105 MILLION YEARS OF IGNEOUS ACTIVITY, WRANGELL, AK TO PRINCE RUPERT, BC

The pioneering and energetic work of Maria Luisa Crawford has significantly improved our knowledge of the magmatic, metamorphic, and tectonic development of the coastal British Columbia-southeastern Alaska Coast Mountains orogen. She is exceptional in her efforts to introduce students and colleagues to this complex terrane and cultivate new working relationships to advance understanding of terrane accretion dynamics. Below is a summary of the authors' interpretation of the tectonomagmatic history of the region based on almost two decades of collaboration with this extraordinary geologist.

The Coast Mountains orogen records extensive Mesozoic and Cenozoic igneous activity that can be divided into two distinct episodes: Mid-Cretaceous through Eocene (~105-50 Ma) and Oligocene (~30 Ma) to Recent. Magmatic activity can be related to plate motion and associated subduction, orogen uplift, and extension since the mid-Cretaceous. The 105-50 Ma period is linked to terrane accretion and subduction during normal convergent plate motion which resulted in formation of an eastward-younging continental calc-alkaline arc. The NW-SE trending Coast Shear Zone divides the 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. They originated from 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 much more abundant in the eastern arc. Igneous activity ceased ~50 Ma and the batholith was uplifted along steep shear zones concomitant with plate reorganization. After 50 Ma, motion along the continental plate boundary became translational. Magmatism resumed at ~30 Ma, peaked in volume ~20 Ma, and has continued to the present. This younger period is characterized by bi-modal igneous suites which formed from upper mantle-derived mafic magmas and coeval crustal melts associated with crustal extension. The youngest volcanic flows are alkaline in nature and formed from small degrees of partial melting of asthenospheric mantle related to continued lithospheric thinning.