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

Abstract

Tonalites to granites of the Tynong Batholith, Lachlan Orogen, southeastern Australia as well as enclaves within them contain primary clinoand orthopyroxenes. These plutons produced very broad (2–10 km) contact aureoles that contain an 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 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 680–750 °C. However, the isopleths of An content of plagioclase indicate depths of up to 14 km at 660–740 °C. We suggest that plagioclase was partially re-equilibrated during melt loss and post-emplacement decompression. Cathodoluminescence (CL) imaging shows that quartz both in the tonalites and hornfels is typically zoned from higher Ti contents in cores to lower in the margins, suggesting a response to falling temperature. Calculated temperatures for quartz crystallization using a Ti-in-quartz thermometer calibrated for 2.5 kbar gave a wide range of values between 900 and 500 °C. 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 previously inferred.