The geology of the Gawler Range Volcanics in the Toondulya Bluff area and U-Pb dating of the Yardea Dacite at Lake Acraman

Abstract

At Toondulya Bluff a sequence of 'older' Gawler Range Volcanics dip in an easterly direction beneath the overlying Yardea Dacite, and are intruded by the comagmatic Hiltaba Granite. The volcanics occur as a series of tuffs and lava flows. Geochemical evidence suggests these volcanics are related to each other by fractional crystallisation, with plagioclase, clinopyroxene, K-feldspar and titan-magnetite, and accessory zircon and apatite controlling differentiation trends. The Si-rich Hiltaba Granite and Yardea Dacite formed from the final, highly fractionated melts. Geothermometry suggests the volcanic and granite crystallised at temperatures within the range 680deg-850degC. The initial magma from which the lithologies were derived, was formed by partial melting of a lower crustal source probably of granulitic composition. Lake Acraman is believed to have been a site of meteoritic impact in the late Proterozoic (~600 Ma ago). Fragments of dacitic ejecta have been identified within the Bunyeroo Formation, Flinders Ranges and dating of these fragments gives an age of c.1575 Ma using single zircon ion probe dating techniques (Gostin et al in prep.). U/Pb dating of the Yardea Dacite at Lake Acraman reveals it to be of comparable age to these fragments (1603-1631 Ma). The lower intercept of the discordia line reveals there has been no resetting of the U/Pb system in response to the postulated meteoritic impact.