I-type granites can be assigned to low- and high-temperature groups. The distinction between those groups is formally based on the presence or absence of inherited zircon in relatively mafic rocks of a suite containing less than about 68% SiO 2 , and shown in many cases by distinctive patterns of compositional variation. Granites of the low-temperature group formed at relatively low magmatic temperatures by the partial melting dominantly of the haplogranite components Qz , Ab and Or in H 2 O-bearing crustal source rocks. More mafic granites of this type have that character because they contain restite minerals, often including inherited zircon, which were entrained in a more felsic melt. In common with other elements, Zr contents correlate linearly with SiO 2 , except sometimes in very felsic rocks, and Zr generally decreases as the rocks become more felsic. All S-type granites are apparently low-temperature in origin. After most or all of the restite has been removed from the magma, these granites may evolve further by fractional crystallisation. High-temperature granites formed from a magma that was completely or largely molten, in which zircon crystals were not initially present because the melt was not saturated in that mineral. High-temperature suites commonly evolved compositionally through fractional crystallisation and they may extend to much more mafic compositions through the production of cumulate rocks. However, it is probable that, in some cases, the compositional differences within high-temperature suites arose from varying degrees of partial melting of similar source rocks. Volcanic equivalents of both groups exist and show analogous differences. There are petrographic differences between the two groups and significant mineralisation is much more likely to be associated with the high-temperature granites. The different features of the two groups relate to distinctive source rock compositions. Low-temperature granites were derived from source rocks in which the haplogranite components were present throughout partial melting, whereas the source materials of the high-temperature granites were deficient in one of those components, which therefore, became depleted during the melting, causing the temperatures of melting to rise.

Granites and related volcanic rocks of the Lachlan Fold Belt can be grouped into suites using chemical and petrographic data. The distinctive characteristics of suites reflect source-rock features. The first-order subdivision within the suites is between those derived from igneous and from sedimentary source rocks, the I- and S-types. Differences between the two types of source rocks and their derived granites are due to the sedimentary source material having been previously weathered at the Earth’s surface. Chemically, the S-type granites are lower in Na, Ca, Sr and Fe 3+ /Fe 2+ , and higher in Cr and Ni. As a consequence, the S-types are always peraluminous and contain Al-rich minerals. A little over 50% of the I-type granites are metaluminous and these more mafic rocks contain hornblende. In the absence of associated mafic rocks, the more felsic and slightly peraluminous I-type granites may be difficult to distinguish from felsic S-type granites. This overlap in composition is to be expected and results from the restricted chemical composition of the lowest temperature felsic melts. The compositions of more mafic I- and S-type granites diverge, as a result of the incorporation of more mafic components from the source, either as restite or a component of higher temperature melt. There is no overlap in composition between the most mafic I- and S-type granites, whose compositions are closest to those of their respective source rocks. Likewise, the enclaves present in the more mafic granites have compositions reflecting those of their host rocks, and probably in most cases, the source rocks. S-type granites have higher δ 18 O values and more evolved Sr and Nd isotopic compositions, although the radiogenic isotope compositions overlap with I-types. Although the isotopic compositions lie close to a mixing curve, it is thought that the amount of mixing in the source rocks was restricted, and occurred prior to partial melting. I-type granites are thought to have been derived from deep crust formed by underplating and thus are infracrustal, in contrast to the supracrustal S-type source rocks. Crystallisation of feldspars from felsic granite melts leads to distinctive changes in the trace element compositions of more evolved I- and S-type granites. Most notably, P increases in abundance with fractionation of crystals from the more strongly peraluminous S-type felsic melts, while it decreases in abundance in the analogous, but weakly peraluminous, I-type melts.

The Lachlan Fold Belt in southeastern Australia comprises rocks ranging in age from Cambrian to Devonian. Granitoid emplacement and related volcanic activity occurred in Silurian and Devonian times, with minor development of Carboniferous plutons in the most easterly part of the belt. The belt is at least 800 km wide, which is much wider than the Mesozoic and Cenozoic fold belts of the circum-Pacific. Granitoids are extensively developed in the Lachlan belt and make up 36 percent of exposed Paleozoic rocks in the relatively well-exposed easternmost part east of longitude 148° E, a strip up to 200 km wide. Granitoids in the Lachlan Fold Belt can be grouped into suites, where each suite has a distinctive chemical character, consistent with its having been derived from source rocks of unique composition. Most of the variation within suites can be ascribed to varying degrees of separation of material residual from partial melting, or restite, from melt. The differences between suites result from differences in source rock composition. The first-order subdivision between suites is between those granitoids derived from sedimentary and from igneous source rocks, the S- and I-types. These two types have chemical, mineralogical, and isotopic characters reflecting the distinctive features of their sources, specifically the fact that the S-type source rocks have been through at least one cycle of chemical weathering at the earth’s surface. There is an eastern limit to the occurrence of S-type granitoids, called the I-S line, which is thought to represent the eastern limit of thick crystalline basement. A late-formed group of felsic granitoids, the A-types, are thought to have been derived from crust that had previously produced I-type magmas so that the source rocks were residual from that prior melting event.