Early Precambrian crustal evolution at Mount Narryer, Western Australia

Abstract

The oldest known rocks in Australia occur at Mount Narryer and are associated with quartzite that contains detrital zircons far older than any known terrestrial rocks.The region forms part of the Western Gneiss Terrain of the Archaean Yilgarn Block. The main rock unit is a banded biotite adamellite gneiss which gives a Sm-Nd model age of 3630 Ma. It is intruded by a leucocratic granite gneiss which gives a Sm-Nd model age of 3510 Ma and contains abundant inclusions of a meta-anorthosite—gabbro—ultramafic suite. Quartzites and banded quartz—pyroxene—magnetite rocks form concordant layers within both gneisses.Most rocks are strongly deformed, are folded together into large-scale fold interference patterns, and show mineral assemblages in granulite or retrograded granulite facies. One large body of quartzite has internally escaped intense deformation and contains cross-bedding and graded-bedding structures as well as horizons of both polymict and quartz-pebble conglomerates. The quartzites contain detrital zircons which mostly give U-Pb ages of ∼3750–3500 Ma, but a few zircon grains give ages of 4100–4200 Ma. Samples of the adamellite gneiss give a Rb-Sr whole-rock isochron age of 3348±43 Ma which might date a peak of metamorphism.