Source rocks of I- and S-type granites in the Lachlan Fold Belt, southeastern Australia

Abstract

Granites and related volcanic rocks derived from both igneous and sedimentary source materials (I- and S-types) are widely distributed in the Palaeozoic Lachlan Fold Belt of southeastern Australia. Many of the granites contain material residual from partial melting of the source rocks, or restite, which enables attempts to be made to calculate source rock compositions. A few of the S-type granites are closely related to regional metamorphic rocks and are of relatively local derivation. Most, however, are intrusive into low-grade rocks and came from deeper levels in the crust; and volcanic equivalents are extensively developed. These dominant S-type rocks have chemical and isotopic properties unlike any known locally exposed sediments. For most of the S-types, and perhaps all of them, no mantle-derived component was present in the source. Chemical and isotopic data on the I-type granites suggest a variety of deep crust sources consisting of mantle-derived material showing differing amounts of isotopic evolution, according to the time since extraction from the mantle. These data do not favour a significant sedimentary component in the sources of even the most isotopically evolved I-type rocks. An origin of the I-type source rocks by crustal underplating is favoured, so that these sources were generally infra-crustal, whereas the S-type sources were of supra-crustal origin.