Magma mixing and the production of compositional variation within granite suites: Evidence from the granites of southeastern Australia

Granite suites are groups of plutons possessing characteristic features that are a result of their derivation from source material of a specific composition. Variation within suites has been ascribed to a variety of processes. Magma mixing or mingling is a popular hypothesis, generally proposed in terms of blending between a crustal melt and mafic material from the mantle that caused that melting. When the compositions of pairs of suites from the Bega Batholith of southeastern Australia are compared, any differences seen at either end of the range in composition are also seen at the other limit, so that both the most mafic and most felsic rocks show similar relative abundances of particular elements. Similar relationships are seen for other granites in the region. These observations are not consistent with large scale magma mixing or mingling and, although those processes may operate on a small scale, they cannot have been responsible for the major compositional variations. Likewise, assimilation of country rocks had no significant role in producing variation in the granites of southeastern Australia. The production of variation by differential separation of melt from residual solid source material, or restite, must be favoured for many of the granite suites of this region.