Exhumation of the lower crust during crustal shortening: an Alice Springs (380 Ma) age for a prograde amphibolite facies shear zone in the StrangwaysMetamorphic Complex (central Australia)

Foliated garnet-bearing amphibolites occur within the West Bore Shear Zone; cutting through granulite facies gneisses of the Strangways Metamorphic Co mplex. In the amphibolites, large euhedral garnet (up to 3 cm) occurs withi n fine-grained recrystallized leucocratic diffusion haloes of plagioclase-q uartz. The garnet and their haloes include a well-developed vertical foliat ion, also present in the matrix. This foliation is the same as that cutting through the unconformably overlying Neoproterozoic Heavitree Quartzite. Th e textures indicate syn- to late kinematic growth of the amphibolite facies mineral assemblages. All mineral assemblages record an arrested prograde reaction history. Notew orthy is the growth of garnet at the expense of hornblende and plagioclase, and the breakdown of staurolite-hornblende to give plagioclase-gedrite. Th ese dehydration reactions indicate increasing P-T conditions during metamor phism, and suggest heating towards the end of a period of intense deformati on. Temperature estimates For the garner-amphibolite and related staurolite -hornblende assemblages from the shear zone are about 600 degreesC. Pressur e is estimated at about 5 kbar. An Sm-Nd isochron gives an age of 381+/-7 Ma for the peak metamorphism and associated deformation. This age determination confirms that amphibolite fa cies conditions prevailed during shear zone development within the Strangwa ys Metamorphic Complex during the Alice Springs Orogeny. These temperature conditions are significantly higher than those expected at this depth assum ing a normal geothermal gradient. The Alice Springs Orogeny was associated with significant crustal thickening, allowing exhumation of the granulite f acies, Palaeoproterozoic, lower crust. Along-strike variations of the tecto nic style suggest a larger amount of crustal shortening in the eastern part of the Alice Springs Orogeny.