North American margin origin of Quesnel terrane strata in the southern Canadian Cordillera: Inferences from geochemical and Nd isotopic characteristics of Triassic metasedimentary rocks

摘要

New geochemical and Nd isotopic data from Triassic (ca. 220 Ma) clastic metasedimentary rocks assigned to the Quesnel terrane (Quesnellia) in southeastern British Columbia show that detritus was derived from both primitive and evolved sources. Twenty-four samples yield Nd₂₂₀ values between–9.1 and +5.6, independent of sample location. These samples have Eu anomalies between 0.6 and 1.0, La_N/Yb_N ratios between 3.8 and 11.4, La/Sc ratios ranging from 0.38 to 4.76, and Th/Sc ratios between 0.09 and 1.62. This range of isotopic and geochemical values overlaps that of samples from upper Paleozoic to Mesozoic rocks of the North American miogeocline and Upper Triassic Nicola Group volcanic rocks of the Quesnel terrane. The range in Nd isotopic and geochemical values supports the interpretation that detritus was derived from both continental North America (evolved cratonic source) and a volcanic arc (primitive source) developing on the continental margin. These data do not support previous interpretations of the Quesnel terrane as an intraoceanic arc. The inferred depositional link between North America and the Quesnel arc in Triassic time is ca. 40 Ma older than the hypothesized obduction of the Quesnel terrane onto the North American continental margin and, when combined with existing geological information, strongly supports a pericratonic, nonexotic origin for the Quesnel terrane in southeastern British Columbia.