ND, SR, AND PB ISOTOPIC EVIDENCE FOR CONTRASTING ORIGINS OF LATE PALEOZOIC VOLCANIC-ROCKS FROM THE SLIDE MOUNTAIN AND CACHE CREEK TERRANES,SOUTH-CENTRAL BRITISH-COLUMBIA

The Slide Mountain and Cache Creek terranes are two prominent oceanic sutures in the Canadian Cordillera. Petrological and isotopic variatio ns between volcanic rocks in these terranes support earlier interpreta tions from stratigraphic evidence that the Slide Mountain terrane repr esents the remnant of a late Paleozoic basin situated marginal to west ern North America, whereas the Cache Creek terrane represents a remnan t of a much larger, open-ocean basin. Slide Mountain terrane volcanic rocks, represented by Late Pennsylvanian basalts of the Fennell Format ion, resemble normal mid-oceanic ridge basalts but possess an unusual kaersutite- or augite-dominated mineralogy. Their epsilon(Nd(300) (Ma) ) values of + 7.7 to + 10.2 are among the highest observed for Paleozo ic basalts. The hydrous mineralogy can be reconciled with eruption on a spreading ridge in either a back-are or marginal basin setting. The latter is preferred from Pb isotope compositions (Pb-206/Pb-204 = 17.7 - 18.5, Pb-207/Pb-204 = 15.51 - 15.61, Pb-208/Pb-204 = 37.2 - 38.8), which suggest exchange with high Th/U continental-derived sediment dur ing hydrothermal alteration. Volcanic rocks, probably middle Mississip pian, in the Bonaparte subterrane of the Cache Creek terrane include p icrites and basalts belonging to a within-plate tholeiite suite. The i ntraplate suite broadly resembles Hawaiian basalts in major and trace element composition. However, moderate positive epsilon(Nd) values (ep silon(Nd(340) (Ma)) + 4.2 to + 5.6) and a transition toward DUPAL sign atures in Pb isotop ic composition (Pb-206/Pb-204 = 18.1 - 19.1, Pb-20 7/Pb-204 = 15.54 - 15.61, Pb-208/Pb-204 = 37.8 - 38.6) are features mo re similar to volcanic rocks from modern South Pacific ocean islands. Basaltic andesite and andesitic tuffs, also found in the Bonaparte sub terrane, are tentatively correlated with Late Triassic to Early Jurass ic low-K tholeiitic volcanic rocks of the Nicola Group on the Quesnel terrane.