Stephenson, D., Loughlin, S.C., Millward, D., Waters, C.N. & Williamson, I.T. 2003. Carboniferous and Permian Igneous Rocks of Great Britain North of the Variscan Front. Geological Conservation Review Series, No. 27, JNCC, Peterborough, ISBN 1 86107 497 2.
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I.T. Williamson
The Dinantian volcanic rocks that crop out in the south of the Kintyre peninsula belong to the Clyde Plateau Volcanic Formation (Strathclyde Group), the most extensive lava succession within the Carboniferous–Permian Igneous Province of northern Britain. The succession is typical of continental lava plateaux formed by the accumulation of overlapping lava sequences. However, unlike the other lava fields that make up the Clyde Plateau Volcanic Formation the Kintyre sequence lies entirely north of the Highland Boundary Fault and, like the nearby Visean volcanic rocks at Ballycastle in Northern Ireland, is therefore structurally outside the graben of the Midland Valley. The Kintyre sequence is also separated by a considerable distance from coeval lava fields in central Scotland and it is not known if these were ever in physical continuity.
The Machrihanish Coast and South Kintyre GCR site
The earliest accounts of the igneous rocks of this district are rather sketchy and appear in the writings of John MacCulloch (1819); they were shown on his general geological map of Scotland in 1840 as a band extending from Campbeltown to the west coast. These igneous rocks figure briefly in papers by Nicol (1852) and Thomson (1865). Nicol divided them into 'Porphyries' and Augitic traps' and noted their association with the 'red sandstones and Carboniferous strata' of the district. However, it was Geikie (1897) who was the first to recognize that the Kintyre lava field forms an outlying portion of what he termed the Clyde Plateau.
The Kintyre area was first surveyed in detail by R.G. Symes and the map was published at the 1:63 360 scale by the Geological Survey in 1894. Details of the volcanic succession were not known until considerably later, when many of the area's pertinent features were described by McCallien (1927). The most recent mapping is by M.M. Avison and M.K. Carr in 1976, and H.M. Ayles and S.P. Duckworth in 1977, undergraduates at the University of Lancaster supervised by R. Macdonald. Their work has been incorporated in the 1:50 000 scale Provisional Series map of the British Geological Survey (Sheet 12, Campbeltown, 1996).
The Kintyre lavas have been featured in several wide-ranging studies of the geochemistry of Dinantian volcanism across the Scottish part of the Carboniferous–Permian Igneous Province of northern Britain by Macdonald (1975, 1980) and Smedley (1986a,b, 1988a) and consequently over 60 whole-rock analyses are available. They were also the main subject of a specific geo-chemical investigation by Smedley (1988b), highlighting differences in the mantle source of Dinantian magmas across the Highland Boundary Fault.
There are five outcrops of the Clyde Plateau Volcanic Formation in south Kintyre. They comprise a broad belt extending south-eastwards from Machrihanish on the west coast to Tirfergus Hill and Killellan (the 'Machrihanish Lavas' of McCallien, 1927), three relatively small outliers north of Campbeltown, and an outlier on the south-east coast, east of Southend (the 'Southend Lavas' of McCallien, 1927 and Smedley, 1986a). The Machrihanish Coast and South Kintyre GCR site is a composite site made up of three areas selected from the first, and largest, of these outcrops
In all three areas of the GCR site the Kintyre lavas overlie an unconformity. Along the western margin of the Machrihanish coast section and in the Chiscan Water on the eastern flank of Black Hill, the volcanic sequence rests upon the Kinnesswood Formation of the Inverdyde Group. This comprises white and red cross-bedded sandstones, and red-brown siltstones with nodules and beds of pedogenic limestone ('cornstone'). At Skerry Fell Fad the Kinnesswood Formation is only present in a small area in the west. Elsewhere, the lavas rest directly upon the Stonefield Schists, a unit of the Neoproterozoic Dalradian Supergroup.
The top of the Clyde Plateau Volcanic Formation in Kintyre is marked by a non-sequence, above which there is, in places, a reddish bauxitic deposit. This, the Kirkwood Formation, mostly comprises a diachronous lateritized, coarse- to fine-grained detrital volcaniclastic deposit that is interpreted as having formed by the contemporaneous weathering of the volcanic rocks. Elsewhere, the overlying rocks belong to the Lower Limestone Formation and Limestone Coal Formation of the Machrihanish Coalfield (McCallien and Anderson, 1930).
The Clyde Plateau Volcanic Formation in southern Kintyre comprises up to 400 m of volcanic rocks, predominantly lavas, dipping to the south-east
The lower part of the lava succession is dominantly olivine basalt that is commonly macro-porphyritic, along with some interbedded olivine-pyroxene-macrophyric (ankaramitic) basalt. The upper part contains most of the more evolved rock-types, including hawaiite, mugearite, benmoreite, trachyandesite and trachyte. Some of these lavas may be composite bodies. Several plugs, sills and dykes with compositions ranging from olivine-dolerite to microsyenite and dacite are also exposed (Macdonald, 1975).
The coastal section from Machrihanish Bay westwards offers the most readily accessible part of the GCR site. Within it, a range of lithologies and structures typical of the Clyde Plateau Volcanic Formation may be examined.
A west to east traverse along the coast from Eudan nan Gallan
Most of the lavas are tabular, sheet-like bodies. The wide range of characteristic lava-flow features exhibited include glassy (in places devitrified) flows, porphyritic variants including some with glomeroporphyritic textures, amygdaloidal and massive flow-units, blocky autobreccias, flow-base and flow-top breccias, ramp structures, and a variety of flow-induced structures. Some flows are remarkably fresh, but most are severely weathered or intensely altered. Olivine is almost always seen as pseudomorphs.
The lower part of the volcanic succession also crops out on the slopes of Skerry Fell Fad
Apart from supplementing the range of lithologies and volcanic structures observed in the Machrihanish coastal section, this area exhibits the following features:
Black Hill, SSW of Campbeltown, is separated from the Skerry Fell Fad area by an extensive tract of relatively poorly exposed ground. The Clyde Plateau Volcanic Formation sequence around Black Hill is broadly similar to that in the other areas, with the proportion of lavas of more evolved composition generally increasing strati-graphically upwards. However, in contrast to the other areas, the lower part of the succession also includes some evolved flows. Trachyte near the base of the formation in the Chiscan Water
Between Killellan Park
The Kintyre sequence of the Clyde Plateau Volcanic Formation shows a broad, two-fold subdivision, with basic lithologies more common in the lower part and more evolved rocks dominating the upper part. Smaller scale variations in lithostratigraphy, such as those suggested by the different fault-block successions in the Black Hill area, may be explained in part by pre-existing topography. Lavas may have been channelled through low-lying areas between older flows and in graben-like structures between contemporaneous volcanic fault scarps. Some of the faults may be re-activations of older, deep-seated (?Caledonian) structures. Similar abrupt varia tions in local successions are also characteristic of the Lower Dinantian Birrenswark Volcanic Formation in the Anglo-Scottish Borders Region (see Langholm–Newcastleton Hills GCR site report). Small-scale fault-control of this type within the lava field may be common throughout the Clyde Plateau Volcanic Formation in the Midland Valley of Scotland.
The basal contact relationships of the trachytic body capping Skerry Fell Fad suggest that it is either a shallow intrusive body or that the underlying lavas had been eroded prior to its emplacement. Trachytic lavas are normally highly viscous and typically do not travel any great distance from their source. Hence, the trachyte could be interpreted as a lava dome capping its feeder pipe.
The south Kintyre lavas are typical of the transitional tholeiitic to mildly alkaline suite of Dinantian age in northern Britain. The sequence in the Macrihanish Coast and South Kintyre GCR site preserves some of the most basic (olivine-pyroxene-phyric basalt) and the most evolved rocks in the formation. Overall, this is a mildly silica-undersaturated and mildly sodic suite, which includes low Fe-variants and exhibits strong P2O5 and TiO2 enrichment. Macdonald (1975) drew an important petrographical distinction between flows from the Kintyre and Campsie sequences of the Clyde Plateau Volcanic Formation, stating that, while in the Kintyre rocks clinopyroxene persisted as a phenocryst phase into the hawaiites, it is completely absent as phenocrysts from even the most basic Campsie lavas. Further sampling and analytical work by Macdonald (1980) and Smedley (1986a, 1988b) established that the Kintyre lavas may also be discriminated from many other Dinantian suites by geochemical parameters such as incompatible trace elements, rare-earth elements and isotopes. The additional data reported by Smedley (1988b) showed that the basaltic lavas from Kintyre are isotopically distinct from their counterparts in the Midland Valley and southern Scotland, having, in particular, lower Nd and higher Sr values. This correlates well with differences seen in the Siluro–Devonian calc-alkaline igneous rocks over the same terranes (e.g. Thirlwall, 1986) and implies long-term differences in the mantle source regions, which were more enriched in incompatible elements north-west of the Highland Boundary Fault.
Xenocrysts of quartz, surrounded by complex reaction rims of clinopyroxene, are a feature of some Kintyre basalts, and according to Smedley (1986a) represent tangible evidence that a degree of crustal assimilation may have occurred. However, isotopic evidence for crustal contamination has been detected in only one trachyte sample.
The low-potassium tholeiitic basalt flow within the lowest part of the Kintyre sequence is unique in the Clyde Plateau Volcanic Formation and its presence is fundamental to the overall understanding of the development of the Carboniferous magma types of the Carboniferous–Permian Igneous Province of northern Britain. Macdonald (1975) and Smedley (1986a) have both shown that it is relatively depleted of incompatible elements. This suggests that it was derived from a depleted mantle source, probably from the same type of spinel lherzolite as the other Clyde Plateau Volcanic Formation basalts, but by substantially greater degrees of partial melting. Contamination by crustal material is not thought to have been significant in this case.
Visean lavas in the Kintyre peninsula, represented by the Machrihanish Coast and South Kintyre GCR site, are the only significant occurrence of the widespread Clyde Plateau Volcanic Formation north of the Highland Boundary Fault. The 400 m-thick succession comprises at least 20 lavas mainly of olivine basalt, but more evolved rock compositions, including mugearite, benmoreite and trachyte, are more common in the upper part. Most of the lavas are extensive sheet-like bodies, but some are lenticular. Some appear to have flowed into contemporaneous topographical hollows, possibly small fault-bound grabens. A trachyte forming the summit of Skerry Fell Fad is a splendid example of a lava dome.
Geochemically, the Kintyre lavas are broadly similar to the transitional to mildly alkaline rocks that constitute the Clyde Plateau Volcanic Formation in the Midland Valley of Scotland, though with some subtle differences. In particular, a single lava of tholeiitic basalt with unusually low potassium content, near the base of the sequence on Skerry Fell Fad, is unique within the formation.