−8 Ma) of the Ballantrae Ophiolite metamorphic sole (Oliver, 2001). Oliver (2001) interpreted the latter to be the age of contemporaneous island-arc collision and obduction in Scotland and the cause of the Grampian Event of the Caledonian Orogeny. Sm-Nd age determinations from garnet in the sillimanite zone of Glen Clova confirmed that the peak metamorphic temperatures in Barrow’s zones occurred penecontemporaneously (Baxter et al., 2002) at 472.9+/−2.9 Ma (early stage) and 464.8+/−2.7 Ma (late stage). Baxter et al. suggested that local igneous intrusions provided additional heat beyond relaxation of over-thickened crust.

1.6 Faulting

1.6.1.1 Lineaments/early deduced faults

The north-easterly trend of the Glen Banchor ‘high’ is a persistent feature in the architecture of the north-west of the district associated with slides and unconformity and it appears to control the Grampian D3 deformation and some of the brittle faulting.

The Deeside Lineament has no surface expression but is believed to have controlled the intrusion of the inferred East Grampian Batholith (Stephenson and Gould, 1995).

1.6.1.2 Brittle Faults

Major faults

The Newtonmore area is cut by the north-easterly-trending Ericht–Laidon Fault. This major late- Caledonian fracture is traceable for about 170 km north-eastwards from Tayvallich, through Loch Ericht and across Glen Truim towards Grantown-on-Spey.

In the Dalmally district, south-west of the Etive Pluton, Treagus (1991) recorded an early dip- slip component of about 1.3 km down to the north-west followed by sinistral strike-slip of 4–5.5 km. The fault movement largely pre-dates the eruption of Siluro-Devonian (Gradstein et al., 2004) Lorn lavas (424–415 Ma). About 25 km south-west of the Newtonmore district the sinistral offset on the fault as it affects the Moor of Rannoch Granite is 6–7 km and dip-slip is apparently absent there (Hinxman et al., 1923; Treagus, 1991).

Nearly 4 km south-west of Newtonmore, the Falls of Phones Semipelite Formation is repeated across the Ericht–Laidon Fault and there appears to be a component of dip-slip down to the north-west of up to 3 km (see Cross-section on Newtonmore Bedrock sheet). This throw is probably less to the south-west of the junction with the Glen Truim Fault as it downthrows to the east. The amount of sinistral displacement on the fault cannot be quantified in this district as the stacking of flat-lying folds makes matching of the semipelite units difficult in three dimensions. To the north-east, in the Tomatin and Aviemore districts it is estimated that up to 8 km of sinistral displacement occurred on the fault, although it appears to have decreased to approximately 1 km around Grantown-on-Spey (Highton, 1999) and it has not been recognised on the Knockando Sheet (85W) to the north-east.

The Ericht–Laidon Fault is not directly exposed in the district but exposures along the Allt a’ Bhinnein [NN 6694 9100] show numerous minor faults and crush zones, which are associated with brittle-fractured red granite veins. The main fault is inferred to lie partly along the line of Allt a’ Bhinnein and, to the east of the A9 road, along the length of Loch Etteridge. A subordinate synthetic fault with a similar north-easterly trend lies just to the west of Meall Odharaich [NN 6865 9039]. To the north-west, in Coire Mhoraich [NN 658 913] and south of Creag na Sanais [NN 656 920], minor north-easterly trending faults offset the Creag na Sanais Semipelite Formation sinistrally.

The Glen Banchor Fault, trending north-east along Glen Banchor, is probably part of the fault- set subparallel to the Ericht–Laidon Fault (cf. Treagus, 1991). It intersects the Craig Liath Fault to the south-west, but appears to continue north-eastwards through Loch Gynack with only a minor offset on the Glen Truim Fault. Since younger rocks lie on its south-east side, a component of downthrow to the south-east has occurred.

The north-north-easterly-trending Glen Truim Fault is a complex splay of the Ericht–Laidon Fault, on its north-western side, controlling the orientation of Glen Truim. It is similar to the Riedel shears trending N010°-015° related to other major northeast-trending faults with sinistral displacement. Faulting and brecciation is exposed near the Falls of Truim [NN 6807 9230], and along the Glen Truim gorge east of Crubenbeg [NN 6833 9273] and east of Poll Uaigh. The downthrow on the fault is considered to be to the south-east.

The northerly-trending Craig Liath Fault is inferred from mapping to fault out the Glen Banchor Subgroup in the north-west corner of the Newtonmore Sheet. It may also be a splay of the Ericht–Laidon Fault.

Farther south, the Glen Garry Fault system trends north-east cutting the Gaick Psammite on the Gaick plateau (Leslie et al., 2006; Leslie et al., 2003). This fault system is one of a set of the prominent north-east-trending faults crossing the Grampian Highlands. The system is a complex of related fractures that extends across the region from the south-west corner of Sheet 64W (Newtonmore) into Cama Choire [NN 688 785], passing just south of Gaick Lodge, across Mullach Coire nan Dearcag [NN 779 862], towards Glen Feshie. It intersects the Cairngorm Pluton just south of Loch Einich on Sheet 64E (Ben Macdui). The north-eastern end of the fault system may constrain the granite contact locally but no significant post-emplacement faulting occurs along this north-east–south-west-trending segment of the granite contact (pers. comm. M. Gillespie, 2002). A north-north-east-trending fault occurs south of Loch Einich. The complex of Reidel shears and normal faults is produced by sinistral strike-slip. To the south-west on Sheet 54E (Loch Rannoch) the fault system passes along Loch Garry to link with the Bridge of Balgie Fault (Johnstone and Smith, 1965; referred to as the Killin Fault by Treagus, 1991). South-east of the Glen Garry Fault N010°-trending faults are mapped along the steep slopes confining Loch an Duin, and to the north-west a similar fault is located along the west side of Loch an t-Seilich with the aid of aerial photography. Normal faulting with a N010° trend is exposed in the River Tromie at [NN 76456 88997] and throws down to the east. Easterly downthrow would be consistent with the general down-to-the-east component of displacement associated with the major north-east-trending strike-slip faults (Treagus, 1991). Granite sheets and dykes east of Gaick Lodge at [NN 7695 8522] exploit a N010°-trending pre-existing set of faults. Brecciation on a N010° trend at Sronphadruig Lodge [NN 7149 7831] clearly post-dates the emplacement of granite sheets and later fine-grained lamprophyre sheets. This argues for an extended fault history, as well as localisation of the igneous intrusions along this fault system (Leslie et al., 2003).

In the south-east quadrant of the Ben Macdui district, the orientation of Glen Tilt (Plate 19), (Figure 7) is largely controlled by the major north-east-trending Loch Tay Fault system which, when traced north-east of Bedford Bridge, swings north-north-eastwards, although several splay faults with subparallel north-east-trends extend for over 3 km south-east of the main fault. This sinistral strike-slip fault system has been estimated to have a net sinistral wrench component of 6–10 km and evidence of an episode of dextral movement (Treagus, 1991). It continues south- west to join the Highland Boundary Fault (Anderson, 1942). The fault system comprises a series of anastomosing near vertical planes with local near horizontal slickensides and brecciated zones. The fault system lies close to the Boundary Slide between the Grampian Group and the younger Dalradian rocks, indicating that the difference in competence may also have been a factor in its development. The fault system also appears to ‘control’ the south-east margin of the Glen Tilt Pluton, particularly the diorite intrusion which projects about 4 km to the north-east along the main fault line.

Shattered minor intrusions and slickensided fractures in diorite and granite belonging to the Glen Tilt Pluton indicate that at least some movement on the Loch Tay Fault post-dated their intrusion. Within the Sron a’ Chro granite/granodiorite close to the fault, much carbonate alteration is apparent as veinlets and patchy granular aggregates and the rock is distinctly sheared, with relict grains of K-feldspar and quartz surrounded by swathes of carbonate and chlorite. The Sron a’ Chro granite/granodiorite at [NN 918 728] was sampled and dated, using the high-resolution ion-microprobe method (Oliver et al., 2008), at 390±5 Ma. This Mid Devonian granite shows weak east-west-striking, subvertical foliation and parallel ellipsoidal enclaves of unfoliated diorite and foliated psammite. This evidence for deformation and the elongate outcrop pattern of the diorite suggested to Oliver et al. (2008) that the intrusion was controlled by the sinistral Loch Tay Fault active at 390±5 Ma. They attributed the sinistral movement (reactivation?) on the Loch Tay Fault to a far-field effect of the Mid Devonian Acadian Event caused by the effects of flat-slab subduction of Rheic Ocean lithosphere (Woodcock et al., 2007). Other minor intrusions show little or no deformation and appear to post-date the main movement on the fault. An element of downthrow, possibly of only a few hundred metres, on the south-east side of the fault was postulated by Treagus (1991) as the pluton is not obviously offset laterally. This is supported by the fact that evidence for a metamorphic aureole on the south-east side of the fault is limited and also that the Glen Banvie Formation is not represented on the south-east side.

One of the best exposed sections of the Loch Tay Fault and the associated fault intrusions is that near Forest Lodge, extending for 1.2 km from the junction of Allt Torcaidh with the River Tilt [NN 9304 7386] to Hutton’s classical locality at the ruins of Dail-an-eas Bridge [NN 9386 7465]. At the junction of Allt Torcaidh, brecciated quartzite is intruded by coarse-grained red granite which contains large xenoliths of psammite and/or quartzite and is much fractured. In the south-east bank of River Tilt opposite Forest Lodge, highly brecciated quartzite includes yellow- brown weathering bands which may be silicified carbonate rock (?dolomitic) in the fault zone. Here, minor buckle folds almost at right angles to the fault plane are probably related to movements on the fault. Just below the waterfall about 250 m north-east of Forest Lodge a fault breccia consisting of fragments of quartzite, psammite and/or silicified limestone is exposed on the south-east bank of the river. Adjacent to this breccia, the fault plane is seen in the river bed, trending N045°, where brecciated sedimentary rock is in contact with brick-red granite. At the bend in the River Tilt just below the waterfall at Dail-an-eas, there are two fault parallel planes. The south-eastern plane is marked by a thin dyke of brecciated granite, to the north-west of which are banded quartzite/psammites dipping 26° to the south-south-west. The second fault plane lies 20 m to the north-west and on its north-western side, red coarse granite extends to the top of the waterfalls at Hutton’s locality (Stephenson, 1999). Here the granite encloses and veins large screens of grey, banded limestone and calcareous phyllites dipping 66° to the south-south- west and belonging to the Glen Banvie Formation.

Details of the exposures adjacent to the Loch Tay Fault and the associated fault intrusions to the north-east of Dail-an-eas were given by Stephenson (1991; 1995; 1999). Between the junction of the River Tilt and An Lochain north to Bedford Bridge over the Tarf Water the trace of the main fault branch trends N020° but farther north it trends N030°, following the steeply incised valley of Allt Garbh Buidhe. This stream generally follows a marked lithological change from greenish grey quartzites and psammites (Gaick Psammite Formation) on the west-north-west bank, to a variety of quartzites, semipelites and limestones (Appin Group) on the east-south-east bank separated by fault breccia. Exposures of intensely brecciated brown quartzitic rock on the west- north-west bank at the junction with the Caochan Dubh Mor [NN 9966 8201] and 300 m north- north-east at [NN 9979 8233] are considered to be on the fault plane (Stephenson, 1995).

Minor north-east-trending faults

The Coire Breac Fault [NN 990 782]; (Figure 7) has a well-marked topographical expression in places, such as in Coire Breac at [NN 981 775]. It probably throws down to the north-west as indicated by the outcrop pattern of the Glen Clunie Graphitic Schist. It appears to follow, and is possibly controlled by, the axial trace of a major F4 synform.

The Fealar Fault system farther to the south-east [NO 010 796] extends to the south-east of Meall Gharran where it appears partly responsible for the apparent downthrow to the north-west of the An Socach Quartzite. This north-easterly system appears to be offset by later, near east- west-trending faults (see below).

About 9 km north-west of the Loch Tay Fault, and roughly parallel to it, the Chapan Mor Fault (Figure 7) cuts through Gaick Psammites at [NN 930 900] and offsets the margin of the Cairngorm Pluton by nearly 800 m in a sinistral direction.

Within the Glen Tilt Pluton, the fault pattern (Beddoe-Stephens, 1997) consists of a set trending just east of north; a set parallel to the Loch Tay Fault and a set approximately perpendicular to it. Since these faults cut the intrusions and are subvertical, late block faulting is probably related to post-orogenic (Acadian) uplift (see Oliver et al., 2008).

Some post-tectonic Siluro-Devonian porphyritic felsites (e.g. at [NO 007 764]) occupy north- north-east-trending faults (Stephenson, 1990). Both the felsites and the north-north-east-trending faults are cut by a later set of minor north-east-trending faults.

North-north-east-trending faults

North-north-east-trending faults result in considerable displacement of outcrops locally. Some are probably related to the major north-east-trending fault-set, such as the Loch Tay Fault which has segments and associated splays with this orientation. In the east of the area, the quartz- feldspar-phyric microgranodiorite of Carn Dearg [NO 023 799] was emplaced into a large-scale north-north-east-trending discontinuity and brecciated quartzite occurs in places against its margin [NO 0084 7693]. The discontinuity coincides with a major change in regional strike and dip direction (the general dip is to the north-east on the west side and to the south-east on the east side) Displacement of the Carn an Righ Slide and An Socach Quartzite indicates a significant downthrow to the east. Other marked examples lie along the valley of Glas Leathad [NN 960 731] and cut the Glen Tilt Pluton in Gleann Mhairc [NN 887 760] as well as to the east of Meall Tionail [NN 920 775], and so are later than the intrusion age of 390±5 Ma. A less prominent fault-set strikes nearly north-south.

Near east-west faults

A smaller set of near east-west-, east-north-east-, or west north-west-trending faults appear to be relatively late brittle structures, producing minor offsets on north-east-trending faults and minor Siluro-Devonian intrusions. The intrusion of the quartz-feldspar-phyric microgranitic dyke on Meall na Spionaig [NO 002 775] may have followed one of these east-north-east-trending faults.

The Fealar Fault system is cut by an easterly-trending fault set, for example at [NN 990 771]. These later faults and a northerly set cause notable displacements in the Appin Group south-east of the Loch Tay Fault.

A near east-west-trending fault crops out in Allt a’ Chama Choire [NN 7050 7962]. This fault has a 30 m-wide zone of cataclasis within the Gaick Psammite Formation (Plate 20) and may have some reverse movement (Leslie et al., 2003).

Farther up this glen at [NN 7002 7938], a microgranitic dyke trending N160° has a strongly sheared margin, effectively turning the dyke margin into a gneiss. The fabric in this sheared margin contains epidote and chlorite, suggesting that both fluid influx and deformation occurred at elevated temperatures (c. 300°C?).

1.7 Geophysics

The Bouger gravity anomaly in this district is generally negative, partly as a result of the prevalence of the thick low-density siliciclastic Grampian Group.

The Cairngorm Pluton has a strong negative Bouger gravity anomaly indicating significant mass deficiency within the upper 10 km of crust. It also has an annular magnetic anomaly which is implied to reflect zoning within the intrusion (Trewin and Rollin, 2002). Over the Cairngorm Pluton maximum aeromagnetic anomalies are around 180 nT, generally within the granite outcrop (Rollin 1993). The Bouger gravity anomalies over the Cairngorm Pluton are less than -65 mGal and lineations picked up from the regional gravity data trend N040° and N120° (Rollin, 1993). 2.5D integrated gravity and magnetic modelling has explained the gravity anomaly across the Cairngorm Pluton in terms of granite to a depth of 6–8 km below OD within Grampian Group rocks to depths of about 12 km. Magnetic anomalies have largely been explained by magnetic phases of the granite (Rollin, 1993). Gravity modelling indicates the Cairngorm Pluton is connected at depth with the nearby Glen Cairn, Lochnagar, Ballater and Mount Battock plutons (Rollin, 1984).

The Cairngorm Pluton is considered to extend eastwards on a structural lineament, the east-west Deeside Lineament (Fettes et al., 1986). This lineament is well defined in the gravity and magnetic anomaly data (B. Chacksfield, pers. comm.) and is coincident with the northern limit of the pluton. It is not necessarily a deep-seated lineament and may relate to east-west structures affecting the North East Grampian Basic Suite. Trewin and Rollin (2002 p.21) found no geophysical evidence for the Deeside Lineament, but they considered an east-south-east-trending East Grampian Lineament to be the main control on the Cairngorm Suite of granites.

Brown (1979) noted the coincidence of the negative gravity and positive aeromagnetic anomalies over the Cairngorm Pluton. This suggested a deep rooted magnetic anomaly source comparable in size to the outcropping intrusion since the rocks at surface have too low a remanent magnetisation. Possible sources considered were either mafic crystal cumulates at depth or metamorphic effects in the basement around the granite root zone. Brown and Locke (1979) favoured the latter explanation as there is no gravity evidence for high-density crystal cumulates and because some British Caledonian granites farther south lack the aeromagnetic anomaly and penetrated a different type of lower crust. Locke (1980) modelled the Cairngorm Pluton and its neighbouring intrusions down to at least 12 km below surface and with outward sloping margins. Substantial volumes of low-density rock in the crust are also indicated by the prominent gravity low extending from the Cairngorm Pluton eastward forming part of the inferred Eastern Grampian Batholith.

The Glen Tilt Pluton does not show up at all in the gravity anomaly data but has a very distinct residual magnetic anomaly (B. Chacksfield, pers. comm.). Trewin and Rollin (2002) suggested that the Mid-Grampian line, separating the Cairngorm Suite of granites from the southern suite, might be related to a deeper basement boundary separating the Grampian Highland and Midland Valley terranes at depth. The Allt Bhran granodioritic intrusion lacks a clear geophysical signature and therefore does not seem to be voluminous at depth.

An elongate lobe with a positive anomaly of about 30 nT on the aeromagnetic plot extends south-south-east from Loch an’t-Seilach. Although there is nothing specific in the surface geology to explain this, it may relate to a concealed intrusion, perhaps a smaller diorite related to the Glen Tilt Pluton. However, there is a similar shaped anomaly in the geochemical data in this area and the anomaly is approximately strike parallel so it may be that the local psammites are slightly more magnetic than normal (B. Chacksfield, pers. comm.).

References