Tectonic affinity of Nisutlin and Anvil assemblage strata from the Teslin tectonic zone, northern Canadian Cordillera: Constraints from neodymium isotope and geochemical evidence

Abstract

We present geochemical and isotopic data for Nisutlin assemblage metasedimentary rocks and Anvil assemblage greenstones from the Teslin tectonic zone of the northern Canadian Cordillera. This study aims to establish the tectonic setting of formation for the sedimentary and basaltic protoliths of these highly deformed and metamorphosed rocks and thereby place constraints on the origin of these enigmatic rocks for which differing tectonic models have been proposed. For the Nisutlin assemblage metasedimentary rocks, the geochemical and isotopic data show that two widely different source regions contributed detritus to the original sediments. One source region was felsic, upper crustal material with Nd isotopic compositions compatible with ultimate derivation from the North American continent (depleted mantle model age (TDM) 2.5–2.8 Ga). The second source region is deduced to be chemically primitive crust (basaltic‐andesitic) with a short crustal residence history (<0.9 Ga). For the Anvil assemblage greenstones, immobile trace element abundances are dissimilar to within‐plate, oceanic arc and normal mid‐ocean ridge basalts and similar to calc‐alkaline basalts in active continental margin settings. We interpret the paleosetting for the Nisutlin assemblage to be at the outermost margin of the ancestral North American continent, in an area which received detrital input from the distal North American craton in the early Paleozoic. However, this area also received detritus from a chemically and isotopically juvenile magmatic arc source, a source type not known from Paleozoic metasedimentary rocks from the miogeoclinal sequence. On the basis of similar Nd isotopic relationships recorded elsewhere in deformed Paleozoic rocks of the orogen, we infer that these geochemical signals reflect tectonic processes of regional extent. The trace element geochemistry of the Anvil assemblage greenstones does not support a correclation with known Paleozoic greenstones of the Slide Mountain terrane, which some tectonic models have advocated.