Silurian volcanism in the wollondilly basin, eastern lachlan fold belt, new south wales

Abstract

The Wollondilly Basin, east of Goulbum in the Southern Highlands of New South Wales, contains a mafic to silicic volcanic succession together with limestone and clastic deposits. The oldest unit, the Gundary Formation, includes clastic deposits, silicic pyroclastic flows, and mafic-intermediate lavas and was emplaced, at least in part, in a terrestrial environment. Fresh hornblende separates from two samples of dacitic ignimbrite have an average K-Ar age of 426 ± Ma (late Early Silurian). The mafic-intermediate flows have a shoshonitic geochemical signature that reflects the nature of their source material. The Gundary Formation is in faulted contact with the marine Boxers Creek Formation which conformably underlies a Late Silurian turbidite unit, the Towrang Formation. Cessation of volcanic activity is reflected by decreasing input of volcanic detritus up section and the increase in detritus derived from uplifted Ordovician basement rocks. The succession demonstrates that rocks with shoshonitic affinity were emplaced during the Silurian and that Silurian subaerial mixed rhyolitic-dacitic-andesitic-basaltic volcanism was widespread along the western margin of the Wollondilly Basin. Modification and enrichment of the source for these shoshonitic rocks may have been coeval with westdipping Silurian subduction or may have occurred during a pre-Silurian subduction episode. Show more