Granitoids from the moonbi district, new england batholith, eastern Australia

The late Palaeozoic granitoids of the Moonbi district are derived both from igneous (I-type) and sedimentary (S-type) sources. Field and petrographic observations and chemical data on the I-type granitoids show that they are derived from four separate and distinct source-rock compositions and that, consequently, these granitoids may be grouped into four suites. Mafic xenoliths and microxenoliths are relatively more abundant in more mafic I-type granitoids. Such xenoliths are interpreted as restite, or material residual from partial melting of the source rocks. Variation within the granitoids is ascribed to varying degrees of separation of restite from the melt produced during each fusion event. The source material of the I-type granitoids is considered to have been material underplated beneath the crust during an earlier subduction event. Two suites of S-type granitoids can be recognized. These are derivatives of pelitic materials that have undergone only a small amount of chemical weathering.