Geochemical and Nd-Pb-O isotope systematics of granites from the Taltson Magmatic Zone, NE Alberta: implications for early Proterozoic tectonics in western Laurentia

Abstract

The early Proterozoic Taltson Magmatic Zone (TMZ) comprises the southern part of the Taltson-Thelon orogenic belt of northwestern Canada. The TMZ is dominated by 1.96–1.99-Ga I-type and 1.93–1.95-Ga S-type suites of granitoids. Previous workers attributed the formation of the I-type granitoids to subduction of oceanic crust beneath the Churchill craton in an Andean-type setting, and generation of the S-type granitoids to subsequent collision between the Churchill craton and the Buffalo Head Terrane. To test this hypothesis, we carried out a geochemical and isotopic (Nd, Pb and O) study of granitoids and associated metasedimentary rocks and basement gneisses from the southern part of the TMZ. Both I- and S-type suites of granitoids are characterized by relatively high SiO 2 content, with the great majority of samples containing more than 64 wt.% SiO 2. The two suites also have strongly crustal εNd (T) values of −3.8 to −9.8 and an average T DM of 2.8 Ga. The initial Pb isotope composition of the granitoids as determined from leached feldspar mineral separates forms a linear array that is bracketed by the isotopic compositions of basement orthogneisses and metasediments. Similarly, δ 18O values of the granitoids overlap those of the orthogneisses and metasediments. These isotopic data are consistent with derivation of the granitoids from a mixture of metaigneous and metasedimentary source rocks similar to those presently exposed in the TMZ. Importantly, the data suggest that both I- and S-type granitoids of the southern TMZ had an exclusively intra-crustal origin unlike Phanerozoic examples of subduction-related magmatism where a significant mantle contribution is easily recognized. Therefore, we argue that the existing Andean-type model for early TMZ magmatism is not viable. The TMZ granitoids show geochemical and isotopic similarities to the Cordilleran interior granitoids of western North America, which formed in response to crustal thickening in the distant hinterland of a convergent plate margin. We propose that these hinterland granitoids are a more appropriate analogue for TMZ magmatism than are the granitoids of plate margins. The tectonic implication of this proposal is that the Taltson-Thelon orogenic belt does not mark the location of the plate boundary in western Laurentia at 1.9–2.0 Ga. Rather, the tectonic setting of the TMZ at that time may have been more akin to the present-day intra-continental mountain belts of central Asia (e.g. Tian Shan Mountains) where compressional tectonism and crustal thickening are occurring well inboard from a convergent plate margin.