DEVELOPMENT OF P-RICH GRANITES BY SEQUENTIAL RESTITE FRACTIONATION AND FRACTIONATIONAL CRYSTALLISATION: THE KOETONG SUITE IN THE LACHLAN FOLD BELT

B.W. Chappell1 and A.J.R. White2

1 GEMOC ANU

2 Victorian Institute of Earth and Planetary Sciences (VIEPS), School of Earth Sciences, University of Melbourne,

Among the S-type granites of south-eastern Australia, the extensive Koetong Suite shows a wide range of compositions. The variation resulted from two processes. At more mafic compositions, it was induced by varying degrees of separation of a primary melt containing ~ 70% SiO2 from restite with which it formed in equilibrium in the source. There was no significant change in phosphorus contents during that stage of evolution. In the more felsic granites variation resulted from fractional crystallisation of the primary melt. Elements such as rubidium and tin increased in abundance while others such as strontium and zirconium decreased in amount. In these strongly peraluminous compositions, fractional crystallisation lead to significantly higher phosphorus contents.

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