Structure, detrital zircon U-Pb ages and 40 Ar/ 39 Ar geochronology of the Early Palaeozoic Girilambone Group, central New South Wales: subduction, contraction and extension associated with the Benambran Orogeny

Abstract

The Girilambone Zone is located in the northern part of the central subprovince of the Lachlan Fold Belt and its bedrock exposure includes the widespread Girilambone Group. This unit consists of two main components, low-grade, metamorphosed quartz-rich turbidites and bedded chert with Middle to Late Ordovician conodont ages in the west (Ballast Formation), and low to moderate-grade schist, psammitic schist, mafic schist and phyllite that occur in the east. Detrital zircon grains from samples of a quartzite and a quartzose psammite from the eastern part of the Girilambone Group around Girilambone, have age spectra (600-500 Ma) typical of Ordovician turbidite successions in southeastern Australia. In the Girilambone district, an intense, steeply dipping differentiated layering (S2) formed in the D2 deformation and has largely masked the effects of earlier deformation apart from foliation (S1). S2 is overprinted by mainly north-trending, moderately to steeply west-dipping crenulation cleavage (S3) that is axial planar to abundant open to close folds (D3 deformation). Locally, S3 cleavage and fold axial planes have a low dip. In the Tottenham district, the main foliation is also a differentiated layering (S2) with low to moderate dips to the southeast in a northern domain and subhorizontal in a larger southern domain due to folding by abundant near-isoclinal, recumbent folds (F3). These reflect ductile thinning of the crust and are attributed to extensional deformation. Weak to locally intense crenulation cleavage and associated folds (D4) affect the S2 in the northern domain and are weakly developed in the southern domain. Three muscovite 40Ar/39Ar ages from both the Girilambone and Tottenham regions are ca 435 Ma consistent with deformation and metamorphism during the Late Ordovician-Early Silurian Benambran Orogeny. The Girilambone Group in the Late Ordovician was accreted in an inferred subduction complex related to an east-dipping subduction zone in the Wagga marginal sea. The subduction complex in the Early Silurian was affected by contractional and extensional deformation prior to the migration of igneous activity from the Molong island arc to the Wagga-Omeo Zone.