NEW U-PB SHRIMP EVIDENCE FOR MULTIPLE METAMORPHIC EVENTS IN THE NORTHERN U.S. CORDILLERA: EOCENE, CRETACEOUS, AND LATE PRECAMBRIAN (GRENVILLE) EVENTS

U-Pb dating of metamorphic zircon rims from rocks in the Priest River complex and Clearwater complex in northern Idaho reveal the timing of metamorphism and tectonism in this part of the Cordillera. These dates were obtained from high U/Th ratio metamorphic rims on older zircons with the USGS/Stanford SHRIMP-RG machine. In the Clearwater complex, reliable analyses of metamorphic zircon rims in the core of the complex give Pb²⁰⁶/U²³⁸ dates of 77-70 Ma (2), 66-60 Ma (14), and 54 Ma (1). One rim yields a slightly discordant (5%) age of 65.7 Ma. These data are interpreted to suggest that M2 and M3 metamorphism are Cretaceous-Eocene in age. In contrast, zircon rims from above the bounding Jug Rock shear zone give ages of 83-69 Ma (8), and are interpreted to record protracted Cretaceous-Paleocene metamorphism. These data support the interpretation of the Clearwater complex as an Eocene metamorphic core complex. In the Priest River complex, six zircon rims give Pb²⁰⁶/U²³⁸ dates of 86 Ma (1) and 59-48 Ma (5). These record metamorphism during Cretaceous contraction and Eocene extension, respectively. Surprisingly, three rims give nearly concordant (<5%) Pb²⁰⁷/Pb²⁰⁶ ages between 993 Ma and 1096 Ma. One rim yields a concordant Pb²⁰⁷/Pb²⁰⁶ age of 1052 Ma. These dates, and those of Sha et al. (2004), reveal the presence of a cryptic Grenville-aged metamorphic event along this part of western Laurentia. Because the overlying Belt Supergroup contains no deformation of this age, we infer that the Grenville metamorphism was primarily a static thermal event.