Aluminium saturation in I- and S-type granites and the characterization of fractionated haplogranites

Abstract

Granites of the Lachlan Fold Belt resulted from partial melting of the crust. In most cases, fusion involved mainly quartz and feldspar, producing felsic melts. Varying degrees of separation of those melts from the unmelted source rock (restite) were responsible for much of the compositional variation seen in the granites of the belt. Less commonly, melting occurred at higher temperatures forming more mafic melts, such as for the I-type Boggy Plain Supersuite and the S-type Koetong Suite. Hence, the felsic haplogranites of the Lachlan belt dominantly formed initially as primary melts that separated from restite and less often by the fractionation of more mafic melts. Source rocks of the I- and S-type granites were undersaturated or oversaturated in Al, respectively, and the more mafic granites share that characteristic with their source. As the magmas of the Boggy Plain Supersuite evolved progressively by fractional crystallization, the rocks trended towards saturation in Al, to eventually form a mode close to Al saturation. Other felsic I-type magmas, formed directly by partial melting, were generally more oversaturated in Al, as were the corresponding S-type melts derived from peraluminous source rocks. In an unfractionated state, there are some overlaps in the degree of Al saturation in these magmas produced by partial melting. However, when extended fractional crystallization of these felsic partial melts took place, an almost complete separation in Al-saturation developed between I-type and more peraluminous S-type melts. Because apatite is soluble in peraluminous melts, P became progressively more abundant in the S-type melts as they fractionated. This led to contrasts in the abundances of P and of elements such as Y, the rare earth elements, and Th, between the strongly fractionated I- and S-type granites. Hence, such granites can easily be distinguished from each other.