Granitoid types and their distribution in the Lachlan Fold Belt, southeastern Australia

Abstract

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.