U-PB GEOCHRONOLOGY AND LU-HF ISOTOPIC DATA OF MAGMATIC AND METAMORPHIC ZIRCONS FROM THE SAWTOOTH METAMORPHIC COMPLEX: IMPLICATIONS FOR CRUSTAL THICKENING IN THE CENTRAL IDAHO HINTERLAND OF THE U.S. CORDILLERA

U-Pb ages and Hf isotopic data of zircons from Mesozoic melts (intrusive and anatectic) and metasedimentary rocks of the Sawtooth metamorphic complex (SMC) are crucial for understanding the magmatism and metamorphism in the central Idaho Cordilleran hinterland. To date, one of the oldest intrusions is a mylonitic granitic dike with zircons that yield concordant ages (²⁰⁶Pb/²³⁸U) of ~169-153 Ma, ~131 Ma, and ~114 Ma, and ƐHf_i of 0 to -12. Zircon rims from a garnet-amphibolite yield an upper intercept age of 102.8±5.9 Ma, and those from a migmatitic sillimanite-garnet-biotite gneiss give an age of 100.2+1.5/-1.0 Ma (ƐHf_i: -21 to -27). Metamorphism of these rocks might be characterized by high temperature and high pressure on the basis of the mineral assemblages. A granitic leucosome (anatectic) from a migmatitic gneiss contains zircons that give an age of 100.6+1.0/-0.6 Ma (ƐHf_i: -9 to -16). A gabbroic intrusion yields a similar age of 99.0+1.2/-0.7 Ma (ƐHf_i: -6 to -11). Five magmatically foliated granitic intrusions yield ages of ~95-92 Ma with ƐHf_i of -6 to -15 for the magmatic zircons, and ~154-100 Ma with ƐHf_i of -1 to -13 for thirteen zircon xenocrysts. Zircons from three unfoliated granitoids yield ages of ~88 Ma, ~84 Ma, and ~77 Ma, with ƐHf_i of -12 to -25, -23 to -28, and -28 to -42, respectively. The ~169-153 Ma zircon xenocrysts from multiple intrusions suggest the presence of Jurassic igneous rocks in the source region. They tend to give less negative ƐHf_i values (0 to -10), indicating involvement of mantle-derived melts. The ~100 Ma thermal event that grew zircons in metamorphic rocks, along with the ~100 Ma felsic/mafic intrusions reflect the extensive mid Cretaceous crustal thickening and anatexis as recorded elsewhere in the North American Cordillera hinterland. Anatexis that produced the magmatically foliated intrusions likely continued through ~95-92 Ma and involved mostly crust based on the Hf data. Younger unfoliated intrusions (~88-77 Ma) increasingly incorporated ancient crust as they tend to have more negative ƐHf_i values (-12 to -42). The SMC magmatism, therefore, has documented steadily increased crustal components and a continuous process of crustal thickening in the central Idaho hinterland of the U.S. Cordillera.