Detrital Chromian Spinel Compositions Used to Reconstruct the Tectonic Setting of Provenance: Implications for Orogeny in the Canadian Cordillera

Abstract

ABSTRACT Detrital chromian spinel is an important accessory mineral in chert and volcanic lithic sandstones from the northern Bowser Basin. Microprobe analyses show that Bowser Basin detrital chromian spinels have Cr/(Cr + Al) between 0.21 and 0. 89, Mg/(Mg + Fe2+) between 0.24 and 0.70, and Fe3+/(Fe3+ + Al + Cr) values below 0.12. Comparison with spinels from literature on ultramafic rocks indicates that the compositional range of the detrital spinels closely matches that of spinels from Alpine-type peridotites emplaced by obduction of marginal-basin crust and island-arc complexes. Furthermore, Bowser Basin spinels are compositionally distinct from spinels derived from either mid-ocean ridge, stratiform ultramafic complex, or Alaskan-type peridotite. The composition of the detrital spinel demonstrates that the source for Bowser Basin sediments included extensive Alpine-type peridotite, which probably originated as marginal-sea lithosphere. In addition, there is no evidence for 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 consistent with the detrital modal analysis of sandstones, which calls for a source terrane consisting of obducted oceanic crust and island arc. The chemistry of the detrital spinels further suggests the source area was eroded prior to emplacement of Alaskan-type peridotites or any regional gree schist or higher-grade metamorphic events. Thus, microprobe compositional analysis of detrital spinels adds important detail to the provenance interpretation that is not available through analysis of detrital modes alone. The inference that marginal-basin lithosphere rather than material derived from mid-ocean ridges was the sediment source suggests that the obducted source terranes were closely associated with North America.