A BAROMETRIC RESPONSE TO LATE COMPRESSION IN THE STRANGWAYS METAMORPHIC COMPLEX, ARUNTA BLOCK, CENTRAL AUSTRALIA

Evidence for four major deformation events (D1-D4) and two metamorphic events (M1, M2) is shown by granulite- and amphibolite-facies rocks of the Strangways Metamorphic Complex, central Australia, which is divided into two domains: the Ongeva Granulites to the east, and the Anamarra Granite domain to the west. D1 and D2 produced layer-parallel foliations, with many of the effects of D1 overprinted by intense non-coaxial shear during D2. Low-pressure granulite-facies metamorphism, M1, accompanied D1-2, at conditions of P = 5.3 ± 1.2 kbar and T > 750°C. The effects of M1 are characterized by S1 spinel + quartz-bearing assemblages, with overprinting S2b sillimanite + biotite + magnetite assemblages, implying approximately isobaric cooling. Effects of M2 are characterized by S1-2 cordierite pseudomorphed by orthopyroxene + sillimanite + biotite + magnetite symplectites, at conditions of P = 7.5 ± 0.8 kbar and T ≊ 800°C. This retrograde increase in pressure was probably the result of crustal thickening during D3, which produced NE-plunging F3 and F4 folds in the Ongeva Granulites, and a widespread shear foliation in the Anamarra Granite domain. The increase in pressure with M2 may reflect the beginning of a clockwise P-T-t path typical of collisional-style tectonics. Sillimanite and orthopyroxene-bearing ultramylonites were produced during D4, an extension event parallel to the D3 transport axis and probably related to crustal collapse after thickening during D3. Wide N-dipping shear zones (D5) containing sillimanite, kyanite, staurolite and overprinting greenschist-facies assemblages bound the Strangways Metamorphic Complex. These zones indicate a S-directed sense of shear and were probably responsible for bringing the Strangways Metamorphic Complex to higher crustal levels, during both a probable late Proterozoic event and the Mid Carboniferous Alice Springs Orogeny. © 1990.