Geology and geochemistry of the Tantangara and Brindabella 1:100 000 Sheet areas, New South Wales and Australian Capital Territory

The Tantangara and Brindabella 1:100 000 Sheet areas cover 5030 km2 between latitudes 35' and 36Â°S and longitudes 148Â°30' and 149Â°E, in the southern part of the Lachlan Fold Belt in New South Wales, The earliest record of sedimentation in the two Sheet areas is of mid-Ordovocian quartzrich distal flysch, becoming more proximal in the Late Ordovician, In the west, a volcanic arc erupted tholeiitic basalis and (?Iater) shoshonites during the middle to Late OrdovicialL A phase of deformation in the latest Ordovician mainly to the west of the mapped area interrupted flysch sedimentation, which recontinued in a meridional trough-the source being the deformed Ordovician flysch to the west, After a further deformational event-which destroyed the trough during the L1andoverian-a shelf environment became established in the east and centre, and shelf sedimentation continued in the centre almost to the end of the Silurian, when major deformation terminated it, Elsewhere, S then ?I-type felsic volcanism-mostly subaerial, depositing ignimbrites-was widespread during the Wenlockian and Ludlovian; comagmatic S and I-type granitoids intruded the volcanic piles, and tholeiitic magmas intruded, simultaneously in the west, during the Late Silurian. The felsic volcanics and granitoids are the anatecticproducts of a prolonged period of high heat flow in the crust which peaked in the Early Silurian and resulted in metamorphism to at least the upper greenschist facies grade. At the start of the Early Devonian, S-type volcanics-again subaerial felsic ignimbriteserupted in the southwest, slightly later, I-type volcanics erupted from at least two large stratovolcanoes in the north, and a comagmatic granitoid suite intruded concurrently in the north and south. These Early Devonian volcanics and granitoids are believed to be the final products of the high heat flow in the Silurian. In the north during the late stage of volcanism, black mud accumulated locally in a restricted marine environment, and arenites, some originating as mudflows on the sides of the stratovolcanoes, were deposited in alluvial fans. During the latter half of the Early Devonian, open-marine sedimentation became established briefly in this northern area; it was terminated by a thick f1uviatile sequence of conglomerate, sandstone, and shale-much of it deposited in cycles-in the northwest during the early Middle Devonian. This marked the final phase of Palaeozoic sedimentation. After further folding and faulting, probably in the Carboniferous, an extensive peneplain developed during the late Palaeozoic and Mesozoic. Sandwiched between two episodes of Tertiary uplift, which progressively elevated the peneplain, lacustrine sediments accumulated locally in the west and south during the Miocene. Extensive colluvium has formed on the mountain slopes during the Quaternary, and alluvium is being deposited along the courses of many streams at the present time. Known mineral deposits are small and mostly uneconomical. They include Mississippi Valley-type lead-zinc in Upper Silurian limestone; skarn deposits containing magnetite and minor lead-zinc associated with Silurian S-type granitoids; tungsten-bismuth and magnetite associated with Lower Devonian I-type granitoids; base metals associated with SilurianDevonian acid volcanics; and gold in Tertiary gravel and Holocene alluvium.

Tags: AU-ACT, AU-NSW, Bulletin, Earth Sciences, GA Publication, Published_External, geochemistry, geology
Organization: Geoscience Australia Data