Cathodoluminescence and micro-structural evidence for crystallisation and deformation processes of granites in the Eastern Lachlan Fold Belt (SE Australia)

Abstract

Trace elements (Al, K, Ti, Fe), growth and deformation pattern in quartz of the multiple deformed Carcoar, Barry and Sunset Hills granites were investi- gated by electron micro probe and cathodolumines- cence. Zoned quartz phenocrysts with high Ti concentrations (>70 ppm) that show blue cathodolu- minescence originated from the early stage of magma crystallisation. Multiple deformation of quartz causes the redistribution of Al and K in the quartz lattice, which results in the accumulation of these elements in submicroscopic inclusions (<0.5 lm) of muscovite-like composition. In contrast, structural Ti in quartz is mostly kept in the lattice. Common halos of defect-poor, secondary quartz around fluid inclusions result from re- equilibration of fluid inclusions because of differences between fluid pressure and lithostatic pressure, e.g. duringuplift (isothermal decompression) and/or a/b- transition of quartz. Duringhealing , defect-poor sec- ondary quartz grows at the cost of the host quartz and releases or replaces defect centres. The results of micro- structural investigations combined with Al-in-horn- blende thermobarometry allow the reconstruction of regional processes. Carcoar and Barry granodiorites and Sunset Hills granite were intruded in the Late Ordovi- cian-Early Silurian at depths of 4-8.6 and 10-12 km, respectively. In contrast to the continuous crystallisation of the granodiorite magmas, the magma of the Sunset Hills granite ascended in a stepwise fashion, causing multiple quartz nucleation. The two granodiorites were multiple, post-magmatically deformed, first, during Early Devonian under more brittle conditions at temperatures of 350-400 � C, whereas the Sunset Hills granite experienced more ductile deformation at tem- peratures of around 550 � C.