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

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Abstract
Volcanic rocks associated with the continental sediments known as the Old Red Sandstone outcrop widely in northern Britain and form a dominantly calc-alkaline suite comparable to those developed on modern continental margins. Rocks rich in Ni and Cr are unusually abundant, and are thought to have incompatible element concentrations little different from those of primary magmas. Such rocks show pronounced spatial chemical variation, with concentrations of Sr, Ba, K, light rare earth elements and P 2 O 5 , and the ratio La/Y showing up to a six-fold increase in a northwestward traverse across Scotland. In more evolved rocks a similar increase is shown by Sr, Ba and La/Y, but regional comparisons are made difficult by the varying effects of fractional crystallization. The chemistry of all Old Red Sandstone volcanic rocks N of the Southern Uplands Fault is consistent with the former existence of a subduction zone that changed in strike from ENE in Ireland and southern Scotland to N in the North Sea. This orientation, in conjunction with almost E–W plate motion, may explain the great contrast in end-Silurian deformation styles between Britain and Scandinavia. Old Red Sandstone volcanic and plutonic rocks S of the Southern Uplands Fault are thought to be younger and are perhaps unrelated to subduction activity.