JURASSIC – TERTIARY EVOLUTION OF THE CENTRAL COAST PLUTONIC COMPLEX: CONTROLS ON OVER 120 MILLION YEARS OF MAGMATISM

The magmatic evolution of the Bella Coola region (51-54°N) provides insight into temporal and spatial variations in igneous petrogenesis within a long-lived magmatic arc. Analysis of pluton lithology, petrography, field relationships, geochemistry, geochronology and isotopic geology constrain the origin and evolution of arc magmas in the western Canadian Cordillera, and permit an analysis of the role that regional tectonism and changing plate configuration play in arc magmatism.

Magmatic evolution is characterized by several plutonic suites, including: 1) Howe Lake suite (ca. 189-183 Ma), heterogeneous, foliated diorites to tonalites subvolcanic to the Jurassic Hazelton Group; 2) Firvale-Stick Pass suites (ca. 133-149 Ma), pervasively altered granitic rocks with abundant metavolcanic screens and basaltic dykes; 3) Desire suite (ca. 123-110 Ma), a texturally and compositionally diverse assemblage of foliated diorite to tonalite partly coeval with Albian volcanic rocks; 4) Big Snow suite (ca. 87 Ma) a coarse-grained, fresh biotite tonalite to granodiorite; 5) Fougner suite (ca. 67-76 Ma), a distinct salt and pepper, sphene-bearing diorite to granodiorite; and, 6) Four Mile suite (ca. 72 Ma), a homogeneous, coarse-grained, garnet-bearing muscovite-biotite granite. Cross-cutting relationships among the younger suites are complex; magmatism was pre-, syn- and post-kinematic with respect to the Paleocene Coast Shear zone.

Temporal and spatial variations between the magmatic suites correspond remarkably to regional tectonic events. The Howe Lake suite represents subduction-related uncontaminated island arc magmatism. The Firvale-Stick Pass suites were emplaced during regional transpression, evidenced by a strong structural control on magmatism, rapid post-emplacement uplift and regional basin development. The Desire suite corresponds to a return to orthogonal subduction-related arc magmatism. Over 50% of plutonic rocks were emplaced during protracted Late Cretaceous to Paleocene magmatism, producing a diverse assemblage of locally high-T adakitic melts and distinct A-type granitoids. The shift from pre-Late Cretaceous essentially subduction-related magmatism to more complex, multiple source magmatism appears related to asthenospheric upwelling following orogenic collapse associated with initiation of transpression in Late Cretaceous-Paleogene time.