DETRITAL CHROMIAN SPINEL COMPOSITIONS USED TO RECONSTRUCT THE TECTONIC SETTING OF PROVENANCE - IMPLICATIONS FOR OROGENY IN THE CANADIAN CORDILLERA

Detrital chromian spinel is an important accessory mineral in chert an d volcanic lithic sandstones from the northern Bowser Basin, Microprob e analyses show that Bowser Basin detrital chromian spinels have Cr/(C r + Al) between 0.21 and 0.89, Mg/(Mg + Fe2+) between 0.24 and 0.70, a nd Fe3+/(Fe3+ + Al + Cr) values below 0.12, Comparison with spinels fr om literature on ultramafic rocks indicates that the compositional ran ge of the detrital spinels closely matches that of spinels from Alpine -type peridotites emplaced by obduction of marginal-basin crust and is land are complexes, Furthermore, Bowser Basin spinels are compositiona lly distinct from spinels derived from either mid-ocean ridge, stratif orm ultramafic complex, or Alaskan-type peridotite, The composition of the detrital spinel demonstrates that the source for Bowser Basin sed iments included extensive Alpine type peridotite, which probably origi nated as marginal-sea lithosphere. In addition, there is no evidence f or material derived from mid-ocean ridge, Alaskan-type peridotites, or stratiform complexes. This provenance interpretation is in agreement with earlier interpretations that related chert pebbles in the Bowser Basin to chert-rich strata of the Cache Creek terrane and is consisten t with the detrital modal analysis of sandstones, which calls for a so urce terrane consisting of obducted oceanic crust and island are. The chemistry of the detrital spinels further suggests the source area was eroded prior to emplacement of Alaskan type peridotites or any region al greenschist or higher-grade metamorphic events, Thus, microprobe co mpositional analysis of detrital spinels adds important detail to the provenance interpretation that is not available through analysis of de trital modes alone, The inference that marginal-basin lithosphere rath er than material derived from midocean ridges was the sediment source suggests that the obducted source terranes were closely associated wit h North America.