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 characteristicfeatures that are a result of their derivation from source materialof a specific composition. Variation within suites has beenascribed to a variety of processes. Magma mixing or minglingis a popular hypothesis, generally proposed in terms of blendingbetween a crustal melt and mafic material from the mantle thatcaused that melting. When the compositions of pairs of suitesfrom the Bega Batholith of southeastern Australia are compared,any differences seen at either end of the range in compositionare also seen at the other limit, so that both the most maficand most felsic rocks show similar relative abundances of particularelements. Similar relationships are seen for other granitesin the region. These observations are not consistent with large-scalemagma mixing or mingling and, although those processes may operateon a small scale, they cannot have been responsible for themajor compositional variations. Likewise, assimilation of countryrocks had no significant role in producing variation in thegranites of southeastern Australia. The production of variationby differential separation of melt from residual solid sourcematerial, or restite, must be favoured for many of the granitesuites of this region. KEY WORDS: assimilation; enclaves; granite suites; magma mixing; restite