VOLCANIC, SEDIMENTARY AND TECTONOSTRATIGRAPHIC ENVIRONMENTS OF THE SIMILAR-TO-3.46 GA WARRAWOONA MEGASEQUENCE - A REVIEW

The Warrawoona Megasequence comprises a approximately 3.46 Ga assemblage of tholeiitic and calc-alkaline volcanic rocks interlayered with cherty sedimentary rocks. Deposition occurred in a range of shallow- to deeper-water environments ranging from the shoreline to distal deposits in sediment starved basins. Volcanism provided the dominant, or only, source of clastic sediment. Sedimentary rocks also contain evaporites, probable stromatolites, other evidence for the existence of microbial life and possible evidence of meteorite impact. The tectonostratigraphic assemblage and its contained facies associations are most similar to those developed in younger volcanic-arc or near-arc settings. The compositions of volcanic rocks are also comparable with those of younger tholeiitic and calc-alkaline suites developed above subduction zones. The calc-alkaline suite has a complex petrogenesis which probably involves mantle-derived basaltic melts, intermediate to silicic melts derived from subducted mafic crust, magma mixing and fractional crystallization. Intense hydrothermal alteration and a distinctive association of metal deposits are also compatible with the interpretation that the Warrawoona Megasequence contains examples of early Archaean volcanic-arc and near-arc assemblages.