Fractionation vs. magma mixing in the Wangrah Suite A-type granites, Lachlan Fold Belt, Australia: Experimental constraints

The Wangrah Suite granites (Lachlan Fold Belt, Australia) reflect different stages of differentiation in the magmatic history of an A-type plutonic suite. In this study we use experimentally determined phase equilibria of four natural A-type granitic compositions of the Wangrah Suite to constrain phases and phase compositions involved in fractionation processes. Each composition represents a distinct granite intrusion in the Wangrah Suite. The intrusions are the Danswell Creek (DCG), Wangrah (WG), Eastwood (EG) and Dunskeig Granite (DG), ordered from "most mafic" to "most felsic" by increasing SiO₂ and decreasing FeO_total. Experimental investigation show that the initial water content in melts from DCG is between 2-3 wt. % H₂O. If the DCG is viewed as the parental magma for the Wangrah Suite, then (1) fractionation of magnetite, orthopyroxene and plagioclase (∼ 20 wt. %) of the DCG composition, leads to compositions similar to that of the EG; (2) further fractionation of plagioclase, quartz, K-feldspar and biotite (∼ 40 wt. %) from the EG composition, leads to the DG composition. These fractionation steps can occur nearly isobarically and are confirmed by bulk rock Ba, Sr, Rb and Zr concentrations. In contrast, the generation of the most abundant WG composition cannot be explained by fractional crystallisation from the DCG at isobaric conditions because of the high K₂O content of this granite. Magma Mixing could be the process to explain the chemical distinctiveness of the Wangrah Granite from all the other granites of the Wangrah Suite.