CONSTRAINING RELATIVE POSITIONS OF ISLAND ARC TERRANES IN THE CANADIAN CORDILLERA USING CONODONT OXYGEN ISOTOPE THERMOMETRY

The Canadian Cordillera is a composite of crustal terranes with various stratigraphic and tectonic histories. The relationships between these terranes, and their relative positions throughout the late Paleozoic and Mesozoic, are an ongoing field of research. In British Columbia and southern Yukon, the oceanic Cache Creek terrane is structurally juxtaposed between the Stikine and Quesnel island arc terranes; the exotic Wrangell island arc terrane lies further to the west. Multivariate statistical analysis of Late Triassic (late Carnian – early Norian) conodont faunas from the Stikine and Quesnel terranes and the North American margin has suggested that the terranes were likely situated to the south of their present location prior to accretion to the North American margin in the Early – Middle Jurassic. An independent test of this hypothesis can be provided by oxygen isotope analysis of conodont phosphate. Oxygen isotope ratios are primarily controlled by sea-surface temperature and salinity, therefore analysis of the oxygen isotope ratios preserved in Late Triassic (early Norian) conodonts of the Quesnel, Stikine and Wrangell terranes, as well as the northern part of the North American margin, may allow relative temperatures of each of these tectonic entities to be determined for this time interval. Previous studies of the oxygen isotope ratios of conodonts have suggested that the Wrangell terrane was hotter than the North American margin during the early Norian. The present study expands on these preliminary results by extending the geographic range of the dataset. In addition to confirming higher temperatures for the Wrangell terrane, the δ¹⁸O ratios of early Norian elements of the conodont genus Ancyrogondolella from both the Stikine and Quesnel terranes are notably lower than those of the same genus from the North American margin. If salinity was consistent across the tectonic entities, then this may imply that all both Stikine and Quesnel were also hotter than the North American margin in the early Norian, and were either located to the south of their present location, similarly to Wrangell, or oceanic currents were bringing warm water north at this time. A southern location for the terranes would be consistent with the results of the previous multivariate statistical analysis.