TECTONIC AFFINITY OF NISUTLIN AND ANVIL ASSEMBLAGE STRATA FROM THE TESLIN TECTONIC ZONE, NORTHERN CANADIAN CORDILLERA - CONSTRAINTS FROM NEODYMIUM ISOTOPE AND GEOCHEMICAL EVIDENCE

We present geochemical and isotopic data for Nisutlin assemblage metas edimentary rocks and Anvil assemblage greenstones from the Teslin tect onic zone of the northern Canadian Cordillera. This study aims to esta blish the tectonic setting of formation for the sedimentary and basalt ic protoliths of these highly deformed and metamorphosed rocks and the reby place constraints on the origin of these enigmatic rocks for whic h differing tectonic models have been proposed. For the Nisutlin assem blage metasedimentary rocks, the geochemical and isotopic data show th at two widely different source regions' contributed detritus to the or iginal sediments. One source region was felsic, upper crustal material with Nd isotopic compositions compatible with ultimate derivation fro m the North American continent (depleted mantle model age (T-DM) 2.5-2 .8 Gal. 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 a bundances are dissimilar to within-plate, oceanic are and normal mid-o cean ridge basalts and;similar to calc-alkaline basalts in active cont inental margin settings. We interpret the paleosetting for the Nisutli n assemblage to be at the outermost margin of the ancestral North Amer ican continent, in an area which received detrital input from the dist al North American craton in the early Paleozoic. However, this area al so received detritus from a chemically and isotopically juvenile magma tic are source, a source type ndt known from Paleozoic metasedimentary rocks from the miogeoclinal sequence. On the basis of similar Nd isot opic relationships recorded elsewhere in deformed Paleozoic rocks of t he orogen, we infer that these geochemical signals reflect tectonic pr ocesses of regional extent. The trace element geochemistry of the Anvi l assemblage greenstones does not support a correlation with known Pal eozoic greenstones of the Slide Mountain terrane, which some tectonic models have advocated.