Lithosphere structure and evolution in southeastern Australia

Geochemical tomography sections based on mantle-derived pyrope garnet xenocrysts have been constructed for different tectonic domains along a southern transect crossing the Tasman Line. At the regional scale, these sections reveal thicker lithosphere and lower geothermal gradients with increasing tectonothermal age of the lithosphere domains. In a transect from the easternmost locality (Jugiong) in Phanerozoic eastern Australia to the Eyre Peninsula (Gawler Craton) in Proterozoic South Australia, the palaeogeotherm decreases from greater than 50 mW/m² at Jugiong to around 40 mW/m² In South Australia and the depth to the lithosphere- asthenosphere boundary changes from -100 to 180 km. Geochemical tomography sections for the Gawler Craton region sampled in the Permian and the Jurassic, indicate lithosphere thinning by about 10-15 km, attributed to the effect of Pangean rifting. These physical changes are paralleled by higher MgO and lower CaO and AI₂O₃ contents in progressively older lithospheric mantle, consistent with the global secular variation in mantle composition previously documented. Olivine is the most abundant mantle mineral in all sections and the Fe/Mg ratio of olivine is important in controlling the physical properties of lithospheric regions (density, Vp, Vs). Because olivine is rarely preserved, an inversion of the garnet-olivine Fe-Mg-exchange geothermometer has been used to calculate the Fe/Mg of olivine coexisting with each garnet grain. Application of this inversion to the southern Australian transect shows high Mg# (Mg/(Mg+Fe)) for olivine in the shallow parts of the Proterozoic western sections and more Fe-rich olivine in the Phanerozoic eastern sections. There also is an overall trend to lower Mg# with increasing depth in each section, which may reflect infiltration by asthenospherlc melts near the lithosphere-asthenosphere boundary. Olivine density varies inversely with Mg#, and the Proterozoic lithosphere in this traverse therefore is more buoyant, relative to the asthenosphere, than the Phanerozoic sections despite the lower geotherm beneath the craton. Vp and Vs vary directly with Mg# (the higher the Mg#, the higher the Vp and Vs) and thus these compositional differences are important in the interpretation of seismic tomography.