Eocene igneous complexes of the U.S. and Canadian Cordillera have long been thought to represent a broad and somewhat discontinuous subduction-related magmatic arc, extending from northern British Columbia to Wyoming. This model has become increasingly untenable as knowledge of the structural and tectonic settings of these rocks has expanded. New major, trace, and rare earth element and isotope data, presented here, virtually preclude the interpretation of a contemporaneous subduction-related source for mantle-derived magmas in the Colville Igneous Complex of northeast Washington State. We suggest an alternative model whereby post-Laramide orogenic collapse resulted in partial melting of a mid-crustal source, followed by partial melting of a distinctly different lower crustal source, the magma from which mixed with magma derived from a lithospheric mantle source. The subduction signature, and the calc-alkalic nature of the magmas, was inherited from previous Proterozoic subduction events.

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