A petrographic and geochronological assessment of the Aileron Metamorphics, Mount Boothby area, Central Australia
thesisposted on 28.03.2022, 21:21 authored by Mark Eastlake
The Aileron Metamorphics form kilometre-scale enclaves of granulite facies metasedimentary rock in voluminous megacrystic granitoid (Boothby Orthogneiss) in the SE Reynolds Range, Arunta Region, central Australia. Early tectono-stratigraphic reconstructions of the Arunta Region isolated the Aileron Metamorphics from the regional stratigraphy on the basis of metamorphic grade and post-depositional history. A revised chrono-stratigraphic framework of the Arunta Region permits correlation of the Aileron Metamorphics with regional sedimentary packages (Lander or Reynolds packages). The interbedded metapelites and metapsammites of the Mount Stafford Beds preserve a suite of metamorphic isograds and progressive evolution of migmatite morphology as a result of a steep lateral geothermal gradient of 75°C / km across a 10 km wide regional aureole. Migmatite banding at Mount Stafford is controlled by the interbedded turbidite protolith, with differentiation due to leucosome/melanosome segregation. The melting history at Mount Stafford has been found to be controlled by a series of biotite breakdown reactions, at different P/T conditions and contrasting whole rock. Melt mobilisation is limited across wide areas, which is confirmed by the constant bulk rock composition of metapelites and metapsammites across the area. U-Pb geochronology of detrital zircon limits sediment deposition to after 1846 Ma. Sillimanite-cordierite assemblages indicate granulite facies conditions prevailed during foliation development accompanying N–S directed shortening that produced moderately (39°) east plunging, tight folds (D1) in the Aileron Metamorphics prior to granitoid emplacement. Magmatic zircon growth during crystallisation of the Boothby Orthogneiss protolith is dated at 1801±8 Ma, coeval with the StaffordEvent (1810–1800 Ma), although limiting factors on this age are discussed. Hf-isotope values indicate significant contributions of recycled crust to local magma production, and equate Hf-isotope signatures of Stafford Event granitoids in the adjacent Anmatjira Range. The intrusive relationship of the Boothby Orthogneiss with the Aileron Metamorphics determines the Aileron Metamorphics are high-grade Lander Package equivalents. Subsequent NNE–SSW oriented deformation (D2) produced WNW–ESE striking upright folds that plunge 43° to the east in paragneiss and a penetrative gneissosity in orthogneiss. D3 structures are associated with NNE–SSW striking shear bands and crenulations that overprint D1 and D2 structures. A static sillimanite overprint of D1–D3 microstructures accompanied partial melting in both igneous and sedimentary protoliths. Two populations of low-Th/U rim domains on detrital protolith zircon separated from post-D3 diatexite record HT metamorphism during the 1590–1560Ma Chewings Orogeny. A late period of metamorphic zircon growth is recorded at 1531±3 Ma by a population of highly concordant low-Th/U rim domains, which suggest the current duration of the Chewings Orogeny in the SE Reynolds Range requires revision. Palaeozoic reactivation of the area during the Alice Springs Orogeny (450–300 Ma) produced shallowly (16°) west plunging open folds and steeply dipping east-west striking high-strain shear zones characterised by Bt+Qz±Mus±Ky±Rt (upper greenschist–amphibolite facies) assemblages. Kinematic indicators determine a reverse sense of movement along these structures that overprint earlier high-grade fabrics in both igneous and sedimentary protoliths.