Progressive and retrogressive metamorphism in the Tumbarumba-Geehi district, N.S.W

Regional metamorphism of a group of pelites, psammopelites and psammites in the Tumbarumba-Geehi district has produced a sequence of metamorphic zones centred about foliated granitic rocks. It is considered that the mineral phases originated during four main pulses in the metamorphism. Two progressive metamorphic pulses are evident. These have produced a white mica -opaque oxide assemblage at low grade, whereas biotite, andalusite and cordierite are present at medium to higher grades. Mineral assemblages allow sub-division of the metasediments into four zones, namely, low-grade, biotite, knotted schist and high-grade zones. Two retrogressive metamorphic pulses are characterised by the development of chlorite, or of biotite closer to the metamorphic focus. The distinctive chemistry of the original sediments (high Al, high K : Na ratio) and the presence of graphite in such rocks are believed to have been significant factors in producing the style of metamorphism that is typical of large areas in south-east Australia.