Abstract

Mesozoic two-mica granites and I-type granodiorites from northeastern Washington have initial 87Sr/86Sr ratios around 0.7100, similar to many other Cordilleran granitoids. Metapelite and calcsilicate country rocks, equivalent to the Belt Supergroup, have measured 87Sr/86Sr ratios in the range of 0.91 to 0.98. Unlike many of their Cordilleran counterparts, the northeastern Washington twomica 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-element modeling shows that it is feasible to derive the two-mica granites by 60% partial melting of a granodioritelike source. The close association of Cordilleran metamorphic core complexes and older I-type granodiorites with two-mica granites supports this model.

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