U-PB AND HF ISOTOPIC ANALYSIS OF ZIRCON IN LOWER CRUSTAL XENOLITHS FROM THE NAVAJO VOLCANIC FIELD: 1.4 GA MAFIC MAGMATISM AND METAMORPHISM BENEATH THE COLORADO PLATEAU

Major unknowns in models for the Proterozoic evolution of the southwestern U.S. are the degree to which the hidden lower crust was modified by ~1.4 Ga granitic magmatism and metamorphism that was widespread in the exposed middle crust and whether a regionally extensive high-velocity (7.xx) layer resulted from basaltic underplating and melting of the lower crust. We addressed these issues by analyzing zircon in lower crustal xenoliths from the Navajo volcanic field on the Colorado Plateau for U-Pb and Lu-Hf isotopic compositions. Xenoliths of mafic and felsic granulite are interpreted as being derived from the lower crust based on the primary mineralogy of Cpx-Grt-Pl ± Qtz and thermobarometry estimates of ~1.3 GPa and 800^oC. CL imaging guided our attempts to isolate homogeneous, moderate sized (50-100 mm) zircon domains through grain fragmentation and air-abrasion. Mafic xenoliths contain a dominant population of CL-bright, weakly zoned grains and rims that are typical of metamorphic zircon, and the common association of zircon with Grt between Cpx and Pl suggests thatit grew during the breakdown of Cpx and the formation of Grt + Pl. Metamorphic zircon yielded U-Pb IDTIMS dates of 1420-1414, 1410-1395 (most prevalent), 1385, and 1360 Ma. The significant variability within and between xenoliths in zircon initial Hf isotopic compositions (e_Hf=-0.7 to +13.6) and depleted mantle model ages (T_DM=1.8-1.3 Ga) indicates that it grew from a variety of sources with diverse time-integrated Lu/Hf ratios. Xenoliths also contain oscillatory- and sector-zoned zircon cores and whole grains that likely grew during igneous crystallization of the protoliths. Igneous cores in felsic granulites have U-Pb dates of ~1710 and 1640 Ma, and unradiogenic Hf isotopic compositions (e_Hf=+7.8-11.4, T_DM=1.7-1.6 Ga). Agreement between crystallization ages and depleted mantle model ages indicate that protoliths of the granulites were derived from “juvenile” Proterozoic crust. Igneous cores in mafic xenoliths have U-Pb dates of ~1435 Ma and unradiogenic Hf isotopic compositions (e_Hf=+4.1-7.8, T_DM=1.7-1.6 Ga), consistent with ~1435 Ma mafic magmas having interacted with older crust. These data suggest a strong link between ~1.4 Ga mafic magmatism and metamorphism in the lower crust and granitic magmatism and metamorphism in the middle crust.