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 feature s that are a result of their derivation from source material of a spec ific composition. Variation within suites has been ascribed to a varie ty of processes. Magma mixing or mingling is a popular hypothesis, gen erally proposed in terms of blending between a crustal melt and mafic material from the mantle that caused that melting. When the compositio ns of pairs of suites from the Bega Batholith of southeastern Australi a are compared, any differences seen at either end of she range in com position 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 reg ion. These observations are not consistent with large-scale magma mixi ng or mingling and, although those processes may operate on a small sc ale, they cannot have been responsible for the major compositional var iations. Likewise, assimilation of country rocks had no significant ro le in producing variation in the granites of southeastern Australia. T he production of variation by differential separation of melt from res idual solid source material, or restite, must be favoured for many of the granite suites of this region.