Abstract

The 600-km-long Anahim volcanic belt of upper Miocene-Quaternary alkalic and peralkalic volcanic centers trends east-west along approximately lat 52°N in British Columbia, in contrast to the Miocene Pemberton volcanic belt of calc-alkalic centers and the Pliocene-Quaternary Garibaldi volcanic belt of calc-alkalic centers, which follow the northwest-trending continental margin. Anahim belt rock types range from alkali basalt and nephelinite, found as small cinder cones and flows, to oversaturated (and undersaturated) peralkalic varieties found in evolved central volcanoes and in their erosion-exposed roots. In contrast to the usual subduction-related calc-alkaline volcanism in the Pemberton and Garibaldi belts, volcanic activity in the Anahim belt has been linked with lithospheric fracturing above the northern edge of the subducted Juan de Fuca plate or interpreted as an edge effect of the subducted plate in the mantle. Available isotopic ages from the oldest centers in the Anahim belt become younger eastward at a rate of 2 to 3.3 cm/yr, suggesting that volcanic activity there may well be related to a mantle hot spot beneath British Columbia. Volcanic chemistry and isotopic composition do not distinguish between either a rift or a hot-spot setting.

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