COMPOSITIONAL VARIATION WITHIN GRANITE SUITES OF THE LACHLAN FOLD BELT - ITS CAUSES AND IMPLICATIONS FOR THE PHYSICAL STATE OF GRANITE MAGMA

Granites within suites share compositional properties that reflect fea tures of their source rocks. Variation within suites results dominantl y from crystal fractionation, either of restite crystals entrained fro m the source, or by the fractional crystallisation of precipitated cry stals. At least in the Lachlan Fold Belt, the processes of magma mixin g, assimilation or hydrothermal alteration were insignificant in produ cing the major compositional variations within suites. Fractional crys tallisation produced the complete variation in only one significant gr oup of rocks of that area, the relatively high temperature Boggy Plain Supersuite. Modelling of Sr, Ba and Rb variations in the I-type Glenb og and Moruya suites and the S-type Bullenbalong Suite shows that vari ation within those suites cannot be the result of fractional crystalli sation, but can be readily accounted for by restite fractionation. Dir ect evidence for the dominance of restite fractionation includes the c lose chemical equivalence of some plutonic and volcanic rocks, the pre sence of plagioclase cores that were not derived from a mingled mafic component, and the occurrence of older cores in many zircon crystals. In the Lachlan Fold Belt, granite suites typically evolved through a p rotracted phase of restite fractionation, with a brief episode of frac tional crystallisation sometimes evident in the most felsic rocks. Evo lution of the S-type Koetong Suite passed at about 69% SiO2 from a sta ge dominated by restite separation to one of fractional crystallisatio n, Other suites exist where felsic rocks evolved in the same way, but the more mafic rocks are absent. In terranes in which tonalitic rocks formed at high temperatures are more common, fractional crystallisatio n would be a more important process than was the case for the Lachlan Fold Belt.