CONSTRAINTS ON EXTENSION AND EXHUMATION OF THE PRIEST RIVER COMPLEX, NORTHERN IDAHO, FROM ZIRCON U-PB GEOCHRONOLOGY

Zircon U-Pb geochronology of Cretaceous and Eocene igneous bodies that intrude the footwall of the Priest River metamorphic core complex (PRC) in northern Idaho constrains the timing of crustal extension and exhumation of the complex, and has implications for the crustal conditions necessary for the initiation of core complex formation. Zircon tips and single grains were analyzed by U-Pb CA-TIMS from four samples that represent a suite of intrusive bodies with varied ages relative to deformation of the PRC footwall.

The Spokane granite, positioned at the western margin of the PRC, is mylonitized along its eastern margin. The Silver Point quartz monzonite cross cuts the Newport fault, which is partially responsible for the exhumation of the PRC. The Silver Point pluton is weakly mylonitized along the Newport fault trace, and is interpreted as late-to-synkinematic. The Wrencoe granodiorite, although previously conflated with the Silver Point pluton, is undeformed. A leucocratic dike cuts the metapelitic, migmatitic Hauser Lake gneiss. Zircon ²⁰⁶Pb/²³⁸U CA-TIMS dates for these samples are: 85.57 ± 0.07 to 76.49 ± 0.06 Ma (n = 7) for the Spokane granite; 50.52 ± 0.04 to 50.09 ± 0.04 Ma (n = 5), with a weighted mean date of 50.13 ± 0.02 Ma for the four youngest analyses (MSWD = 2.4) for the Silver Point pluton; 48.17 ± 0.04 to 47.85 ± 0.03 Ma (n = 7) for the Wrencoe pluton; and 65.94 ± 0.06 to 49.92 ± 0.04 Ma (n = 8) for the leucocratic dike.

The Spokane granite predates deformation and peak metamorphism of the Hauser Lake gneiss. Robust crystallization ages for the deformed Silver Point and undeformed Wrencoe plutons, as well as dates from the leucocratic dike, bracket the timing of extension and exhumation to c. 50-48 Ma. The timing of crystallization of Eocene plutons, which is consistent with the timing of crystallization of partial melt in the Hauser Lake gneiss, is late relative to the timing of extension and exhumation of the PRC. This suggests that large volumes of partial melt were not prerequisite to the initiation of the Priest River metamorphic core complex, as has been suggested for other northern North American complexes.