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

Granites of the Lachlan Fold Belt resulted from partial melting of the crus t. In most cases, fusion involved mainly quartz and feldspar, producing fel sic melts. Varying degrees of separation of those melts from the unmelted s ource rock (restite) were responsible for much of the compositional variati on seen in the granites of the belt. Less commonly, melting occurred at hig her temperatures forming more mafic melts, such as for the I-type Baggy Pla in Supersuite and the S-type Koetong Suite. Hence, the felsic haplogranites of the Lachlan belt dominantly formed initially as primary melts that sepa rated from restite and less often by the fractionation of more mafic melts. Source rocks of the I- and S-type granites were undersaturated or oversatu rated in Al, respectively, and the more mafic granites share that character istic with their source. As the magmas of the Baggy Plain Supersuite evolve d progressively by fractional crystallization, the rocks trended towards sa turation in Al, to eventually form a mode close to Al saturation. Other fel sic I-type magmas, formed directly by partial melting, were generally more oversaturated in Al, as were the corresponding S-type melts derived from pe raluminous source rocks. In an unfractionated state, there are some overlap s in the degree of Al saturation in these magmas produced by partial meltin g. However, when extended fractional crystallization of these felsic partia l melts took place, an almost complete separation in Al-saturation develope d between I-type and more peraluminous S-type melts. Because apatite is sol uble in peraluminous melts, P became progressively more abundant in the S-t ype 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 s trongly fractionated I- and S-type granites. Hence, such granites can easil y be distinguished from each other. (C) 1999 Elsevier Science B.V. All righ ts reserved.