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

Abstract

The Slide Mountain and Cache Creek terranes are two prominent oceanic sutures in the Canadian Cordillera. Petrological and isotopic variations between volcanic rocks in these terranes support earlier interpretations from stratigraphic evidence that the Slide Mountain terrane represents the remnant of a late Paleozoic basin situated marginal to western North America, whereas the Cache Creek terrane represents a remnant of a much larger, open-ocean basin. Slide Mountain terrane volcanic rocks, represented by Late Pennsylvanian basalts of the Fennell Formation, resemble normal mid-oceanic ridge basalts but possess an unusual kaersutite- or augite-dominated mineralogy. Their εNd(300 Ma) values of +7.7 to +10.2 are among the highest observed for Paleozoic basalts. The hydrous mineralogy can be reconciled with eruption on a spreading ridge in either a back-arc or marginal basin setting. The latter is preferred from Pb isotope compositions (206Pb/204Pb = 17.7–18.5, 207Pb/204Pb = 15.51–15.61, 208Pb/204Pb = 37.2–38.8), which suggest exchange with high Th/U continental-derived sediment during hydrothermal alteration. Volcanic rocks, probably middle Mississippian, in the Bonaparte subterrane of the Cache Creek terrane include picrites and basalts belonging to a within-plate tholeiite suite. The intraplate suite broadly resembles Hawaiian basalts in major and trace element composition. However, moderate positive εNd values (εNd(340 Ma) +4.2 to +5.6) and a transition toward DUPAL signatures in Pb isotopic composition (206Pb/204Pb = 18.1–19.1, 207Pb/204Pb = 15.54–15.61, 208Pb/204Pb = 37.8–38.6) are features more similar to volcanic rocks from modern South Pacific ocean islands. Basaltic andesite and andesitic tuffs, also found in the Bonaparte subterrane, are tentatively correlated with Late Triassic to Early Jurassic low-K tholeiitic volcanic rocks of the Nicola Group on the Quesnel terrane.