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

[摘要] Tonalites to granites of the Tynong Batholith, Lachlan Orogen, southeastern Australia as well as enclaves within themcontain primary clino- and orthopyroxenes. These plutons produced very broad (2–10 km) contact aureoles that containan anatectic zone within metagreywackes. The very broad contact aureoles can be related to the 3-D shapes of the plutons and we assume that the Cpx and Opx are remnants of higher temperature crystallization that were preserved dueto water loss or low water content in the magma. Estimates of P and T based on x(Fe) values for coexisting cordieriteand 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 680–750°C. However, the isoplethsof An content of plagioclase indicate depths of up to 14 km at 660–740°C. We suggest that plagioclase was partiallyre-equilibrated during melt loss and post-emplacement decompression.Cathodoluminescence (CL) imaging shows that quartz both in the tonalites and hornfels is typically zoned from higherTi contents in cores to lower in the margins, suggesting a response to falling temperature. Calculated temperatures forquartz crystallization using a Ti-in-quartz thermometer calibrated for 2.5 kbar gave a wide range of values between900 and 500 °C. This suggests that although the granitoids contain two pyroxenes and have produced a broad contactaureole, they were not emplaced at temperatures as high as previously inferred.

[学科分类] 自动化工程

[关键词] TitaniQ;pseudosection modelling;Tynong Batholith;P–T estimation