IN-SITU MIGMATITE AND HYBRID DIATEXITE AT MT STAFFORD, CENTRAL AUSTRALIA

Metasedimentary gneisses show a rapid change in grade within a 10-km-w ide low-P/high-T regional aureole at Mt Stafford, Arunta Block, centra l Australia. Migmatites occur in all but the lowermost of five metamor phic zones, which are characterized by: (1) muscovite-quartz schist; ( 2) andalusite-cordierite-K-feldspar granofels with small melt segregat ions; (3) spinel-sillimanite-cordierite-K-feldspar migmatite; (4) garn et-orthopyroxene-cordierite migmatite and minor diatexite; and(5) biot ite-cordierite-plagioclase diatexite that shows a transition to granit e. A subsolidus unit comprising interbedded sandstone and siltstone is equivalent to bedded migmatite, the main rock type in Zones 2-4. Meso scopic textures and migmatite classification of this unit vary with gr ade. In Zone 2, metatexite is developed in siltstone layers that are s eparated by quartz-rich, unmelted metapsammite layers. Melt segregatio n was less efficient in Zones 3 and 4, where the dominant migmatite la yering is a modified bedding. High proportions of melt were present in Zone 4, in which schlieren migmatite is transitional between bedded m igmatite and metapelite-sourced diatexite. The preservation of sedimen tary structures and coexistence of melt reactants and products in Zone 4 metapelite imply that molting proceeded ill situ without substantia l migration of melt. Zone 5 biotite-cordierite-plagioclase diatexite c arries rafts of bedded migmatite with strongly resorbed edges, as well as large K-feldspar and quartz augen. This unit of comparatively Ca-r ich migmatites is inferred to have been formed by the mixing of locall y derived and injected granitic melt.