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 I‐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.