Relations in space and time between major Precambrian shield units: An interpretation of Western Australian data

Abstract

Vertical metamorphic and age variations appear to occur in the Early Precambrian Yilgarn craton in Western Australia. The Wheat Belt gneiss-granulite terrain in the southwest of Western Australia is interpreted as the exposed equivalent of coeval infracrustal roots of the low-grade granite-greenstone Kalgoorlie System. An eastward tilt of the upper layers of the Yilgarn craton, indicated by seismic data, accounts for the exposure of progressively deeper levels of the Archaean crust from east to west. This transition involves increases in metamorphic grade, Na/K ratios of granites, and isotopic ages. The crustal level exposed in the Pilbara craton (north of the Yilgarn craton) is believed to be intermediate between that of the Kalgoorlie System and the Wheat Belt. It is emphasised that no igneous ages are available for the metamorphosed pre-granite ultramafic-mafic assemblages of the greenstone belts, and we consider that these constitute the oldest identifiable rocks of the Western Australian shield. Although post-granite greenstones also occur in the Kalgoorlie System, as well as in Rhodesia, Canada, and India, they cannot be used as evidence against the existence of a primordial oceanic crust as they could well represent temporally distinct upper phases of mafic and ultramafic volcanism. It is suggested that the presently available data from Western Australia can be interpreted in terms of successive formation of ultramafic-mafic oceanic crust, sodic granites, and potassic granites. The spatial relations between the Yilgarn and Pilbara cratons and the surrounding mobile belts are considered, and an ensialic origin of at least some mobile belts is favoured. Possibly they evolved from circum-cratonic and intracratonic rift-valley systems, where subsidence and underlying mantle diapirism resulted in extensive metamorphism, anatexis, and igneous activity.