PALEOCENE-EOCENE MIGMATITE CRYSTALLIZATION, EXTENSION, AND EXHUMATION IN THE HINTERLAND OF THE NORTHERN CORDILLERA: OKANOGAN DOME, WASHINGTON

The Okanogan gneiss dome, Washington, is located in the hinterland of the N. American Cordillera and is part of a chain of metamorphic core complexes containing gneiss domes (including migmatites) exhumed during Eocene extension of thickened crust. U-Pb SHRIMP analyses of zircon, monazite, and titanite and Ar-Ar analyses of biotite from migmatites exposed in the footwall of the Okanogan detachment document the timing and duration of crustal melting and indicate coeval partial melting, extensional deformation, and exhumation of the dome. Okanogan migmatites are folded and deformed, and preserve successive generations of leucosomes generated by syn-kinematic anatexis.

Zircons from a granodiorite in a high melt fraction (diatexite) subdome near Stowe Mtn. preserve rim overgrowths with a mean ²⁰⁶Pb/²³⁸U age of 51.1 ± 0.9 Ma; ²⁰⁶Pb/²³⁸U ages in the core domains range from ~70 to 85 Ma. Two zircon samples from variably folded and cross-cutting granitic leucosomes in the diatexite and structurally overlying metatexite yield ²⁰⁶Pb/²³⁸U ages of 53.5 ± 0.4 Ma and 59.8 ± 0.4 Ma, respectively, for both core and rim domains. Within the metatexite, a few analyses yield ages as young as 53 Ma. Monazite from 2 samples give ²⁰⁶Pb/²³⁸U ages of 52.9 ± 0.5 Ma for the granodiorite diatexite and 52.0 ± 0.5 Ma for nearby boudinaged and foliated layers of biotite-granodiorite. One sample of folded granitic leucosome in metatexite contains titanite with a mean ²⁰⁶Pb/²³⁸U age of 47.1 ± 0.4 Ma. The 47 Ma age for titanite is similar to biotite Ar-Ar ages of 48.0 ± 0.1 Ma, 47.9 ± 0.2 Ma, and 47.1 ± 0.2 Ma for samples collected from the upper detachment surface downward over 3 km of structural thickness into the migmatite domain.

These new data suggest that rocks of the Okanogan dome underwent partial melting (migmatization) for at least 8 m.y. which was in part coeval with upper crustal extension and ductile flow of the mid-crust. Leucosome crystallization had ceased by 52 Ma followed by rapid cooling of footwall rocks through ~350 ºC by 47 Ma. These data are similar to crystallization ages in migmatites from other domes in the northern Cordillera hinterland, suggesting that crustal anatexis was laterally widespread over much of the mid-crust during Paleocene to Eocene time and nearly coeval with extension and exhumation of orogenic middle crust.