Pegmatite mylonites: origin and significance
Résumé
Pegmatites generally behave as resistant objects compared to their host rocks during solid-state deformation. They usually show folding or boudinage within their host rocks, features observed from low-grade to high-grade deformations, up to upper-amphibolite metamorphic conditions. Here we describe mylonitic pegmatite veins emplaced within S-C fabric-bearing two-mica granites. Pegmatites consist mainly of feldspar and quartz ribbons, with remnants of coarse-grained Kfs phenocrysts. Quartz ribbons are made of large grains elongated in the shearing direction, with evidence of dynamic recrystallization involving grain boundary migration. Ribbons show very strong lattice-preferred orientations of c-axes at low angles to the stretching lineation. From these, we propose that quartz ribbons are formed by oriented growth during syn-shearing crystallization of the pegmatite. Subsequent syn-shearing solid-state deformation involved grain size reduction accompanied by prism slip. The occurrence of pegmatite mylonites within granitoid intrusions should be considered as a univocal argument for synkinematic emplacement.