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 thatextends from Yukon across British Columbia to the northern UnitedStates, running approximately parallel to the margin of the NorthAmerican continent. The complex is associated with the formation of afault-bounded basin controlled by strike-slip faults. The volcanicrocks were emplaced during two short-lived (<1-2 My) periods. Thefirst 52 Ma old period produced high-K calc-alkaline mafic andintermediate lavas and related subvolcanic intrusions of the GooslyLake Formation. Their age coincides with the age of two silverdeposits, which are related to the intrusions. The second period whichtook place 50+ or -1 Ma ago, produced high-K calc-alkaline mafic andintermediate lavas (Buck Creek Formation), and overlying intraplatetholeiitic to high-K calc-alkaline basalts (Swans Lake unit). Therocks of the three units have similar Sr and Nd isotopic ratios andmantle-normalized trace element patterns characterized by a depletionof Nb, Ta and Ti suggesting a similar mantle source, garnet-bearingsubcontinental lithosphere, for the lavas of all three units. Theaverage contents of SiO (sub 2) and strongly incompatible traceelements in the lavas decrease from the Goosly Lake rocks towards thetop of the complex (Swans Lake unit). The compositional changes couldbe, in part, related to an increase of the degree of partial meltingtowards the top. Volcanic rocks of similar composition and age arewidespread in this part of the Eocene Challis-Kamloops belt in centralBritish Columbia. The magmatic activities are related to subduction ofthe Farallon/Kula plate under the North American continent in a regimecharacterized by strike-slip faulting and oblique plate movement alongthe continental margin of western Canada. It is inferred that obliqueplate convergence decoupled into a component of convergence normal tothe plate margin, and a component of strike-slip faultingapproximately parallel to the margin within the overriding NorthAmerican plate. These events triggered a melting in the mantle wedgeoverlying the subduction zone and generated the parent magmas for therocks of this part of the Challis-Kamloops belt.

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