The Neoproterozoic-Early Paleozoic Tectonic Evolution of the Peri-Gondwanan Margin of the Appalachian orogen: An Integrated Geochronological, Geochemical and Isotopic Study from North Carolina and Newfoundland

Abstract

LAM-ICP-MS U-Pb dates are reported from more than 1000 detrital zircons from the Neoproterozoic-early Paleozoic clastic sequences of Carolinia and Avalonia. The majority of analyzed zircon grains from Carolinia are late Neoproterozoic with minor amounts of Mesoproterozoic⁄Paleozoic and accessory Archean grains. The overall distribution of age populations of detrital zircons is consistent with derivation from the Amazonian craton and its peripheral orogenic belts on the margin of west Gondwana. The dominance of Ediacaran-early Paleozoic zircons in the Albemarle sequence suggests an underlying local protosource and deposition occurring synchronously with arc magmatism.

Neoproterozoic rocks of the type area of Avalonia are dominated by Ediacaran zircons. Early Paleozoic platform units contain an abundance of grains that have ages between 1.0-1.6 Ga, with lesser Paleoproterozoic and Archean zircons. These ages are inconsistent with recycling from underlying Neoproterozoic rocks and indicate that Avalonia's provenance is a Gondwanan craton that contains Archean, Paleo-, Meso- and Neoproterozoic rocks. These data are inconsistent with a West African provenance and suggest that Avalonia originated along the margin of the Amazonian craton. The prominent change in provenance is interpreted to be related to separation of Avalonia from Gondwana during the Early Ordovician opening of the Rheic Ocean.

In central North Carolina, mafic rocks of the Stony Mountain gabbro are characterized by variable LREE enrichment, prominent negative Nb anomalies and low Nb⁄Th ratios; features of tholeiitic basalts in modern island-arc, subduction related lavas. Nd isotope data indicate juvenile magmas consistent with derivation from lithospheric and asthenospheric sources during decompression melting of the mantle. The Stony Mountain gabbro can be modeled as the product of 10–15% hydrous partial melting of variable mixtures of MORB- and OIB-like mantle sources overprinted by a minor subducted-slab derived hydrous fluid component. By analogy with modern settings the Stony Mountain gabbro are comparable to MORB-like to OIB-type enriched rocks from the Lau Island and Sumisu Rift and are interpreted to have formed within an evolving early Paleozoic island arc–back arc rift-basin system. The presence of an Early Cambrian arc-back arc rift system in Carolinia is broadly coeval with arc-back arc volcanism in other peri-Gondwanan blocks of the Appalachians and may be related to the Early Paleozoic opening of the Rheic Ocean.