ABSTRACT
The Buck Creek volcanic complex of the Intermontane Superterrane of the Canadian Cordillera records a long history of volcanic activity from the Cretaceous through to the Eocene, when mag-matic activity peaked, to the Miocene. Its basement includes primitive continental arc volcanic rocks of the Jurassic Hazelton Group (~200 Ma; mainly basalt and andesites) emplaced prior to the accretion of Stikinia during the Mesozoic. In contrast, in the post-accretionary volcanic complex, the major pulse of volcanism started with the extrusion of continental margin calc-alkaline rocks (basaltic andesites to rhyolites) of the Cretaceous Tip Top Hill Formation (~85 Ma). Their Nd-Sr isotopic compositions resemble those of the pre-accretionary Hazelton Group. However, this post-accretionary Cretaceous arc was more evolved and its crust was significantly thicker than the Hazelton. Mafic rocks of both the Hazelton and Tip Top Hill suites were generated from a spinel-bearing mantle source.
The Eocene volcanics (~50 Ma) evolved from typical high-K calc-alkaline mafic/intermediate rocks to flows that resemble intraplate tholeiitic basalts. Collectively, the Eocene rocks had a common (and comparatively deep) source, likely garnet-bearing subcontinental lithosphere. These compositional variations are probably related, in part, to an increase in the degree of partial melting. They record a gradual change from a compressional to an extensional tectonic environment, and overall represent an arc setting that matured and thickened over ca. 35 m.y., culminating in Eocene extension coincident with the cessation of compression in the foreland fold-and-thrust belt. overlying intraplate alkali basaltic rocks of Miocene age appear to represent the final stage of this transition, as subduction-modified lithospheric mantle was replaced by a new asthenospheric mantle source. The association of Cretaceous, Eocene, and Miocene volcanic rocks in the Buck Creek complex is widespread in the region, suggesting that this model of tectono-magmatic evolution can be applied elsewhere in central British Columbia.
