Chapter 2 Previous studies

Cauldron subsidence

Published in 1909, the paper entitled The cauldron-subsidence of Glen Coe, and the associated igneous phenomena, by C T Clough, H B Maufe and E B Bailey, provided the first detailed description of a deeply dissected caldera volcano; it included four geological cross-sections. The striking occurrence of an Old Red Sandstone (Siluro-Devonian) volcanic and sedimentary succession, over 1 km thick, juxtaposed with Neoproterozoic to early Proterozoic (Dalradian Supergroup) metasedimentary rocks along a steep ‘boundary-fault’ was interpreted as recording syn- to post-eruptive piston-like subsidence of a virtually cylindrical block of crust (Figure 5)a. The authors presented considerable evidence that subsidence was accompanied by the complementary rise of magma, which formed semi-continuous ‘fault-intrusions’ around the downthrown block. It was demonstrated that the subsidence occurred in at least two stages. Along northern outcrops, where there are two subparallel strands of the boundary-fault, subsidence was found to have taken place first on the outer ‘Early Fault’ and then on the inner ‘Main Fault’. In the east, the earlier fault was found to be the inner of the two strands there. The authors described in detail their findings of ‘flinty crush-rock’, which they took to represent frictionally melted rock (pseudotachylite; Shand, 1916) along the boundary-faults. At Stob Mhic Mhartuin [NN 208 575], excellent and relatively accessible exposures of the Early Fault and Main Fault, flinty crush-rock, and fault-intrusions, led to this relatively small and accessible peak (707 m) becoming the type locality of the boundary-fault and thereafter probably the most visited and vigorously debated locality of the entire volcano complex.

The ‘cauldron-fill’ type succession (see (Table 2), p. 24) of Clough et al. (1909) was described from the precipitous crags between Aonach Dubh [NN 148 559], at the western entrance to the Pass of Glencoe, and the lofty Bidean nam Bian [NN 143 542], 2 km to the south. The succession was thought to consist mainly of lavas, and the complicated internal structure that was detected across the cauldron led the authors to conclude that eruptions had occurred at more than one centre. From their interpretations of the Glencoe cauldron, together with their analysis of the cross-cutting Clach Leathad Pluton (referred to by them as the ‘Cruachan Granite’; see p.98) and the Etive Pluton, Clough et al. (1909) formulated models of various styles of piston-like cauldron subsidence (Figure 5)a. These early, detailed and perceptive studies of a dissected volcano were also recorded in a memoir of the Geological Survey, Scotland, entitled The Geology of Ben Nevis and Glen Coe, and the surrounding country (Bailey and Maufe, 1916). This included the previously published four cross-sections of the cauldron (Bailey and Maufe, 1916, fig. 15, p.105) and was an explanation of the geology depicted on the one-inch to one-mile geological map, Sheet 53 (Glen Coe), the first edition of which was published in 1921. A second edition of the memoir was published in 1960 (Bailey, 1960), with some minor advances on interpretation of the Glencoe volcano. This edition included in a footnote a rather important correction concerning one of the cross-sections of the cauldron (Bailey, 1960, fig. 21, p.134), noting that dips on the boundary faults shown dipping inwards in fact dip outwards (for the significance of this see pp.14; 29). It is worthy of note, and admiration, that H B Maufe mapped the greater part of the volcanic rocks of the Glencoe cauldron as well as Ben Nevis, and also that E B Bailey’s huge contribution to unravelling the complicated geology of this region spanned some sixty years. This is not to belittle the important and wide-ranging contributions in this area by other Geological Survey geologists such as C T Clough, G W Grabham and H Kynaston.

Explosive volcanism

Roberts (1963, 1966a) recognised two major rhyodacitic welded ignimbrites within the volcanic succession; as was usual before the first recognition of welded ignimbrite (Marshall, 1935), the Geological Survey geologists had originally considered the rocks to be rhyolite lavas (see (Table 2)).

This first identification of major explosive activity at the Glencoe volcano heralded renewed interest both in the volcanic succession and in the boundary fault and its associated intrusions. Roberts (1963, 1966a) considered that the two ignimbrites recorded major cycles of volcanism that correlated with caldera subsidence and emplacement of fault-intrusions at depth. Like Clough et al. (1909), he attributed the two subparallel strands of the boundary fault to separate subsidence episodes, although he used the more prevalent term ‘ring-fault’ rather than ‘boundary-fault’. In a review, Roberts (1974) incorporated his own work with that of Taubeneck (1967) and Hardie (1968) and concluded that early explosive volcanism involved asymmetrical, trapdoor-like subsidence of the caldera floor, with magma withdrawal and consequent fault-block subsidence greatest at the north-eastern margin, towards Stob Mhic Mhartuin [NN 208 575] and Stob Dearg [NN 22 54] (Figure 5)b. He also interpreted certain steep contacts between metamorphic rocks and breccias inside the ring-fault in this north-eastern part of the volcano complex as part of a topographical wall of an early caldera.

Bailey (1960) suggested that some of the breccias adjacent to metamorphic basement just inside the ring-fault system in the north-east, between Stob Mhic Mhartuin [NN 208 575] and Meall a’ Bhùiridh [NN 25 50], might represent volcanic vents; thus he revised earlier Geological Survey conclusions that these were outliers of basal sedimentary rocks, with some representing infills of ‘landslip-cracks and earthquake-rents’ (Clough et al., 1909). However, Bailey offered no firm evidence for the revised interpretation. Hardie (1963, 1968) described the various breccias composed predominantly of fragments of metamorphic basement and postulated that they represent parts of a linear north-west-trending fissure-vent system; he interpreted them as explosion breccias. Taubeneck (1967) considered that breccias towards the north-west along the linear system discussed by Hardie (1968) were sedimentary, and he speculated that to the south-east some dyke-like tuffaceous breccias with invasive rhyolite represent vents. Moore and Kokelaar’s (1998) work, discussed more fully below, indicates that all of the earlier authors were partly correct. The breccias towards the north-west, on the slopes beneath Stob Mhic Mhartuin [NN 20 56] and beneath Stob Dearg [NN 228 547], are indeed mainly parts of the volcanic succession, including sedimentary deposits. The breccias farther south-east are in places invasive into the faulted caldera floor of metasedimentary rocks, and elsewhere occur in deep crevasses between metamorphic basement and volcanic rocks. These south-eastern occurrences probably do represent vents. Much of the earlier uncertainty was a consequence of the rather poor quality of exposure in the low but relatively accessible ground where those who followed the Geological Survey geologists chose to devote their attentions.

Ring-fault rocks and geometry

The ring-fault and its associated intrusions became the subject of close scrutiny by several geologists as processes of explosive volcanism in general were becoming topical. The view that the veins of flinty crush-rock had been produced by frictional melting of rocks along the ring-fault during large-scale subsidence, as originally interpreted by Clough et al. (1909), fell from favour as models involving streaming of particulate debris in a flow of escaping magmatic gas (fluidisation) were preferred. Reynolds (1956) suggested that the very fine-grained material represented intrusive tuff, whereas Hardie (1963) considered it was much-modified explosion breccia and Roberts (1966b) argued that it represented fluidised microbreccia, a view with which Taubeneck (1967) tended to agree. These problematic rocks are described later in this book along with fuller accounts of the previous interpretations, following which a new unifying model is proposed. Suffice it to say here that problems concerning the flinty crush-rock are not fully resolved, although an origin involving frictional melting is back in favour.

Taubeneck (1967) and (Roberts 1974) took the dips on the ring-fault mainly to be near-vertical or towards the inside of the volcano complex. Like Reynolds (1956), they interpreted inward-dipping strata within the Glencoe volcanic succession as evidence for compressional shortening of the subsided block owing to downthrow between inward-dipping faults. In addition, they suggested that the ring-fault, which they described as being shaped like an ‘upward-opening cone’ (see (Figure 5))b, might have formed initially by upwards-directed magmatic pressure. This fracturing mechanism is like that which leads to the formation of cone-sheets at central volcanoes, but no direct evidence for magmatically induced uplift was presented. Later in this book (p.83) it is demonstrated both that there is no simple cross-section of the ring-fault system that has bounding faults that diverge (‘open’) upwards, and that the inward dips of the volcanic strata resulted from (extensional) downsag rather than compressional shortening. There are long sections of the ring-fault system with outward dipping fault planes, especially along the northern strands, including the type locality at Stob Mhic Mhartuin [NN 208 575]. It is possible that the erroneous view of upwards-divergent bounding faults arose from a mistake in the original paper by Clough et al. (1909). In their Plate XXXIII, three of their four cross-sections of the ‘cauldron-subsidence’ show the bounding faults to be parallel or slightly upwards-convergent, but their main north-west to south-east section shows upwards-divergent faults. The four cross-sections were reproduced showing the same fault geometry in both subsequent editions of the Geological Survey memoir (Bailey and Maufe, 1916, p.105; Bailey, 1960, p.134). Only an easily overlooked footnote in the second edition admitted that faults in the one section showing upwards divergence were wrongly drawn; faults in the south-east shown with inward dips in fact dip outwards, so that this section too should show parallel caldera-bounding faults.

In the far west of the volcano complex, Bussell (1979) mapped various breccias and intrusive rocks on the slopes of An t-Sròn [NN 13 55] and considered that they record the former presence of a volcanic centre at this locality. He found evidence for early intrusions that formed ‘felsite’ (rhyolite) and then granite, with later emplacement of mainly ‘dioritic’ rocks to form the Main Fault intrusion here. He argued that brecciation, involving explosive release of volatiles and followed by metasomatism, occurred in advance of the ascent of both the silicic and the intermediate magmas. Garnham (1988) reappraised the various fault-intrusions and studied their petrography and geochemistry. Importantly, she concluded that most of the ring-fault intrusions are chemically and mineralogically distinct from the volcanic rocks in the down-faulted caldera-fill succession and thus that the intrusions were unlikely to be their feeders. However, like Bussell (1979), she considered that the An t-Sròn composite intrusion (comprising mainly tonalite, with granite and diorite) might represent the root of a volcano, and that the marked contact metamorphism around the intrusion, which has sillimanite overprinted onto the regional garnetiferous mica-schists (Bussell, 1979), was due to protracted throughput of magma. Major movement on the ring-fault here must have postdated this protracted heating, as the inner (downthrown) rocks are not so metamorphosed; the chilled margin of the intrusion is slightly tectonised along the fault plane.

Sediment provenance

A recurrent topic of discussion in early studies was whether the granitic boulders that occur in several conglomerates immediately beneath and also within the volcanic succession were derived from the Rannoch Moor Pluton see (Figure 3). This pluton is cut by, and therefore predates, fault-intrusions of the eastern part of the Glencoe complex, such as on the slopes of Creag Dhubh [NN 25 52]. It consists predominantly of an outer, foliated K-feldspar-phyric biotite-granite, up to 1 km wide at outcrop, and an inner, foliated hornblende-granodiorite (Bailey and Maufe, 1916; Hinxman et al., 1923; Bailey, 1960); quartz-diorite, monzodiorite, monzogranite and syenogranite occur locally and the term ‘granitic’ is used in the broad sense. Petrographical analyses by Taubeneck (1967) confirmed that the granitic boulders were probably derived from the outer parts of the Rannoch Moor Pluton, which is what the Geological Survey geologists had suspected specifically for occurrences in Cam Ghleann [NN 249 520] (e.g. Bailey, 1960, pp.131, 147). This provenance is significant, because the pluton is one of the suite of numerous large, composite granitic intrusions that are genetically related to, and in some instances cut, the central volcanoes, like those of Glen Coe, Ben Nevis and Starav–Cruachan (see (Figure 3) and Read, 1961). The evidence from the boulders is that the Rannoch Moor Pluton was emplaced, uplifted and unroofed, before caldera volcanism at Glen Coe. As the pluton was probably emplaced at depths no less than some 2 to 3 km (compare with Droop and Treloar, 1981; Key et al., 1993), this unroofing suggests vigorous crustal uplift during the magmatic cycle.

Geochemistry and the origin of the magmatism

Although there has been considerable interest in the genesis of the volcanic and plutonic rocks of the region (Figure 3), particularly concerning the extensive lavas of Lorn (e.g. Groome and Hall, 1974; Thirlwall, 1979, 1981, 1982, 1986, 1988; Fitton et al., 1982; Trewin and Thirlwall, 2002) and the nearby plutons (Harmon and Halliday, 1980; Clayburn et al., 1983; Halliday, 1984; Harmon et al., 1984; Frost and O’Nions, 1985; Halliday et al., 1985; Batchelor, 1987; Holden et al., 1987; Tarney and Jones, 1994), there has until now been little systematic geochemical examination of the Glencoe Caldera-volcano Complex. Nevertheless, it is clear that the caldera volcano formed an integral part of the regional magmatic system (see p.103), sharing the same origin and overall tectonic setting. The geochemical studies of Siluro-Devonian volcanic rocks in northern Britain by Thirlwall (references above) included whole-rock analyses of 23 samples from the Glencoe volcanic succession, and Garnham (1988) presented analyses of 56 samples from the Glencoe fault-intrusions, with a few from the volcanic rocks. These results, together with more recently obtained data (J C Neilson, B P Kokelaar, J G Fitton and M F Thirlwall, unpublished results, 2005) and some regional findings, are briefly synthesised below.

The rocks of the Glencoe Volcanic Formation range from 52 to 78 weight per cent SiO₂ and fall within the range basaltic trachyandesite to rhyolite on a total alkalis versus silica classification (weight per cent Na₂O + K₂O versus SiO₂; see Trewin and Thirlwall, 2002, p.217). They have generally high levels of Ba, Sr and light rare-earth elements and are fairly typical of the high-K calc-alkaline suites that are common in the volcanoes of modern continental margins beneath which there is subduction of oceanic lithosphere. Garnham (1988) found a similar high-K calc-alkaline signature in the fault-intrusions, which range between about 52 and 72 weight per cent SiO₂, from gabbro through diorite and monzonite to granite. However, she thought that the rhyolitic rocks now preserved within the volcanic succession and intruding its basement have no counterpart in the fault-intrusions, except possibly in a granophyric granite that forms part of the An t-Sròn composite intrusion. All of the studies have found many of the rocks to be mildly or strongly altered, apparently mainly by hydrothermal activity.

Thirlwall (1981) identified geochemical features that unite most of the Siluro-Devonian volcanic rocks of northern Britain in a single province, and he interpreted spatial variations in concentrations of elements such as Sr, Ba, K, P and the light rare-earth elements as reflecting magmatism directly related to subduction of the Iapetus oceanic lithosphere beneath the Laurentian continent. He highlighted the existence of rocks that, from their unusually high Mg, Ni and Cr content, seem to have been little-evolved from primary magma compositions: that is, representing partial melts of mantle and having undergone little fractional crystallisation or contamination during relatively rapid ascent. From studies of trace elements and of isotopes of Nd, Sr and Pb in these relatively primitive rocks, Thirlwall (1982, 1986) subsequently invoked the mixing of the mantle melts with subducted oceanic sedimentary material akin to the Lower Palaeozoic strata exposed in the Southern Uplands. He recognised that the mantle source beneath the south-west Highlands was mildly enriched in incompatible elements and that it differed from the depleted mantle that was the source for the majority of the coeval lavas in the Midland Valley of Scotland. To account for these features he invoked layering in the lithospheric mantle, with the south-west Highland magmas originating at relatively deep levels, possibly at depths in excess of 200 km in the lowest lithosphere, or in the asthenosphere, close to an active or very recently defunct subduction zone that supplied the sedimentary material. Contamination of the primitive magmas by continental basement material was considered to be minimal, whereas this appears not to be true regarding the more evolved magmas.

From the geochemistry of the intermediate to silicic plutonic rocks in the south-west Highlands, Harmon and Halliday (1980), Clayburn et al. (1983), Harmon et al. (1984) and Halliday et al. (1985) showed that the derivation of the intrusive magmas did not significantly involve asthenospheric mantle influenced by active subduction, but that the magmas were derived from sources in enriched lithospheric mantle and various levels of the continental crust, commonly with hybridisation of different melts during ascent.

Although it had been recognised for a long time that the regional magmatism was ‘late’ in relation to closure of the ocean adjacent to the Laurentian continent, recent refinements in reconstructions and dating of the collisions of Laurentia with Baltica and Avalonia (e.g. Armstrong and Owen, 2001; Dewey and Strachan, 2003) have tended to suggest that oceanic closure was complete by about 425 million years ago, at just about the same time as the onset of the major magmatic activity that was to continue for some 25 million years. This timing, if it is correct, renders the fundamental cause of the magmatism somewhat problematical. Where there is continuous subduction of oceanic lithosphere, fluids from the down-going slab persistently induce partial melting of the overlying mantle, in the lithosphere or in the convecting asthenosphere, or in both. However, it is not clear for how long the down-going slab may sustain magmatism after oceanic closure and the consequent deformation due to plate collision; ultimately, when subduction of relatively buoyant continental crust becomes impossible, no new slab will be supplied. In the case of the former Laurentian margin, the recently proposed plate-tectonic reconstructions imply that magmatism was sustained with considerable intensity for a long time after obliteration of the ocean. It is not straightforward to explain the cause of such a long-lived and intense thermal anomaly. Conceivably subduction continued for some time after the oceanic basin ceased to exist, but for how long?

The apparent persistence of the the magmatism after continental collision was considered by Zhou (1985), who cited possible modern analogues in Turkey, Iran and Tibet, where post-collision calc-alkaline magmatism is voluminous. He did not, however, explain the fundamental cause of the (putative) long duration of post-collision magmatism. Trewin and Thirlwall (2002, pp.247–249) reviewed evidence critical to understanding the magmatism, and re-emphasised that the volcanic and plutonic rocks in the Midland Valley and Highlands of Scotland are older than and differ isotopically from the superficially similar igneous rocks farther south. It is easier to reconcile the northern suite with continuing subduction of Iapetus lithosphere, whilst accepting that the younger rocks to the south do require an alternative origin. However, Atherton and Ghani (2002) have suggested that the Baltica–Laurentia continental collision, which caused the Scandian orogenic event (Soper et al., 1992), led to break-off and sinking of the subducted slab of oceanic lithosphere into the asthenosphere. They postulated that the substantial thermal event recorded by the regional magmatism in the Highlands could have resulted from buoyant ascent of hot (but relatively ‘dry’) asthenosphere through the gap created by the slab break-off, with consequent heating and partial melting of the enriched (relatively ‘wet’) subcontinental lithospheric mantle, as appears to have occurred in the Alps (see Davies and von Blanckenburg, 1995; von Blanckenburg and Davies, 1995). Ongoing research, including high-precision age determinations, should shed further light on this problematical magmatic episode.

New models

Moore and Kokelaar (1997, 1998) studied the volcanic and sedimentary rocks that comprise the Glencoe volcanic succession and showed that caldera subsidence and infilling were too complex to be reconciled with any simple model of coherent-block movement on a ring-fault. Early subsidence was shown to have been incremental and piecemeal, with the locations and styles of downthrow of caldera-floor rocks profoundly affected by tectonically active, intersecting basement faults. The basement faults facilitated both the movement of magmas towards the surface and the localisation of the tectonic and volcanotectonic subsidence in the vicinity. The localisation of the subsidence in turn caused a major fluvial system to drain across the site during much of the volcanic history.

Consequently, rivers repeatedly incised the intra-caldera rocks, and fluvial and lacustrine deposits became extensively intercalated in the volcanic succession, along with alluvial-fan deposits that formed near active fault-scarps. Two instances of uplift have been detected from the more recent studies; they involve minor doming of the caldera floor due to shallow intrusion of andesitic sills. There is no evidence of cone-sheets or for major doming by resurgent magma (as occurred at the Valles caldera; (Figure 4)a.

The ring-fault system and associated intrusions formed only after the early incremental caldera developments and it seems that even then coherent-block subsidence on a ring-form discontinuity may not have occurred. While there is no doubt that downthrow on the ring-fault system, amounting to some 700 m in places, was an important factor in the preservation of the volcanic succession, it is also clear that early subsidence occurred entirely within the yet-to-form bounding ring-structure and that late caldera subsidence overlapped south-westwards a considerable distance beyond it (see p.81). Thus the role of the ring-fault system is not as originally conceived and, had there been less erosion so that south-western fault strands remained buried, a rather different volcanic structure would have been evident. Both because of its early influence in volcanological interpretations and because this superbly displayed and well-described system yields much in a modern analysis, it is thoroughly reappraised in this book.

References