Geochemistry of Late Cretaceous granitoids from northeastern, Washington: Implication for genesis of two-mica, Cordilleran granites

Abstract

Mesozoic two-mica granites and I-type granodiorites from northeastern Washington have initial /sup 87/Sr//sup 86/Sr ratios around 0.7100, similar to many other Cordilleran granitoids. Metapelite and calcsilicate country rocks, equivalent to the Belt Supergroup, have measured /sup 87/Sr//sup 86/Sr ratios in the range of 0.91 to 0.98. Unlike many of their Cordilleran counterparts, the northeastern Washington two-mica granites are enriched in U, Rb, and other incompatible elements, and therefore cannot have been derived from depleted lower crust sedimentary sources. Both the two-mica granites and the I-type granodiorites have similar rare-earth-element and trace-element enrichment patterns. Trace-elements modeling shows that it is feasible to derive the two-mica granites by 60% partial melting of a granodiorite-like source. The close association of Cordilleran metamorphic core complexes and older I-type granodiorites with two-mica granites supports this model.