OROGENY ASSOCIATED WITH ANTICLOCKWISE P-T-T PATHS - EVIDENCE FROM LOW-P, HIGH-T METAMORPHIC TERRANES IN THE ARUNTA INLIER, CENTRAL AUSTRALIA

Low-pressure, high-temperature metamorphic terranes from the Proterozoic northern Arunta inlier, central Australia, are characterized by anticlockwise pressure-temperature-time paths. The thermal peak was reached before or during the earliest folding, and it was associated with abundant, repeated intrusion of granite sheets. We generalize our observations in the area to infer that low-P, high-T metamorphism is induced principally by focusing of mid-crustal heat from rapid, repeated granite sheet intrusion, which generates large metamorphic aureoles. Intrusion and associated metamorphism produce melting reactions, which increase the fluid pressure and thermally soften the mid-crust, resulting in accelerated, melt-enhanced deformation. Shortening is accommodated generally by thrusting in the mid-crust and by upright folding in the upper crust. Compression occurs during cooling of the terrane, as indicated by near-isobaric cooling paths and the preservation of upper-crust sections. The result is the production of discrete low-P, high-T terranes that are localized around abundant granitoids, show thin-skinned deformation, and have anticlockwise P-T-time paths. Continued granitoid intrusion produces posttectonic, discordant plutons at high crustal levels, within low-P metamorphic belts.