Paleocene-Eocene migmatite crystallization, extension, and exhumation in the hinterland of the northern Cordillera: Okanogan dome, Washington, USA

The Okanogan gneiss dome, Washington, is located in the hinterland of the North American Cordillera and is part of a chain of metamorphic core complexes containing gneiss and migmatite domes exhumed during Eocene extension of thickened crust. U-Pb sensitive high-resolution ion microprobe (SHRIMP) analyses of zircon, monazite, and titanite, and ⁴⁰Ar-³⁹Ar analyses of biotite from migmatites exposed in the footwall of the Okanogan detachment, coupled with a detailed structural analysis, document the timing and duration of migmatite crystallization and indicate coeval crystallization, extensional deformation, and exhumation of the dome. Okanogan migmatites are folded and deformed, and preserve successive generations of leucosomes; generated by synkinematic anatexis. Analyses of migmatite samples from a highmelt fraction subdome near Stowe Mountain suggest that the Okanogan dome records a history of migmatite crystallization spanning at least 12 m.y., as indicated by ²⁰⁶Pb/²³⁸U ages ranging from ca. 61 to 49 Ma for new zircon growth and rim overgrowths attributed to migmatite crystallization. Zircons from a granodiorite in a domain of diatexite near Stowe Mountain preserve rims that have a mean ²⁰⁶Pb/²³⁸U age of 51.1 ± 1.0 Ma for the youngest population attributed to migmatite crystallization. Zircon from folded and discordant granitic leucosome in the diatexite domain yields a calculated ²⁰⁶Pb/²³⁸U age of 53.5 ± 0.5 Ma for migmatite crystallization. Zircon from discordant leucosome of the metatexite domain has a mean ²⁰⁶Pb/²³⁸U age of 59.8 ± 0.5 Ma, with ages as young as ca. 53 Ma attributed to final crystallization of the leucosome. Core domains of zircon samples have ²⁰⁶Pb/²³⁸U ages that range from ca. 85 to 70 Ma and are interpreted to be related to an earlier phase of the orogeny. Monazite from two samples gives ²⁰⁶Pb/²³⁸U crystallization ages of 52.9 ± 0.6 Ma for the granodiorite diatexite and 52.0 ± 0.6 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 ± 0.5 Ma. The ca. 47 Ma age for titanite is similar to biotite ⁴⁰Ar-³⁹Ar 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 1.5 krn of structural thickness into the migmatite domain. Crystallization of the Okanogan migmatites was therefore coeval in part with upper crustal extension and ductile flow of the mid-crust Leucosome crystallization largely ceased by ca. 49 Ma, followed by rapid cooling of footwall rocks through ∼325 °C by ca. 47 Ma. These data are similar to crystallization ages in migmatites from other domes in the northern Cordillera hinterland, suggesting that crustal anatexis was widespread over much of the mid-crust during Paleocene to Eocene time, coeval with extension and exhumation of orogenic middle crust.