XIII. The Igneous Geology of the Cumbrae Islands, Firth of Clyde

The Early Carboniferous volcanic outliers of Little Cumbrae and south Bute: implications for westward attenuation of the Clyde Lava Plateau

“…The volcanic succession in south Bute is dominated by feldspar-macrophyric and feldspar-microphyric basaltic rocks and mugearites, which closely resemble their counterparts of Little Cumbrae in their respective outcrop features and megascopic characteristics. The similarity is reinforced by the match in petrographical details provided by Smellie (1916) and Tyrrell (1918) and by Smedley's (1986a) demonstration that, in both Little Cumbrae and south Bute, the preponderant feldspar-macrophyric basaltic rocks range in chemical composition from basalt, through basaltic hawaiite, to hawaiite. Without repeating details already addressed in considering the lavas of Little Cumbrae, it is notable that (a) rare feldsparmacrophyric basalts also carry augite phenocrysts, which is true also of a few basalts on Great Cumbrae; (b) mugearites are less common, feldspar-microphyric basaltic rocks more common, and transitional varieties between feldsparmacrophyric and feldspar-microphyric basaltic rocks and between feldspar-microphyric basaltic rock and mugearite considerably more common in south Bute than in Little Cumbrae; (c) by a number of them carrying some olivine microphenocrysts (commonly serpentinised), the basaltic rocks of south Bute are closer to those of Little Cumbrae than their inferred correlatives in the lower slopes of the central Renfrewshire Hills; and (d) whereas the south Bute lavas have also been affected by hydrothermal alteration, the resulting minerals seem to be fewer in kind and reduced in incidence of occurrence.…”

“…Tyrrell (1918) noted that the average thickness of the flows lay between 75 and 100 feet (23 and 30 m), but failed to comment that such thicknesses are high for flows in the Clyde Plateau. Whyte & MacDonald (1974) calculated the average thickness of flows in the western Campsie Fells to be about 10 m, with 23 m as a maximum, and the average in the Kilpatrick Hills to be about 17 m. Paterson et al (1990) found that, in the Renfrewshire Hills, flows of similar composition to those in Little Cumbrae have an average thickness of 10 m and a maximum of 20 m; and Richey et al (1930) found similar flows in the Kilbirnie Hills to range in thickness from 3 to 10 m. There can be little doubt that at least some of the successive tiers in Little Cumbrae's topography correspond to single flows: Geikie (1897) and Tyrrell (1918) drew attention to their completely crystalline, hard, massive cores and more amygdaloidal and slaggy tops and bases. In the eroded 'crags' of one or two flows, however, textural variations and what look like internal chilled contacts raise the possibility of some 'flows' recording multiple, short-lived extrusive pulses rather than single uninterrupted, voluminous outpourings.…”

“…In support of this position, sandstones are present between the Traill Isle and succeeding Castle Island flows, and the latter is linked at low tide to the main island, where sandstones, over 15 m thick, overlain by lava, are well exposed along the shore south of the manor house and its outbuildings. This distribution led Gunn (1903) and Tyrrell (1918) to conclude that sandstones and the two thin, lava flows are interstratified in a transitional sequence underlying the continuous pile of flows (eight by Tyrrell's count) responsible for the main island. The notion of a transitional sequence at the base of the lava pile, while possible, seems unlikely, given the regional sub-volcanic unconformity with notable hiatus and lack of any other sequence of this kind in the upper Firth of If the patch of lavas between the faulted extension of the Paisley Ruck and the Dusk Water Fault can be shown conclusively to belong to the Renfrewshire Hills Block, then in their southward spread, the Clyde Plateau lavas ultimately come to rest on the Kinnesswood Formation, lowest formation of the (mainly) Carboniferous Inverclyde Group.…”

XIII.— The Old Red Sandstone Rocks of the West Kilbride-Largs District, Ayrshire

“…The dykes include examples of Lower Carboniferous, Permo-Carboniferous and Tertiary intrusions. They are most abundant along the rocky shores and may be compared as regards mode of occurrence with the dykes of Great Cumbrae Island described by Dr. G. W. Tyrrell (1917a). The first-named are mainly basalts with north easterly or north-north-easterly trends (Portencross shore, Bowen Craigs area, etc.…”

Stratigraphy and geochemistry of the Early Carboniferous Clyde Plateau Lavas in south Bute, Midland Valley of Scotland

“…The purposes of this paper are to provide a description of the field character of the volcanic rocks in south Bute, to place them in a stratigraphic context of associated Carboniferous sedimentary rocks, and to present and interpret the results of geochemical analyses of a representative suite of samples. Petrographic aspects of these rocks and related igneous rock suites on the Cumbrae islands to the east have been described in considerable detail by Smellie (1916) and Tyrrell (1918), and the interested reader should consult these works and Caldwell & Young (in press).…”