U-PB DETRITAL ZIRCON GEOCHRONOLOGY OF AVALONIA: CONSTRAINTS ON THE OPENING OF THE RHEIC OCEAN

The Rheic Ocean is a first-order, although poorly understood, global feature that persisted for most of the Paleozoic. It formed in the wake of crustal blocks rifted from the Amazonian-West African margin of Gondwana and it grew as the early Paleozoic Iapetus Ocean was closing. Despite its prominence in the Paleozoic, our understanding of such a fundamental aspect of the Rheic Ocean as its timing of opening is poorly constrained. Avalonia is the largest peri-Gondwanan crustal block in the Appalachian-Caledonian orogen, and it originated by rifting from Gondwana and thereafter constituted a portion of the leading edge of the Rheic Ocean; consequently it should record the timing of opening of the Rheic Ocean.

Avalonia comprises Neoproterozoic-early Palaeozoic magmatic arc terranes that extend from eastern Massachusetts to the type area in eastern Newfoundland. Most of Avalonia is at low metamorphic grade, generally mildly deformed, and consists of volcanic, plutonic and sedimentary rocks that record at least five distinct groupings of pre-Iapetan tectonomagmatic and depositional events at ca. 760 and 730 Ma, 685-670 Ma, 635-590 Ma, and 590-545 Ma. The Neoproterozoic rocks are overlain by a terminal Neoproterozoic-Early Ordovician cover of fine-grained siliciclastic rocks containing Acado-Baltic faunas.

U-Pb geochronology on detrital zircons from seven samples from the major lithotectonic elements in Avalonia was completed in order to address multiple first-order questions concerning the evolution of both Avalonia and the Rheic Ocean. The data from Neoproterozoic-Early Cambrian strata (Conception, Musgravetown, and Signal Hill groups and Random Formation) are dominated by Ediacaran (c. 580 Ma) ages and suggest derivation from the underlying arc-volcanic sequences. Although the Arenig Bell Island Group comprises mostly Ediacaran and Cambrian zircons, it also contains significant quantities of Mesoproterozoic and Palaeoproterozic zircons. This change in provenance may be related to the rifting and isolation of Avalonia from Gondwana in the Early Ordovician.