Fractionation of metaluminous A-type granites: an experimental study of the Wangrah Suite, Lachlan Fold Belt, Australia

Abstract

Crystallisation experiments were performed on two metaluminous A-type granite compositions (AB412 and AB422) from the Wangrah Suite, a representative A-type granite suite from the Lachlan Fold Belt, south eastern Australia. AB412 is more mafic than AB422 and zircon saturation temperatures are 897 and 843 °C, respectively. Experiments were performed at 200 MPa, fO2 between NNO and NNO−1.05 in a temperature range between 700 and 900 °C, for various melt H2O contents. Experimental results were compared to the natural phase compositions to constrain the water content of the melt and the phases involved in fractionation processes in the Wangrah Suite. With decreasing temperature, Fe–Ti oxide, orthopyroxene and plagioclase are the first phases to crystallise, followed by biotite. The orthopyroxene stability is mainly restricted to temperatures above 750 °C. Hornblende is only stable in the less evolved composition if the water content of the melt is at least 4–5 wt.% H2O. The initial water content in the more mafic melt AB412 was between 2 and 3 wt.% H2O. Although orthopyroxene is not observed in the natural assemblages of the Wangrah Suite, it has influenced the early fractionation of the Wangrah Suite granites. Mass balance calculations show that composition AB422 can be obtained after the crystallisation of 4.7 wt.% iron-rich orthopyroxene plus 8.5 wt.% plagioclase from AB412.