Shear criteria in granite and migmatite deformed in the magmatic and solid states

Abstract

Microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid-state deformation. Sense of shear determination is either direct, deduced from the sense of rotation of markers, or indirect, involving the obliquity between the shear and foliation planes, or between the successive foliations imprinted at different stages of progressive deformation. This study is a by-product of the detailed structural and microstructural investigation of a high-grade metamorphic rock pile (Variscan Vosges Massif, France) which underwent subhorizontal shearing during partial melting and further solidification. Depending on the rock chemistry, on the position in the pile and the relative timing of progressive deformation, layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states. The sense of shear obtained from these rock types, using the criteria presented here, consistently gives a top to SW direction.