Granite suites and supersuites of eastern Australia

Separate granite plutons in southeastern Australia can commonly be grouped into suites on the basis of shared similarities in field, petrographic and compositional data. Granites in different plutons of the same suite share common properties or exhibit a sequence of such features Rocks of the same suite are co genetic, but the details of their genesis need not be known or agreed on, to group granite units in such a way. These rocks are co genetic in the sense that they shared a similar petrogenesis and were derived from source materials of essentially the same composition, whereas differences between suites reflect analogous differences in their source rocks. The term suite is lithologic or lithodemic in a stratigraphic sense and is closely analogous to the lithostratigraphic term group As such, the plutons within a suite need not be of the same age, and age is not a factor in recognising a suite. However, the fact that the petrogenesis of the components of a suite resulted in such similar products means that their ages are likely to be similar Granite plutons that share many similar features, but which also show distinct differences and which may be assigned to more than one suite, may be grouped into supersuites. The allocation of granites to suites is fundamental to understanding their petrogenesis. Suites vary in the complexity of their compositional variation. Simple suites show variations in element abundances that are highly correlated and the dispersion of composition within such suites is considered to result from varying degrees of fractionation of entrained restite from a melt. Intricate suites vary in composition in more complex ways and their variation is considered to be a consequence of processes such as fractional crystallisation. Any mineralisation is generally associated with intricate suites, and the occurrence of mineralisation and its precise character is generally specific to particular suites. If isotopic compositional dispersion within a granite suite is observed, a process of magma mixing or mingling is commonly invoked. A particular case where isotopic dispersion would occur is that when a mantle-derived component partly melts and then mingles with a crustal magma (Barbarin 1991); the more felsic rocks would be isotopically more evolved than the mafic ones. Granite derived from heterogeneous source rocks, particularly granites formed from relatively low degrees of partial melting from metasedimentary rocks, might be expected to reflect that lack of isotopic uniformity in their source compositions. For example, Chappell et al. (1999) noted that the initial 87Sr/86Sr values for 10 granites sampled from various parts of the Jillamatong pluton show a range of 0.0039, approximately 50-fold that of analytical uncertainty. For this very mafic unit of the S-type Bullenbalong Suite, there is no systematic variation of strontium isotopic composition with SiO2, although samples of more felsic members of the Bullenbalong Suite and Supersuite show a systematic decrease in initial 87Sr/86Sr with increase in SiO2. This observation indicates that variation in initial isotope ratios is here source related, and not magma-mingling related. Copyright (2001) Geological Society of Australia