A peri-Gondwanan arc-back arc complex in iapetus: Early-mid Ordovician evolution of the Exploits Group, Newfoundland

Abstract

The early-middle Ordovician Exploits Group, well-preserved in the westernmost Exploits Subzone of the northern Newfoundland Dunnage Zone, accumulated in a peri-Gondwanan intraoceanic basin. The depositional, early diagenetic, chronostratigraphic, and variable petrochemical features of this group are interpreted to record the ∼25 Ma development of a primitive island arc-backarc complex. Evolution of this southern Iapetan arc took place, without significant interruption, seaward of the convergent Ordovician margin of Avalonia. The Exploits Group spans the late Tremadoc to early Caradoc, although most rocks are Arenig and Llanvirn in age. New chronostratigraphic controls provided by U/Pb isotopic dating and by graptolite and conodont biostratigraphy demonstrate a protracted history of intermittent eruptive and intrusive magmatism at ∼486, 470, and 464 Ma. These were separated by periods of dominantly deep-marine volcaniclastic and hemipelagic sedimentation in the early-late Arenig and late Arenig-Llandeilo, respectively. All the ductile phases of regional deformation observed in the Exploits Group postdated early and mid Ordovician deposition. Well-developed chemostratigraphic associations in the ∼486 Ma Tea Arm Formation document a progression from arc-tholeiite pillow lavas with mafic dike swarms to high-silica sodic rhyolites and LREE-depleted basalts to low-K calc-alkalic tholeiites and, finally, to refractory andesites, all controlled by subduction-related processes. In contrast, the subalkaline to alkaline, LREE-enriched pillow lavas of the ∼70 Ma Lawrence Head Formation represent within-plate flows with no influence of subduction that are locally fed by diabase dikes. Laccoliths of ∼464 Ma Thwart Island gabbro intrude Llanvirn and Arenig formations of the Exploits Group and suggest a return to active subduction in the early Llandeilo. The depositional architecture and bathometric relief of the Exploits back arc is thought to have controlled lithofacies and to reflect the presence of synrift horsts and grabens. The Llanvirn resedimentation of Arenig strata from within the basin implies back-arc reactivation of intra-arc block faults. The paleotectonic history of the Exploits Group and its position within the Exploits Subzone of the Dunnage Zone demonstrate fundamental spatial and temporal variations during Ordovician evolution of the southern margin of the Iapetus Ocean.