Lachlan Fold Belt granites revisited: High‐ and low‐temperature granites and their implications

Abstract

Many of the granites in southeastern Australia possess compositional, petrographic, zircon age inheritance and other features that cannot be accounted for satisfactorily by the classical models of petrogenesis. The restite model was developed to account for these features and recognises that unmelted but magmatically equilibrated source material (restite) may be entrained in a partial melt, together comprising magma. Variation in the degree of separation of those two components, leading to differences in the ratio of melt to restite, is responsible for the variation in composition within many suites of granites. A popular alternative view, that variation within suites resulted from magma mixing or mingling, conflicts with simple observations of the rock compositions and cannot be sustained. Several strong arguments can be made against another alternative view that fractional crystallisation was the dominant process in producing variation within those suites. New and conclusive evidence against that process is provided by the fact that zircon age inheritance is present in most of these granites where that has been examined. That has shown that the S‐type and most of the I‐type granites formed at low magmatic temperatures and confirmed that the compositional variation within those suites must have resulted from restite fractionation. The Ordovician sedimentary rocks exposed in the Lachlan Fold Belt are not sufficiently feldspathic to have produced the voluminous S‐type granites and volcanic rocks of the Bullenbalong Supersuite. The view that those granites and volcanic rocks were derived from more feldspathic metasedimentary rocks is supported by much evidence and confirmed by the fact that the pelitic enclaves in those granites are relatively high in Ca. The presence of a once‐thick metasedimentary basement in those areas in which these S‐type granites occur is inferred. The I‐type granites of the Lachlan belt are not compositionally analogous to the more calcic and less potassic granites found in younger subduction‐related continental margins and mostly formed at low magmatic temperatures through the partial melting of pre‐existing quartzo‐feldspathic igneous crust. This implies that the petrological evolution of the belt during the Silurian and Devonian occurred dominantly by the vertical redistribution of the components of older crust through partial melting and movement of granite magmas. Evolution of the belt at that time was not related to active subduction.