Abstract

The Coast Mountains orogen, which lies along coastal British Columbia and southeastern Alaska, records extensive Mesozoic and Cenozoic igneous activity. Here we focus on the two youngest periods: 105–50 Ma and 30 Ma – present. The history of subduction, orogen uplift, and, finally, extension associated with this magmatic activity can be related to plate motion since the mid-Cretaceous. The 105–50 Ma period is related to terrane accretion and subduction. The northwest–southeast-trending Coast shear zone divides the resulting Coast Mountains calc-alkaline continental margin batholith into a western (105–90 Ma) and an eastern (80–50 Ma) arc. Melting of hydrous mantle overlying a dehydrating slab generated the plutons of the western arc. The plutons of the eastern arc show a wider range of compositions. Their origin consists of mantle melts modified by melts from lower crustal rocks of continental affinity and possibly amphibolitic hydrated basalt. In both parts of the arc, igneous bodies also resulted from crustal melting; these are very abundant in the eastern arc. During the younger period bi-modal igneous suites consisting of mantle-derived mafic magmas and coeval crustal melts are associated with crustal extension and block faulting.

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