PROGRADE METAMORPHIC ASSEMBLAGE EVOLUTION DURING PARTIAL MELTING OF METASEDIMENTARY ROCKS AT LOW PRESSURES: MIGMATITES FROM MT STAFFORD, CENTRAL AUSTRALIA

The Mt Stafford area preserves a low-pressure greenschist to granulite facies regional aureole. The metasedimentary sequence has been divided into 5 zones from greenschist (Zone 1) to granulite facies (Zone 4) and a zone of hybrid diatexite formed from the introduction of granitic magma into the high-grade migmatites (Zone 5). Melt production was dominated by a series of multivariant biotite breakdown reactions, not the univariant reactions suggested by previous studies. Though the three main metasedimentary rock types produced similar amounts of melt at the highest grades, their melt production histories differed markedly as a function of temperature. Aluminous metapelites produced more melt at lower temperatures (Zones 2 and 3), whereas metapsammite and cordierite granofels experienced an additional major melt producing step at higher temperatures (upper Zone 3 & Zone 4). This melting step involved the breakdown of biotite to produce garnet, K-feldspar and melt, and in some rocks the production of orthopyroxene. Melt production in Zone 4 exceeded 25 molar percent, resulting in the formation of in situ diatexites. The complex relationships involving aluminosilicate porphyroblasts resulted in the breakdown of biotite and aluminosilicate being drawn out over a wide temperature range, from subsolidus conditions to temperatures close to 750°C. Initially, much of the melting developed around the aluminosilicate porphyroblasts during the breakdown of coexisting biotite, aluminosilicate and quartz. However, much of the rock was chemically isolated from the porphyroblasts and could not react to produce melt. As temperatures increased, the presence of the large isolated aluminosilicate porphyroblasts controlled the spatial development of quartz-absent, spinel-present compositional domains, the formation of spinel being governed by the silica-undersaturated breakdown of coexisting biotite and aluminosilicate.