Reorientation of early biotites during schistosity formation in andalusite schists, Mount Franks area, Broken Hill, N.S.W.

Abstract

Study of schistosity formation in andalusite—grade mica schists from part of the Pre-cambrian Willyama Complex in New South Wales is facilitated by the coarse grained nature of the rocks, and by the presence of deformation indicators provided by pre- S 2 biotite. S 2, the dominant schistosity, is generally domainal. It is defined by the alternation of mica (M) and quartz + mica (QM) domains. M domains are dominated by biotite aggregates with a very marked shape (but not a crystallographic) orientation parallel to S 2, and by S 2 muscovite laths. QM domains are dominated by kinked biotite grains, biotites aligned across S 2, and biotite lozenges and grains in which (001) traces are aligned oblique to S 2. Biotite grains in these domains are less elongate than those in M domains. Individual biotite grains have been reoriented by kinking and corrosion; some homo- geneous glide on (001) may also have taken place. The variation in these deformation effects indicates that M domains represent zones of high strain with respect to the QM domains. M domains have undergone a history of shortening, rotation, diffusive mass transfer, volume reduction and syntectonic crystallization of muscovite. QM domains have also undergone syntectonic crystallization of muscovite, but their history is marked by less rotation, shortening, mass transfer and volume reduction than that of M domains. The greater activity of mass transfer mechanisms in M domains suggests that they are strain dependent, and proceed more easily in more highly deformed grains. Metamorphic driving forces associated with chemical reaction may play a part in the generation of these mass transfer mechanisms.