Eocene Challis-Kamloops volcanism in central British Columbia: an example from the Buck Creek basin

Eocene volcanic rocks of the Buck Creek basin in central British Columbia a re part of the Challis-Kamloops volcanic belt extending from the United Sta tes across British Columbia to central Yukon. The volcanic rocks include tw o units, the Buck Creek Formation, composed of high-K calc-alkaline rocks w ith predominant andesitic composition, and the overlying Swans Lake unit ma de up of intraplate tholeiitic basalts. Whole rock Ar-40/Ar-39 data for bot h units show that they were emplaced at 50 Ma. They have similar mantle-nor malized trace element patterns characterized by a large-ion lithophile elem ent enrichment and Nb-Ta depletion, similar chondrite-normalized rare earth element patterns with (La/Yb)(n) approximate to 4-14 and heavy rare earth element fractionation, and overlapping epsilon(Nd) values (2.4-3.1) and ini tial Sr-isotope ratios (similar to 0.704). These features suggest derivatio n of these two units from a similar mantle source, probably garnet-bearing subcontinental lithosphere. The differences between tholeiitic and calc-alk aline suites can be due, in part, to differences in the depth of fractional crystallization and the crystallizing mineral assemblage. Fractional cryst allization of the calc-alkaline magmas began at a greater (mid-crustal) dep th and included fractionation of Fe-Ti oxides. The volcanic rocks are proba bly related to subduction of the Farallon plate under the North American co ntinent in a regime characterized by transcurrent movements and strike-slip faulting.