SILURIAN VOLCANISM IN THE WOLLONDILLY BASIN, EASTERN LACHLAN FOLD BELT, NEW-SOUTH-WALES

The Wollondilly Basin, east of Goulburn in the Southern Highlands of N ew South Wales, contains a mafic to silicic volcanic succession togeth er with limestone and elastic deposits. The oldest unit, the Gundary F ormation, includes elastic deposits, silicic pyroclastic flows, and ma fic-intermediate lavas and was emplaced, at least in part, in a terres trial environment. Fresh hornblende separates from two samples of daci tic ignimbrite have an average K-Ar age of 426 +/- Ma (late Early Silu rian). The mafic-intermediate flows have a shoshonitic geochemical sig nature 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 Towra ng Formation. Cessation of volcanic activity is reflected by decreasin g input of volcanic detritus up section and the increase in detritus d erived from uplifted Ordovician basement rocks. The succession demonst rates that rocks with shoshonitic affinity were emplaced during the Si lurian and that Silurian subaerial mixed rhyolitic-dacitic-andesitic-b asaltic volcanism was widespread along the western margin of the Wollo ndilly Basin. Modification and enrichment of the source for these shos honitic rocks may have been coeval with west-dipping Silurian subducti on or may have occurred during a pre-Silurian subduction episode.