Archaean basin development and crustal extension in the northeastern Yilgarn Block, western Australia

Abstract

Abstract Supracrustal sequences in the northeastern Yilgarn Block, Western Australia, form two contrasting lithotectonic associations. Most sequences were deposited in subaqueous to subaerial environments in a basin 120 km in width. The preserved succession reaches a maximum of e. 10 km in thickness in the central part of the basin, although there is no record of the basement on which it was deposited. In the later stages of its development, the basin was intruded by calc-alkaline granitoids, that are in part comagmatic with calc-alkaline volcanic rocks in the upper part of the stratigraphic succession. Brittle fracture (normal strike-faults and conjugate cross-faults), open, upright folds and peak low-strain metamorphism were coincident with granitoid emplacement. The eastern and western margins of the basin were greatly modified during a period of increased crustal extension, marked initially by the generation of a mildly peralkaline volcanic-plutonic suite, the volcanic members of which were deposited in linear, faultbounded depressions. A major growth fault indicated by thin persistent units of oligomictic and polymictic conglomerate and localized intense metasomatism was probably contemporaneous with felsic volcanic activity. Subsequent sediments, derived largely from granitic source areas, were deposited in narrow grabens. The youngest felsic plutonic rocks in the area include small bodies of alkali feldspar syenite many of which are found near the margins of the late, sediment-filled grabens. Tectonism at the eastern basin margin extended into the Proterozoic. In contrast to sequences at the centre of the basin, those at its margin have undergone isoclinal folding, ductile deformation and high-strain metamorphism. The geological framework of the northeastern Yilgarn Block emphasizes the importance of crustal extension in the development of younger Archaean supracrustal sequences and provides valuable support for extension-dependent greenstone belt evolutionary models (e.g., Groves and Batt).