Magmatic evolution of the eastern Coast Plutonic Complex, Bella Coola region, west-central British Columbia

The eastern Coast Plutonic Complex in west-central British Columbia (51-54 degrees N) consists of a broad belt of Jurassic to Eocene granitic to dioritic intrusive rocks that collectively represent more than 140 m.y. of nearly continuous, subduction-related magmatism. Seven distinct intrusive suites have been identified in the Bella Coola region, including, from oldest to youngest, the Howe Lake, Stick Pass, Firvale, Desire, Fougner, Big Snow, and Four Mile suites. Pre-Late Cretaceous rocks were produced by episodic subduction-related magmatism characterized by partial melting of preexisting lower-arc crust, with minimal incorporation of evolved continental material. The eastern Coast Plutonic Complex experienced a magmatic lull between ca. 110 and 95 Ma, coincident with sinistral transpression potentially linked to translational displacement of the Wrangellia-Alexander composite terranes. Crustal thickening in Late Cretaceous time (ca. 100-90 Ma), possibly due to underplating of the Wrangellia-Alexander terrane, produced a deep crustal root (> 40 km) that likely extended into the eclogite transition zone. Adakitic magmas (ca. 95-67 Ma) were produced by partial melting of an amphibolitic protolith in this thickened lower-arc crust. Adakitic plutons are pre-, syn-, and postkinematic with respect to Late Cretaceous high-angle shear zones, and they are coeval and spatially associated with postkinematic two-mica granites (ca. 70-75 Ma) having a hybrid A-type geochemistry. We suggest that delamination of this dense crustal root led to voluminous magmatism, extension, and crustal exhumation in Paleocene-Eocene time. The successive intrusive events in the eastern Coast Plutonic Complex may reflect a predictable evolutionary progression common to all continental arc systems.