Origin of enclaves in S-type granites of the Lachlan Fold Belt

Abstract

The more mafic S-type granites of the Lachlan Fold Belt contain a distinctive assemblage of lithic inclusions of deep crustal origin. Two types predominate, the schistose enclaves and the microgranular enclaves. There is also a small proportion of fragments of local country rock. All the features of the schistose enclaves are consistent with an origin as lithic restite fragments from the source. Their ubiquity in the mafic S-type granites and absence from both felsic S-type granites and I-type granites, conform with such an origin. The argument that these enclaves are not in chemical equilibrium with the host granite because they do not complement its composition is not valid, since they represent less fertile parts of the source that had different compositions that, as a result, melted to a lesser extent. Models that derived these enclaves from regions of the crust above the zones of partial melting, so that they are not restites, are complex, unnecessary, do not conform with their ubiquity in the more mafic S-type granites, and are not consistent with their chemical compositions.The origin of the microgranular enclaves is controversial. In most S-type granites these enclaves have been recrystallized to a similar mineral assemblage to the host granite with a higher proportion of biotite. In a few places, such material is seen to be forming by recrystallization of material in the core of the enclave, which has the assemblage quartz+calcic plagioclase+orthopyroxene±cordierite±biotite with accessory ilmenite, sulfide and apatite. All of the microgranular enclave cores show pseudo-doleritic texture in which calcic plagioclase crystals (>An60) and orthopyroxene crystals project into or are enclosed by quartz. Large plagioclase crystals are zoned, with some core compositions as calcic as An94. The low Na contents of the microgranular enclaves are not consistent with an igneous origin. We consider that these enclaves were derived by metamorphism of calcareous mudstones or argillaceous limestones. The pseudo-igneous textures result from the presence of a partial melt during metamorphism. An analysis of a calc-silicate lens of upper amphibolite grade from the Wilsons Group of Victoria Land, Antarctica is remarkably similar in composition to the microgranular enclaves from Lachlan Fold Belt S-type granites. We suggest that rocks equivalent to that Group, which is of appropriate age and general composition, were the source rocks for the S-type granites of the Lachlan Fold Belt.