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: 10550 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 10550 Ma period is related to terrane accretion and subduction. The northwestsoutheast-trending Coast shear zone divides the resulting Coast Mountains calc-alkaline continental margin batholith into a western (10590 Ma) and an eastern (8050 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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