Are A-type granites the high-temperature felsic granites? Evidence from fractionated granites of the Wangrah Suite

Abstract

A-type granites are a minor, but distinctive, component of the granites of the Lachlan Fold Belt of southeastern Australia. They are felsic rocks with SiO2 contents ranging from 69.7 to 77.1%, with an average of 73.8% (55 analyses). When unfractionated, as evidenced by high Ba contents, they are distinguished from felsic l-type granites by a greater abundance of high-field-strength elements, such as Zr. The Wangrah Suite contains a diverse association of A-type granites, comprising four main units with coherent geochemical trends overall, but with textural variation from equigranular through to porphyritic. The least felsic granites from the suite (Danswell Creek Granite ∼70% SiO2) have compositional features that suggest that they represent parental magma compositions. The most felsic granites (Dunskeig Granite ∼76% SiO2) were derived from such compositions by fractional crystallisation. The Wangrah Suite granites were emplaced at shallow levels (∼200 MPa), at high zircon saturation temperatures (>830°C) and relatively low water activity. The chemical composition of the Wangrah granites cannot be easily related to the adjacent mafic magmas. The compositionally variable Wangrah Suite differs from the homogeneous A-type suites, such as the Gabo Suite to the southeast. Its variability is probably related to the efficiency of fractional crystallisation and emplacement along a major fault at shallow levels. We favour a single-stage petrogenetic scheme where the A-type magmas were produced by high-temperature, partial melting of quartzo-feldspathic crustal rocks. The relatively refractory nature of the source rocks may have been due to limited H2O content, relatively low fo2 and relatively high (TiO2 + FeOtotal)/MgO.