Distinction between intrusive and extrusive or sedimentary parentage of felsic gneisses: Examples from the Broken Hill Block, NSW

The parentage of felsic gneisses in high-grade metamorphic terranes is often difficult to determine. Recent workers in the Broken Hill Block of the Willyama Supergroup, western New South Wales, have inferred that most of the felsic gneisses have volcanic, tuffaceous or arkosic precursors. However, megacrystic gneisses in the southern and central parts of the Broken Hill Block have field and chemical characteristics consistent with granitoid precursors. Intrusive granitoids have been reported to occur in the contiguous Olary Block, South Australia, and are common in low pressure metamorphic terranes. Therefore, their presence would be expected at Broken Hill. An intrusive granitoid origin is favoured by local unfaulted transgressive contacts, the sizes and shapes of some of the bodies, the general absence of continuous compositional layering, the presence of aplites, and the common occurrence of K-feldspar megacrysts. If the megacrystic gneisses have intrusive granitoid precursors, they should not be used in stratigraphic correlation. Furthermore, the origin of layering in some non-megacrystic gneisses is sufficiently doubtful that their use in stratigraphic correlation is also questionable.