Was the Tynong Batholith, Lachlan Orogen, Australia, extremely hot? Application of pseudosection modelling and TitaniQ geothermometry

Tonalites to granites of the Tynong Batholith, Lachlan Orogen, southeastern Australia contain primary clino- and orthopyroxenes as well as in enclaves and have produced very broad (2-10 km) contact aureoles that contain an anatectic zone within metagreywackes. The very broad contact aureoles can be related with the 3-D shape of the pluton and the Cpx and Opx we assume as remnants of higher temperature crystallization that were preserved due to water loss or low water content in the magma. Estimates of P and T based on x(Fe) values for coexisting cordierite and biotite in P-T pseudosections for a typical migmatitic hornfels, providing minimum temperature of pluton emplacement, indicate emplacement of the Toorongo tonalite at 4-10 km (1-3 kbar) and 680-750oC. However, the isopleths of An content of plagioclase indicates depths of up to 14 km at 660-740oC. We suggest that plagioclase was partially re-equilibrated during melt loss and post-emplacement decompression. Cathodoluminescence (CL) imaging shows that quartz both in the tonalities and hornfels are typically zoned from higher Ti contentsin cores to lower in the margins, suggesting a response to falling temperature. Calculated temperatures for quartz crystallization using a Ti-in-quartz calibrated for 2.5 kbar gave a wide range of values between 870 and 570 oC. This suggests that although the granitoids contain two pyroxenes and have produced a broad contact aureole, they were not emplaced at temperatures as high as it has been inferred. The new estimates are similar to the crystallization temperature of rapakivi granitoids of the Fennoscandian Peninsula.