THE INFLUENCE OF SLAB, RIDGE AND MANTLE ON LATE CENOZOIC ARC, ADAKITE AND INTRAPLATE MAGMATISM IN THE CANADIAN CORDILLERA

Interplay between subduction, mantle flow and crustal deformation have governed the Cenozoic magmatic evolution of the northern Cordillera. From the start of the Paleocene to the end of the Eocene, plate configurations in the region are uncertain. Oceanic plates to the west of North America included the Farallon and Kula, and possibly the Resurrection and Eshamy. These plates, and the ridges that separated them, were subducted beneath the western edge of North America, resulting in widespread arc magmatism. Magmatism related to slab windows occurred in the forearc and the southern interior. By the Oligocene, a more certain plate tectonic regime was underway, with only the Pacific and Juan de Fuca plates flanking North America. The Juan de Fuca ridge intersected the coastline somewhere from central Vancouver Island to Haida Gwaii, and a large slab window began to open. Concurrently, the tiny Anahim hotspot began to impinge on North America as the continent moved west. Remnants of the Kula, Resurrection and Eshamy plates became scattered microplates that either stagnated in place or fused to the northward-moving Pacific plate. Arc magmatism became restricted to mainly the North Cascade-Pemberton and Aleutian-Wrangell chains. The motion of the Pacific plate, relative to North America, was generally parallel to the coastline (along the dextral Queen Charlotte fault), with some intervals of modest convergence and divergence. By the end of the Oligocene, the hydrated mantle wedge that yielded Paleogene arc magmatism was largely displaced by dry asthenosphere that welled up through the slab window, leading to a broad Miocene to Holocene intraplate field flanked by the Wrangell and Garibaldi arcs. The Northern Cordilleran volcanic province occupied the northern part of the intraplate field, and the Anahim and Wells Gray chains and Chilcotin lava province dominated in the south. Ti/Mn and Nb/La ratios are high but variable throughout the intraplate field, and melts were derived from both the asthenospheric and lithospheric mantle. Anatexis of the slabs that flank the slab window yielded adakite volcanism in the Wrangell and Garibaldi arcs. The tip of the Juan de Fuca plate broke off to form the Explorer plate, which thermally decayed. Magmatism continued in glacial and post-glacial times in both the arc and intraplate domains.