Eocene volcanism in the Buck Creek basin, central British Columbia (Canada): transition from arc to extensional volcanism

Abstract

The Eocene Buck Creek volcanic complex (central British Columbia, Canada) is a part of the Challis–Kamloops volcanic province that extends from Yukon across British Columbia to the northern United States, running approximately parallel to the margin of the North American continent. The complex is associated with the formation of a fault-bounded basin controlled by strike–slip faults. The volcanic rocks were emplaced during two short-lived (<1–2My) periods. The first 52Ma old period produced high-K calc-alkaline mafic and intermediate lavas and related subvolcanic intrusions of the Goosly Lake Formation. Their age coincides with the age of two silver deposits, which are related to the intrusions. The second period which took place 50±1Ma ago, produced high-K calc-alkaline mafic and intermediate lavas (Buck Creek Formation), and overlying intraplate tholeiitic to high-K calc-alkaline basalts (Swans Lake unit). The rocks of the three units have similar Sr and Nd isotopic ratios and mantle-normalized trace element patterns characterized by a depletion of Nb, Ta and Ti suggesting a similar mantle source, garnet-bearing subcontinental lithosphere, for the lavas of all three units. The average contents of SiO2 and strongly incompatible trace elements in the lavas decrease from the Goosly Lake rocks towards the top of the complex (Swans Lake unit). The compositional changes could be, in part, related to an increase of the degree of partial melting towards the top. Volcanic rocks of similar composition and age are widespread in this part of the Eocene Challis–Kamloops belt in central British Columbia. The magmatic activities are related to subduction of the Farallon/Kula plate under the North American continent in a regime characterized by strike–slip faulting and oblique plate movement along the continental margin of western Canada. It is inferred that oblique plate convergence decoupled into a component of convergence normal to the plate margin, and a component of strike–slip faulting approximately parallel to the margin within the overriding North American plate. These events triggered a melting in the mantle wedge overlying the subduction zone and generated the parent magmas for the rocks of this part of the Challis–Kamloops belt.