ZIRCON U-PB GEOCHRONOLOGY AND HF ISOTOPIC CONSTRAINTS ON THE NATURE OF CRUSTAL MELT GENERATION IN THE PRIEST RIVER COMPLEX, NORTHERN IDAHO

Zircon U-Pb and Hf isotope data from a suite of igneous units that intruded the footwall of the Priest River metamorphic core complex (PRC) in northern Idaho constrain the Cretaceous-Eocene history of pluton emplacement and crustal melting. In situ U-Pb and Hf isotope data from the Spokane granite, Silver Point quartz monzonite, Wrencoe granodiorite, and a leucocratic dike that crosscuts the metapelitic Hauser Lake gneiss were analyzed via split-stream LA-MC-ICPMS, followed by U-Pb CA-TIMS analysis of zircon tips and single grains.

CA-TIMS zircon ²⁰⁶Pb/²³⁸U dates from the Spokane granite range from 85.57 ± 0.07 to 76.49 ± 0.06 Ma (n = 7), while Hf isotope compositions document an ancient pluton source. Silver Point pluton ²⁰⁶Pb/²³⁸U dates range from 71.37 ± 0.06 to 50.09 ± 0.04 Ma (n = 7), with a weighted mean date of 50.13 ± 0.02 Ma from the four youngest analyses (MSWD = 2.4). Hf isotope compositions for the Silver Point pluton are more unradiogenic and homogeneous than for the Spokane granite. Wrencoe pluton ²⁰⁶Pb/²³⁸U dates range from 48.17 ± 0.04 to 47.85 ± 0.03 Ma (n = 7) with Hf isotope compositions similar to the Silver Point. Zircon ²⁰⁶Pb/²³⁸U dates from the leucocratic dike range from 65.94 ± 0.06 to 49.92 ± 0.04 Ma (n = 8); Hf isotope compositions are more radiogenic than those of the Eocene plutons.

The Hf isotope data indicate that these units are dominantly crustal melts derived from Archean/Proterozoic sources, with the Spokane granite from a younger, more radiogenic component, and the leucocratic dike from varied crustal sources. These data confirm that the Spokane granite crystallized prior to peak PRC metamorphism. The late-to-synkinematic Silver Point pluton cuts the Newport fault, which is partially responsible for the exhumation of the PRC, and is weakly mylonitized near the fault trace. The post-kinematic Wrencoe pluton was previously conflated with the Silver Point pluton, but distinct crystallization ages suggest multiple pulses of crustal melting related to decompression during extension. Crystallization ages of the Eocene plutons limit extension and deformation in the PRC footwall to no later than c. 50-48 Ma. Late pluton emplacement relative to PRC extension and exhumation suggests that extensive crustal melting and pluton emplacement was not a requirement for the initiation of core complex formation.