U-Pb ages and tectono-magmatic evolution of Middle Ordovician volcanic rocks of the wild bight group, Newfoundland Appalachians

摘要

The Wild Eight Group and spatially associated South Lake Igneous Complex form one of the Ordovician oceanic terranes of the central mobile belt of the Newfoundland Appalachians. An integrated study of these rocks, involving detailed mapping, geochemistry, Sm-Nd isotopic analyses and U-Pb geochronology, has shown that there are two temporally and genetically distinct volcanic sequences within the Wild Eight Group. The younger sequence comprises a lower volcanic succession associated with coarse volcaniclastic rocks and an upper volcanic succession interbedded with argillite, chert, and minor greywacke. The lower volcanic succession has calc-alkaline affinties, and isotopic evidence for minor crustal contamination. It is interpreted to represent a volcanic are formed in proximity to the Gondwanan margin, above an east-dipping subduction zone. The age of this volcanic sequence is confined to 472 +/- 3 Ma by felsic tuffs which occur stratigraphically above and below it. The upper volcanic unit has predominantly enriched tholeiitic to alkaline geochemical characteristics with isotopic signatures indicative of little or no crustal contamination, and is interpreted to represent are rifting. The age of this sequence was determined indirectly by dating two geochemically related gabbro sills (472(-9)(+2) Ma and 471 +/- 4 Ma). This work shows that despite different lithologies and stratigraphic and structural relationships between Early and Middle Ordovician sequences in the northern and southern Exploits Subzone, they have undergone essentially the same tectono-magmatic events. The age constraints on the magmatic events in the Wild Eight Group provide evidence for the timing of "obduction" of Early Ordovician oceanic sequences and the reversal of subduction polarity along the Gondwanan margin, suggested by previous workers. [References: 68]