Ultrametamorphism and granitoid genesis

A model is presented to explain the geochemical and mineralogical characteristics of granitoids and their inclusions. The product of ultrametamorphism is melt + residuum, both of which may move en masse to the site of crystallization. The nature of the source material can be deduced from studies on the granitoids and their inclusions; based on studies of the Lachlan belt of southeastern Australia we recognize granitoids derived from metasedimentary rocks (S-types) and those derived from igneous source rocks (I-types). The straight-line variation diagrams of most granitoid suites is explained by progressive separation of residuum (= restite) and melt. It is shown that some granitoid suites consist of minimum melt + residuum whereas others represent the crystallization of “nonminimum” melts + residuum. Residuum is recognized as metasedimentary xenoliths in S-types and as mafic hornblende-rich xenoliths in I-types. Xenocrystal material is more difficult to recognize petrographically. We suggest that the complexly zoned and twinned plagioclases so characteristic of orogenic rocks are modified residuum. These and xenoliths are absent in granitoids which have largely crystallized from a melt such as those of the Tuolumne Suite of the Sierra Nevada. Mafic minerals of granitoids whether residuum or crystallization products are mostly equilibrium assemblages. Relict mafic phases from the source do not persist and hence P-T conditions of magma generation cannot be deduced.