RAIDERS OF THE LOST ARC: PRE-RODINIA COLLISIONAL EVENTS ALONG LAURENTIA’S WESTERN MARGIN ROB EASTERN AUSTRALIA OF ITS PALEOPROTEROZOIC MAGMATIC ARC AND ACCRETIONARY PRISM

Ocean basin closure followed by arc-continent and continent-continent collision has been invoked to explain late Paleo-early Mesoproterozoic orogenesis in proto-Australia and western and southern Laurentia, competing with earlier suggestions that these two continents were contiguous by 1800 Ma and grew southward along a common convergent margin. Evidence consistent with the former includes a clockwise pressure-temperature time path and Barrovian metamorphism commencing ca. 1650 Ma in Australian basinal sequences preserving an older record of 1800-1655 Ma crustal thinning, bimodal magmatism and backarc extension. Slab rollback and eastward migration of a west-dipping subduction zone are thought to have driven these processes, culminating in arc separation and formation of one or more marginal seas lying inboard of a more expansive ocean that began to close between Australia and Laurentia from 1720 Ma onward. Western Laurentia was then overridden by the arc and more easterly elements of the Australian backarc system, remnants of which still reside in western and southern North America (e.g. Mojave Province) and served as source rocks for sediment shed eastwards into a developing foreland basin (e.g. ≤ 1640 Ma Wernecke Basin). Further plate convergence led to complete collapse of the backarc basin, continent-continent collision and formation of a doubly verging orogenic belt by 1620-1580 Ma (Racklan, Isan, and Mazatzal). In keeping with this tectonic model, all three orogenic belts are floored by a common ≥ 1840 Ma crystalline basement and exhibit similar 1.9-1.59 Ma Nd model ages. Orogenesis was then superseded in Proterozoic Australia by extensional collapse and pluton-enhanced low pressure-high temperature metamorphism from 1540 Ma to 1490 Ma when the locus of lithospheric extension and bimodal magmatism shifted southward, heralding onset of a new tectonic regime and likely start of north-dipping subduction beneath now conjoined Australian and Laurentian continents.