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Journal of the Geological Society

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Discussion on implications for Caledonian plate tectonic models of chemical data from volcanic rocks of the British Old Red Sandstone

DR T. R. ASTIN writes: In his valuable paper on the geochemistry of the Old Red Sandstone volcanic rocks, Thirlwall (1981) brings out the points that (1) the Lower Old Red volcanics in the Midland Valley of Scotland are calc-alkaline and show a spatial chemical variation which may be explained by their being generated at differing distances from an active subduction zone; (2) the Highland Newer Granites are largely of older or equivalent age to the Lower ORS volcanics and these too were thus generated during active subduction. Therefore, Thirlwall infers that the collision orogeny in Scotland that terminated subduction and magmatic activity, took place after Lower ORS deposition, with the resultant mild deformation being explained by oblique closure. However, he goes on to infer that the Lower ORS in Scotland must be Silurian in order to occur before the ‘end-Silurian’ collision, and to dismiss the dating reliability of the fish faunas.

While it is true that the Stonehaven Group (1500 m) is mostly Pridoli (U. Silurian) in age, the main volcanics (Arbuthnot Group) occur approximately 2300 m above the top of the Stonehaven Group (House et al. 1977), and both fish faunas (Westoll 1977) and plant spores (Richardson 1967; and Richardson quoted in Westoll 1977) from the Arbuthnot Group indicate a Siegenian age. Indeed, the presence of volcanics within the Stonehaven Group and the high proportion of volcanic clasts throughout the thick sequence of conglomerates adjacent to the Highland Boundary Fault, including the conglomerates in the Dunnottar and Crawton

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