MAGMATIC EPISODICITY IN THE EASTERN COAST PLUTONIC COMPLEX, BELLA COOLA, BRITISH COLUMBIA

Field mapping, petrographic, geochemical, isotopic, and geochronologic studies demonstrate distinct magmatic episodicity in the eastern Coast Plutonic Complex of west-central British Columbia. The eastern Bella Coola region straddles a broad transitional boundary that separates crystalline rocks of the Coast Plutonic Complex from magmatic arc successions of the Stikine Terrane. Magmatic episodicity in this area is represented by five plutonic suites and their genetically associated volcanic carapaces, including, from oldest to youngest: 1) Howe Lake suite (ca. 189-183 Ma), a heterogeneous, foliated dioritic to tonalitic package that is subvolcanic to the Jurassic Hazelton Group into which it intrudes; 2) Firvale suite (ca. 133-149 Ma), characterized by pervasively altered granitic bodies containing abundant metavolcanic screens and basaltic dykes, is subvolcanic to volcano-sedimentary rocks of the Valanginian Monarch Assemblage; 3) Desire suite (ca. 123-110 Ma), a texturally and compositionally diverse assemblage of diorite to tonalite which is commonly foliated and contains abundant metavolcanic xenoliths, is partly coeval with Albian volcanic rocks. These three suites display trace element geochemical patterns characteristic of uncontaminated volcanic arc affinities; 4) Fougner suite (ca. 68 –47 Ma), a distinct salt and pepper, sphene-bearing diorite to granodiorite that is syn- to post-kinematic with respect to the Paleocene Coast Shear zone; and 5) Four Mile suite (ca. 87-51 Ma), a homogeneous, coarse-grained, garnet-bearing muscovite-biotite granite that clearly post-dates the Coast Shear Zone. The Four Mile suite displays a strongly spiked LILE pattern characteristic of continental volcanic arcs, which contrasts markedly with the largely coeval Fougner suite, which displays geochemical patterns consistent with an uncontaminated volcanic arc genesis. Volcanic equivalents of the Fougner and Four Mile suites have not been identified. Strongly contrasting petrologic and geochemical characteristics between coeval Tertiary plutons indicate a distinct change in magmatic style in Early Tertiary time. This change may represent a shift from simple subduction related magmatism to more complex, multiple source magmatism associated with the establishment of a transpressional regime in Late Cretaceous or Paleogene time.