Adakitic volcanism in Southern BC during the early Eocene: Isotopic and geochemical constraints from the Princeton Group

The Princeton Group is an assemblage of volcanic and clastic sedimentary rocks in south-central British Columbia, and is part of the Challis- Kamloops belt that stretches from central British Columbia to the northwestern USA. Volcanics were deposited largely as composite-volcanoes, and are composed of calc-alkaline basaltic-andesite, andesite, dacite, and rhyolite. 4 0 ~ r / 3 9 ~ r dating indicates that magrnatism occurred during the Early to Middle- Eocene, from 53-47 Ma. 143~d/144~d measurements indicate that the f3G is primarily juvenile, with EN^^^ = 1.2-6.4. Princeton Group rocks geochemically resemble those of many rnodern continental arcs and have an 'adakitic' signature that extends throughout their compositional range. This signature is not derived from melting of subducted oceanic crust, but from an already enriched 'arc-like' source, hypothesized to be mafic dykes emplaced into the lithospheric mantle during Mesozoic magmatism. These dykes subsequently melted during lithospheric heating in the Eocene, probably caused by upwelling asthenosphere related to a slab-window or slabtear.