Microfabric of mica aggregates in partly recrystallized biotite

Abstract

Large biotite grains, up to 1 metre across, in a pegmatite near Broken Hill, Australia, have been heterogeneously deformed and partly recrystallized in zones of relatively high strain, probably under conditions of the lower amphibolite facies. Many of the new aggregates (which consist mainly of more magnesian biotite, with muscovite, and less abundant ilmenite and albite) have a mica preferred orientation. Some of the oriented mica aggregates have grown in kink-like deformation zones (some of which appear to have involved fracturing) and others have grown in dilatation zones (growth of mica probably keeping pace with the opening of the zones). The shapes and preferred orientation of new mica grains appear to be due to varying contributions by (a) mechanical rotation of slices cleaved parallel to (001), (b) coaxial nucleation and/or growth of new mica on rotated portions of deformed biotite, and (c) preferred nucleation and/or growth of new grains in directions of minimum mechanical constraint and maximum transport of chemical components. This preferred growth can also explain the observed high degree of elongation of the oriented mica grains.