PETROGENESIS AND PROVENANCE OF DISTAL VOLCANIC TUFFS IN THE PERMIAN-TRIASSIC KAROO BASIN, SOUTH AFRICA: A WINDOW INTO A DISSECTED, MAGMATIC PROVINCE

We present zircon rare-earth element (REE) compositions integrated with U‒Pb ages of zircon and whole rock geochemistry from 29 volcanic tuffs preserved in the Karoo Supergroup, South Africa, to investigate the history of magmatism in Southern Gondwana. Whole rock compositions suggest a subduction-driven, magmatic arc source for early (>270 Ma) to middle Permian (270-260 Ma) Karoo tuffs. After ~265 Ma, the magmatic source of the volcanic deposits transitioned toward intraplate, shallow-sourced (<40 km) magmatism. Zircon U‒Pb ages and REE chemistry suggest that early to middle Permian magmas were oxidizing, U- and HREE-enriched, melts, while middle Permian to Triassic zircon record HREE-depleted, more reduced magmatism. Middle Permian to Triassic tuffs contain increasingly large volumes of "zircon cargo" derived from assimilated crustal material; therefore magmas may have been zircon undersaturated, resulting in less zircon growth and increased inheritance in late Permian to Triassic, Gondwanan volcanics. Zircon U‒Pb ages and REE chemistry suggest a shift from arc magmatism in the early Permian to extensional magmatism by the late Permian, which may be associated with development of a backarc magmatic system adjacent to Western Antarctica that predates known extensional volcanism elsewhere in Gondwana. Opening of the Southern Ocean in the Jurassic-Cretaceous paralleled this extensional feature, which may be related to reactivation of this Permo-Triassic back arc. This study demonstrates the potential of zircon U‒Pb age and REE compositions from volcanic tuffs preserved in sedimentary strata to provide a more complete record of magmatism, when the magmatic province has been largely lost to active tectonism.