Inclusions in Three S-Type Granites from Southeastern Australia

Abstract

The Jillamatong Granodiorite is one of the most mafic S-type granites in the Kosciusko regidn and is typical of widely distributed, cordierite-bearing S-type granites in the Lachlan Fold Belt of southeastern Australia. The Koetong and Granya Adamellites belong to the Koetong Suite of the Corryong Batholith and are rare examples in the Lachlan Fold Belt of granites that contain primary muscovite. Although subtle differences can be found, inclusions within the Jillamatong Granodiorite and the Koetong Suite are broadly similar despite the fact that the Jillamatong Granodiorite belongs to a different and distinct suite (the Bullenbalong Suite). Mica-rich schistose and micTogranular inclusions dominate but other types occur, including foliated quartzofeldspathic varieties, calcsilicates, quartzites, and pure quartz types. The total abundance of all inclusion types in each granite studied is less than 5.1% although abundance varies from one granite to another. All inclusions are believed to have been derived from metasedimentary or modified metasedimentary lithologies and all inclusions, except some quartzites, were entrained at depth where the host granite magmas were generated by partial melting of heterogeneous sedimentary sources. The inclusions are restite but most are not complementary to the melt component of the magma now represented by the host granite. They represent fragments from different refractory lithologies of a complex metasedimentary source and because their compositions and mineral assemblages were unsuitable for the generation of large quantities of granite melt, they did not melt or were melted only to small and variable extents (less than the rheological critical melt percentage of Arzi, 1978). Such lithologies remained physically coherent and retained their separation from the host granite magma during ascent. Lithologies that did melt extensively were physically disaggregated and are not represented among the inclusions. Since the inclusions do not represent complementary restite controlling compositional variation among the host granites, their compositions cannot be used to precisely estimate the bulk compositions of the source rocks. However, the different, source-rock derived, inclusion types collectively provide information regarding the lithologies present in the source and hence the general character of the source terranes. The dominance of schistose and microgranular inclusions in the Jillamatong Granodiorite and the Koetong Suite indicates that pelitic and quartzofeldspathic compositions are the two dominant components in the source terranes. Inclusions of the same type from the two suites are broadly similar but different in detail. Inclusions reflect the mineralogical and geochemical characteristics of their host granites and there are textural differences between microgranular inclusions of the two suites examined. The differences reflect subtle but significant contrasts in source materials, the conditions prevailing during partial melting and the history of emplacement and crystallization of the host magmas.