USING PAIRED U-PB AND HF ISOTOPES TO CHARACTERIZE THE CRUSTAL EVOLUTION OF THE MOJAVE PROVINCE: COMPARISON OF IGNEOUS AND METASEDIMENTARY LITHOLOGIES IN GRAND CANYON AND DEATH VALLEY

Two distinct Proterozoic provinces of southwest Laurentia, the Mojave and Yavapai, are discriminated in terms of their age, isotopic composition, and metamorphic grade. The crystalline basement rocks of the Mojave province preserve an evolved isotopic signature (Nd, Pb, Hf) that suggests Archean crustal material was involved in the crust-forming process. The origin and tectonic significance of this Archean component, and the nature and location of province boundaries are debated. U-Pb and Hf isotopic analysis of zircons separated from igneous and metasedimentary lithologies along a transect from Grand Canyon to Death Valley provide new insight into the Mojave Province’s evolved isotopic composition, and the Mojave – Yavapai boundary. Comparison of the Hf isotopic composition of zircons separated from granodiorite plutons in the Grand Canyon transect shows that plutons to the west of river mile 96-98 (Crystal shear zone) characteristically contain Paleoproterozoic grains that yield Archean (2.5 Ga) Hf model ages, as well as xenocrystic Archean grains. Plutons east of river mile 96 are derived from juvenile 1.7 – 1.8 Ga crust. Paradoxically, the Vishnu Schist (turbidites) across the entire Grand Canyon transect have a bimodal zircon population (~1.85 and 2.48 Ga), with only 13% juvenile 1.75 Ga grains. New samples with this same signature suggest a regional turbidite basin that extended into Death Valley, parts of the AZ Transition zone and throughout the Mojave province. This basin is interpreted to have been deposited during initial juxtaposition of the Yavapai and Mojave provinces at ca. 1.75 Ga. Paleoproterozoic plutons in western Grand Canyon and Death Valley yield Hf model ages that correspond to 1.85 and 2.48 Ga detrital zircon peaks of Vishnu Schist. Hence both metasediments and granodiorites contain cryptic evidence for the ages of crust that both sourced Vishnu metaturbidites and served as the lower crustal melt source region for Paleoproterozoic arc-magmatism. Although not yet found in surface outcrops, we infer that 1.85 and 2.5 Ga crust occupies parts of the lower crust of the Mojave province.