A sequence of partial melting reactions at Mt Stafford, central Australia

Abstract

Metasedimentary gneisses show a rapid change in grade in a 10 km wide low-P/high-T regional aureole at Mt Stafford in the Arunta Block, central Australia. Migmatite occurs in all but the lowermost of five metamorphic zones, which grade from greenschist (Zone 1) through amphibolites (Zones 2-3) to granulite facies (Zones 4-5). The sequence of partial melting reactions inferred for metapelitic rocks is dependant upon protolith, temperature and fluid conditions. The metapelite solidus in Zone 2 reflects vapour-present melting at P ≃ 3 kbar and T ≃ 640°C, melting having initially been controlled by the congruent breakdown of the assemblage Crd-Kfs-Bt-Qtz. At slightly higher temperature, andalusite in leucosome formed via the reaction Kfs+Qtz+Bt+H 2 O→And+melt; And+melt having been stabilized by the presence of boron. Sillimanite coaxially replaces andalusite in the high-grade portion of Zone 2. In Zone 3, large aluminosilicate aggregates in leucosome are armoured by Spl-Crd±Grt symplectites. Garnet partially pseudomorphs biotite, cordierite or spinel in high-grade portions of Zone 3. Zone 4 Grt-Crd-Opx-bearing metapsammite assemblages and garnet-bearing leucosome reflect T≃800°C and P=2.2±0.9 kbar. In the model KFMASH system the principal vapour-absent melting step reflected significant modal changes related to the breakdown of the As-Bt tie-line and the establishment of the Spl-Crd tie-line; the bulk rock geochemistry of migmatite samples straddle the Spl-Crd tie-line. The aluminous bulk-rock composition of the common bedded migmatite restricted its potential to witness garnet-forming and orthopyroxene-forming reactions, minor textural and modal changes in and above Zone 3 reflecting biotite destabilization in biotite-poor assemblages.