ORIGINS OF MAGMATISM IN THE NORTHERN AND SOUTHERN CORDILLERAN VOLCANIC PROVINCES (NCVP AND SCVP) OF WESTERN CANADA

The Northern Cordilleran volcanic province (NCVP) is a Neogene to Recent petrotectonic province extending from the northernwestern half of British Columbia/southeastern Alaska into the Yukon Territory and the eastern part of central Alaska. Volumetrically the volcanism is dominated by alkali olivine basalt and hawaiite, but also locally has: i) nephelinite-basanite dikes and tephra cones and flows, and ii) peralkaline trachyte-rhyolite lava sequences and domes. The province includes isolated vents and lava fields as well as larger, long-lived volcanoes and volcanic complexes. Constraints on the origins of the magmatism derive from bulk rock chemistry, local diverse suites of xenoliths (derived from crust and mantle), and lithospheric scale structural elements. Trace element and REE geochemistry strongly support derivation of the alkaline basalts from an asthenospheric source, while the nephelinites may represent deep-seated (>50-60 km) mantle melts. The larger masses of evolved rock types (e.g. trachytes) likely result from AFC processes in within the lithospheric crust. While previous workers have shown that a slab window developed beneath the northern Cordillera after the end of Kula plate subduction (e.g. Thorkelson and Taylor, 1989; Thorkelson et al. 2011), we favor tectonic-scale extension/transtension as the driving force for magma production based on spatial-temporal discrepancies between slab window formation and the initiation of NCVP magmatism. Another series of petrologically similar zones of magmatism are located to the south and east of the NCVP, including the Anahim volcanic belt, the Chilcotin Group, the Cheslatta suite, and the Nazko Cone and Wells Gray-Clearwater volcanic fields. Similarities in overall geochemical compositions and the styles and timing of volcanism overlap strongly with the NVCP, and are consistent with a similar petrotectonic environment. We suggest that this southern area of magmatism may comprise a similarly coherent petrotectonic province formed by transtension, and we informally refer to all of these areas of magmatism as the Central Cordilleran volcanic province (CCVP).