Magmatism as a response to exhumation of the Priest River complex, northern Idaho: Constraints from zircon U–Pb geochronology and Hf isotopes

Abstract

Zircon and monazite U–Pb geochronology and zircon Hf isotopes place constraints on the temporal and source relationships between crustal anatexis, magmatism, and exhumation of the Priest River metamorphic core complex, northern Idaho. Granitoids that intruded the migmatitic, pelitic Hauser Lake gneiss include the <76.5±0.1Ma Spokane granite, 50.13±0.02Ma Silver Point quartz monzonite, c. 47.9Ma Wrencoe granodiorite, <46.4±1.8Ma Rathdrum granite, and a <49.8±0.4Ma leucocratic dike. Cretaceous magmatism preceded the c. 64Ma peak metamorphism (recorded by monazite) of the Hauser Lake gneiss, whereas discrete pulses of Eocene magmatic activity post-date the onset of exhumation by 10Ma. The relative timing of pluton emplacement in the Priest River complex indicates that it was primarily a response to decompression rather than a cause. The mylonitized Silver Point and undeformed Wrencoe plutons bracket the end of a rapid phase of exhumation to c. 50–48Ma. Zircon εHf(i) values and Lu–Hf isotope evolution indicate that the Silver Point and Wrencoe plutons crystallized from homogeneous magmas sourced from Archean–Proterozoic basement orthogneisses, whereas the Spokane granite and two leucocratic units appear to have been produced by partial melting of the Hauser Lake gneiss. Comparison of the Priest River complex with other deeply exhumed northern Cordilleran complexes indicates variability in the timing and, therefore, relative influences of partial melting and magmatism on the initiation of exhumation, which must be accounted for in numerical models of metamorphic core complex formation and evolution.