Reply to discussion on the provenance of granite boulders in conglomerates of the Northern and Central Belts of the Southern Uplands of Scotland

Abstract

C. F. Elders writes: In suggesting that the clasts in the Ordovician and Silurian conglomerates of the Southern Uplands could have come from a terrane similar to that exposed in Western Ireland, Winchester & Max (1989) point out that the validity of potential correlations between detritus and source should be confirmed by full geochemical analysis. This is an important part of provenance studies, and for the Southern Uplands granite clasts such information is available elsewhere (Elders 1987). However, the petrography of the clasts and the nature of the detritus with which they are associated also reveal the nature of the source area from which they were derived. Using this information, it can be shown that western Ireland and southern Scandanavia were unlikely to have acted as sources for the Southern Uplands clasts. If, as Winchester & Max suggest, the c . 1200 Ma clasts in the Corsewall Point and Glen Afton conglomerates were derived from migmatitic segregations such as those in the Annagh gneiss, it might be expected that comparable volumes of granitic and gneissic detritus would be present in both conglomerates. In fact only a few clasts of quartzo-feldspathic gneiss were found. The c. 1200 Ma clasts are extremely homogeneous and represent a large proportion of the granitic detritus in each conglomerate. This indicates that they were derived from large single plutons, not small migmatitic segregations. Small xenoliths in the Corsewall Point clasts indicate that they were derived from a pluton intruded into amphibolite facies garnet-sillimanite schists, and not a