Geochemistry, geochronology and tectonic implications of Late Silurian-Early Devonian volcanic successions, Central Lachlan Orogen, New South Wales

The Ural Volcanics and Mt Hope Volcanics in the Central Lachlan Orogen, New South Wales, overlie non-volcanic, sedimentary, below-wave-base submarine facies in two intracontinental rift basins, the Rast and Mt Hope Troughs. The Ural Volcanics and Mt Hope Volcanics consist primarily of felsic (rhyolite to dacite), coherent facies and associated felsic monomictic breccia facies. These volcanic facies are interpreted to represent submarine lava-sill complexes, which define intrabasinal, effusive, volcanic and shallow intrusive centres. Small felsic intrusions and rare intermediate (monzodiorite) to mafic (gabbro) dykes also occur. The rocks experienced chlorite to biotite greenschist-facies metamorphism. LA-ICPMS U/Pb zircon dating indicates the Ural Volcanics and the Mt Hope Volcanics were broadly contemporaneous, but with mean ages of 421 ± 2 and 411 ± 5 Ma, respectively, the Ural Volcanics are slightly older. Given the differences in the degree of metamorphism, the different ages might represent greater Pb loss for the Mt Hope Volcanics. Major- and trace-element data (variably high K+, Zr, Nb, and Y, among others) is most consistent with A-type, within-plate origins for the igneous rocks, although some loss of immobile elements apparently accompanied metamorphism. The geologic setting, based on mapping, indicates that the Ural Volcanics and Mt Hope Volcanics were erupted and deposited in an intrabasinal environment, best described as an elongate, intracontinental submarine rift. This setting, and apparent absence of a proximal contemporaneous arc, supports contentions that A-type magmas are produced in rift settings.