Early Paleozoic ophiolite complexes of the Newfoundland Appalachians as mantle-oceanic crust sequences

Abstract

The layered ultramafic-mafic complexes of the Northwest platform and the Fleur de Lys orthotectonic zones of the Newfoundland Appalachians are comparable to layered Tethyan ophiolite suites of the Piedmont Alps, the Dinarides, and the Oman. They can be interpreted as oceanic crust and mantle: the peridotites, with high-pressure mineral assemblages, kaersutite, and titaniferous phlogopite, represent the upper mantle; the overlying gabbros and ‘sheeted’ diabases represent layer 3 of the oceanic crust; and pillow lava, chert, and the greywacke unit represent layers 2 and 1. As oceanic lithosphere the ophiolites may represent an intrasialic small ocean basin rather than a major ocean basin such as the Atlantic or Pacific. The early Ordovician thrust emplacement of the ophiolite sheets followed tectonism and metamorphism of a thick sequence of late Proterozoic-Cambrian clastic sediments (Fleur de Lys supergroup) that was deposited as a marginal wedge during the early stages of the development of the Appalachian geosyncline within a composite ‘North American-European’ continental plate. Their emplacement may reflect an early phase in the closing of an Appalachian ocean basin by underthrusting of the North American plate along a southeasterly (present geographic coordinates) dipping Benioff zone. Final closing of the ocean basin during the late Ordovician was effected by subduction of oceanic crust, also along a southeasterly dipping Benioff zone, during the northwest drift of the European plate. Alternatively, the ophiolites may have been emplaced directly onto the Fleur de Lys rocks either (1) at the time of inception of an ocean-forming ridge within a geosyncline that was initiated on continental sialic basement, or (2) during an ocean-closing phase of drift while a ridge was positioned relatively close to the continental margin, or was newly developing within the margin. The Newfoundland ophiolites may represent the oldest known oceanic lithosphere within the North American continent.