Petrology and tectonic setting of some Ordovician volcanic rocks from the Southern Uplands of Scotland

SUMMARY: Within the Ordovician sequence of the Southern Uplands of Scotland numerous basaltic units occur, forming local basement to sequences of chert, graptolitic mudstone and greywacke. The basalts are fault-bounded at their base. Although they have suffered secondary alteration to varying degrees, the least mobile elements, Ti, Zr, Y and Nb, show a correlation between relative chemical abundances and supposed tectonic level. The most northern tectonic slice contains alkali basalts; the central slice contains 11 examples of what are apparently ocean-floor tholeiites and 4 mildly alkaline basalts; a third slice, found at one locality only, also contains tholeiites whose chemistry is comparable with the most primitive basalts. Samples of Ordovician extrusives from Bail Hill have calc-alkaline to alkaline affinities, while extrusives from Wrae Hill are quartz-keratophyres. The geochemical pattern is consistent with a model of an opening Iapetus Ocean in which rifting pre-dated spreading such that the basement to the Lower Palaeozoic sequence of the Southern Uplands was oceanic.