The Wologorong Batholith, New South Wales, and the extension of the I‐S line of the Siluro‐Devonian granitoids

Abstract

Abstract New data for the Wologorong Batholith, northwest of Goulburn, New South Wales, indicate that it is S‐type and probably similar in age to the major batholith in the southern part of the Lachlan Fold Belt. Its component rocks are predominantly biotite and biotite‐muscovite adamellite, and have a strong N‐S metamorphic foliation. An isochron, defined by fourteen rocks, gives a Rb/Sr age of 405 ±11 Ma and an initial 87Sr/86Sr ratio of 0.7110 ± 0.0012. Five Rb/Sr biotite ages range between 339 and 381 Ma, the older ages being obtained from the least deformed parts of the Batholith. Whereas uncertainties inherent in dating a deformed S‐type batholith make 405 Ma a reconnaissance value only, the minimum emplacement age for the Batholith, as determined from two biotites, is around 381 Ma. The younger biotite ages, around 339 to 348 Ma, are considered to date a major period of regional metamorphism in the northeastern part of the Lachlan Fold Belt, similar to that recorded from K/Ar ages of metamorphic biotite in the Hill End region 100 km to the north. The I‐S line of Siluro‐Devonian granites, defined in the southern part of the Lachlan Fold Belt, extends in a NNE direction, and passes to the east of the Wologorong Batholith. To the north of the Carboniferous Bathurst, the position of the line is uncertain, because of the absence of Siluro‐Devonian S‐type granites, and establishing its presence may depend on the nature of the volcanic rocks, as yet not investigated.