A B S TR A CT Metasedimentary gneisses show a rapid change in grade within a 10-km-wide low-P/high-T regional aureole at Mt Stafford, Arunta Block, central Australia. Migmatites occur in all but the lowermost of five metamorphic zones, which are characterized by: (1) muscovite-quartz schist; (2) andalusitecordierite-K-feldspar granofels with small melt segregations; (3) spinel-sillimanite-cordierite-K-feldspar migmatite; (4) garnet-orthopyroxene-cordierite migmatite and minor diatexite; and (5) biotite-cordieriteplagioclase diatexite that shows a transition to granite. A subsolidus unit comprising interbedded sandstone and siltstone is equivalent to bedded migmatite, the main rock type in Zones 2-4. Mesoscopic textures and migmatite classification of this unit vary with grade. In Zone 2, metatexite is developed in siltstone layers that are separated by quartz-rich, unmelted metapsammite layers. Melt segregation was less efficient in Zones 3 and 4, where the dominant migmatite layering is a modified bedding. High proportions of melt were present in Zone 4, in which schlieren migmatite is transitional between bedded migmatite and metapelite-sourced diatexite. The preservation of sedimentary structures and coexistence of melt reactants and products in Zone 4 metapelite imply that melting proceeded in situ without substantial migration of melt. Zone 5 biotite-cordierite-plagioclase diatexite carries rafts of bedded migmatite with strongly resorbed edges, as well as large K-feldspar and quartz augen. This unit of comparatively Ca-rich migmatites is inferred to have been formed by the mixing of locally derived and injected granitic melt.

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 amphibolite (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-Opxbearing 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 destablization in biotite-poor assemblages.

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