Metamorphosed Lamprophyres and the Late Thermal History of the Moines

Abstract

AbstractCertain lamprophyres in Moidart exhibit at least three sets of minor structures (cleavages, lineations and small-scale folds). During the formation of the first set the lamprophyres acquired a schistose fabric: hornblende and biotite laths crystallized along the axial surfaces of tightly compressed folds. The deformation episodes recognized in the lamprophyre sheets post-date the main episodes of major and minor folding (F1-F3) that affected the surrounding Moine gneisses. The existence of a metamorphic fabric in these lamprophyres raises interesting problems—was the thermal energy required to “metamorphose” the lamprophyres derived from deformational heat, or by a heat flow from the Moine country rocks, or were the lamprophyres deformed during cooling and consolidation from the magmatic state (autothermal metamorphism) ?