Geochemistry of a peraluminous granitoid suite from North-eastern Victoria, South-eastern Australia

Abstract

The Koetong Suite of Silurian, 2-mica granitoids was derived from a metasedimentary source and emplaced into Ordovician sediments and metasediments along the eastern margin of the Western Metamorphic Belt of South-eastern Australia. Whole-rock geochemical considerations preclude derivation of the magmas represented by the granitoids from exposed Ordovician metasediments. The magmas were generated by partial melting of material similar in composition to garnet-cordierite gneisses exposed in the adjacent metamorphic belt. Melting at pressures in excess of 5 Kb and temperatures about 750°C produced peraluminous magmas and, when the degree of partial melting approached 25–30%, these magmas became mobile and moved vertically into the overlying Ordovician sediments. During movement from the source region to the zone of emplacement, separation of the melt and refractory residue components of the magma resulted in a range of compositions so that whole-rock analyses of the granitoids are linearly related on major and trace element variation diagrams. Processes such as crystal fractionation and crystal accumulation may have operated locally. The magmas were largely composed of solid material throughout their emplacement histories and the amount of melt may not have exceeded 30–45% at any stage. Metasedimentary inclusions are a reflection of source heterogeneity. After emplacement of the magmas, in situ crystallization of a relatively anhydrous assemblage of minerals led to water contents in residual, intercrystalline, melts sufficiently high for muscovite to begin crystallization at pressures around 4 Kb. Subsequent saturation of intercrystalline residual melt and loss of the resultant volatile phase caused the development of eutectoid intergrowths involving muscovitebiotite-quartz and alkali feldspar.