Trace elements in shale as indicators of crustal provenance and terrane accretion in the southern Canadian Cordillera

Abstract

In this study, the trace-element geochemistry of shale is used to infer the nature of adjacent crustal blocks during terrane amalgamation and accretion in southern British Columbia. The geochemistry of about fifty shale samples from one of the best-dated stratigraphic sections in the North America Cordillera is used to (1) infer proximity to source regions characterized by juvenile ocean arcs, continental arcs, and ophiolites and (2) address a controversy concerning the timing of terrane amalgamation and accretion. This study includes analyses of shale from the entire Mesozoic stratigraphic section of the Cadwallader terrane and overlying Tyaughton basin, as well as argillites from the structurally adjacent Bridge River Complex. The mid-Cretaceous (Albian) strata of the upper Taylor Creek Group, show light rare earth (LREE) enrichment (La N /Sm N of ∼3.0) and anomalously high Cr and Ni. Older units (Upper Triassic–Lower Cretaceous) that stratigraphically underlie the upper Taylor Creek Group have less LREE enrichment, high Ba N /La N ratios, and no Cr or Ni anomaly. The mid-Cretaceous strata were probably derived from a continental arc during a period of uplift and erosion of ultramafic rocks (probably ophiolites) and the co-extensive nature of these sediments suggests accretion of many basement terranes to the edge of a continent. The trace-element geochemistry of the older strata is similar to that of sediment derived from an oceanic arc; however, Lower Cretaceous strata of the Relay Mountain Group may record the incipient collision and progressive closure of this ocean basin. The Bridge River Complex, which is dominated by oceanic rocks and is interpreted to be an accretionary complex, contains two suites of argillites, one of which shows significant LREE enrichment, and isotopic data suggest a Jurassic-Cretaceous age for these argillites that compose the matrix in this complex (Leitch et al., 1991). We suggest that the Bridge River Complex was a Jurassic-Cretaceous accretionary complex that formed outboard of the Methow section of rocks and was not juxtaposed with the Cadwallader terrane and Tyaughton basin until the Albian when these terranes were accreted to North America. Paleomagnetic data from these rocks (Maxson et al., 1993) indicate that accretion took place at the latitude of central Mexico and that subsequent strike-slip faulting brought these terranes to their present position by Eocene time.