Characterization and origin of aluminous A-type granites from the Lachlan Fold Belt, Southeastern Australia

The metaluminous to weakly peraluminous A-type granites of the Lachlan Fold Belt are a distinctive group of igneous rocks, on the basis of chemical and mineralogical criteria. Those granites that contain ∼65-72% SiO₂ can be distinguished from other types on the basis of higher abundances of Fe_total/ (Fe_total + Mg), high field strength elements, trivalent rare earth elements, Ga and Zn. Mineralogically, they contain Fe-rich hydrous mafic minerals and primary ilmenite, and hence are reduced relative to the Ni-NiO buffer. However, the extremely felsic A-type granites (SiO₂ > ∼72%) have the same chemical and mineralogical characteristics as felsic, fractionated I-type granites. Recent analyses indicate that the Lachlan Fold Belt A-type granites have Sc, F, alkali element, trace transition element and H₂O contents similar to those of other unfractionated I-type granites. Rb-Sr and Nd-Sm isotopic compositions are highly variable, probably reflecting source region heterogeneity. The metaluminous to weakly peraluminous A-type granites of the Lachlan Fold Belt are distinct from peralkaline rocks in terms of chemical composition, petrography and field associations, although these rocks have been grouped together as a single type in current classification schemes. We propose that the metaluminous to weakly peraluminous A-type granites, such as those of the Lachlan Fold Belt, should be defined as 'aluminous A-type' granites and should not be grouped with peralkaline granites. The Lachlan Fold Belt aluminous A-type granites have relatively high calculated zircon saturation temperatures. We suggest that these granites were produced by high-temperature partial melting of a felsic infracrustal source.