Component migration patterns during the formation of a metamorphic layering, Mount Franks area, Willyama Complex, N.S.W., Australia

Mineral shapes, sizes and proportions in mica rich (M) and quartz-mica (QM) domains of an S₄ metamorphic layering have been compared in two mutually perpendicular orientations with shapes, sizes and proportions in mildly crenulated S₂ enclaves preserved within a rock from part of the Willyama Complex, N.S.W., Australia. The study has shown that data from two orientations are necessary to place constraints on the movement of mineral components dissolved in solution during formation of the layering. Comparison of mineral data has shown that SiO₂ has been dissolved from quartz grains in both M and QM domains. The SiO₂ lost from M domains has not migrated into QM domains. Measurements have also shown that biotite has undergone less solution than quartz. Most of the biotite components have probably undergone reaction to form probable syn-S₄ muscovite and chlorite. The muscovite-forming reaction requíres some silica on the lefthand side of the reaction, probably more than can be supplied by the chlorite-forming reaction. Some of this extra silica was supplied from dissolved quartz grains. However, only a small amount is needed for this purpose and most of the SiO₂ has left the system. Some syn-S₄ muscovite may have formed in M domains where it lies within the crenulated S₂. Direct evidence for its growth is hard to find.