U-PB DETRITAL ZIRCON GEOCHRONOLOGY OF THE AVALON ZONE, NEWFOUNDLAND APPALACHIANS: IMPLICATIONS FOR THE OPENING AND EVOLUTION OF THE RHEIC OCEAN

The Rheic Ocean is a first-order, although poorly understood, global feature that persisted for most of the Palaeozoic. It formed in the wake of crustal blocks rifted from the Amazonian-West African margin of Gondwana and it grew as the early Palaeozoic Iapetus Ocean was closing. Despite its prominence in the Palaeozoic, our understanding of such fundamental aspects of the Rheic Ocean as its time of opening, its mode of opening - as either a single extensive rift or an amalgamation of multiple marginal basins - and its geometry with respect to Palaeozoic continental elements are poorly constrained. The largest Appalachian peri-Gondwanan crustal block, the Avalon Zone, is located on the northeastern edge of the Appalachian Orogen. It originated by rifting from Gondwana and thereafter constituted a portion of the leading edge of the Rheic Ocean; consequently it should record fundamental aspects of Rheic evolution.

The Avalon Zone comprises Neoproterozoic-early Palaeozoic magmatic arc terranes that extend from eastern Massachusetts to its type area in eastern Newfoundland. Most of the zone is at low metamorphic grade, generally mildly deformed, and consists of late Neoproterozoic to early Palaeozoic volcanic, clastic sedimentary, and plutonic rocks. These rocks record at least four distinct groupings of pre-Iapetan tectonomagmatic and depositional events at 760 Ma, 685-670 Ma, 635-590 Ma and 590-545 Ma that are overlain by a terminal Neoproterozoic-Early Ordovician cover of fine-grained siliciclastic rocks containing Acado-Baltic faunas.

It is unclear whether the Avalon Zone formed as an independent single crustal block or as part of a larger single peri-Gondwanan microcontinent and its source area(s) in Gondwana is poorly constrained. The resolution of these uncertainties bears directly upon our understanding of the timing, nature of opening, and the geometry of the Rheic Ocean. In order to address these critical uncertainties, we conducted U-Pb geochronology on detrital zircons from seven samples from the major lithotectonic elements in the Avalon Zone of Newfoundland.