SURPRISING PREDOMINANCE OF 1.84 GA AND >2.5 GA DETRITAL ZIRCONS IN THE 1.75 GA VISHNU SCHIST, UPPER GRANITE GORGE, GRAND CANYON, ARIZONA

U-Pb dating of detrital zircons in the oldest metasedimentary rocks of Grand Canyon, the Paleoproterozoic Vishnu Schist, was completed using LA-MC-ICPMS at the University of Arizona LaserChron center. The resulting data, for samples that span the entire ~200 km Grand Canyon transect, revealed a consistent bimodal ²⁰⁷Pb/ ²⁰⁶Pb age distribution of: 1) ~1.84 Ga detritus and 2) a significant amount of previously unrecognized Archean detritus. The 1.84 Ga grains are dominant west of the Crystal shear zone, whereas Archean and 1.84 Ga grains are in nearly equal abundance east of the Crystal shear zone; and therefore, weakly consistent with models for a significant crustal boundary at Crystal shear zone. Although youngest grains agree with the depositional age of 1750 ±2 Ga for the Vishnu Schist (Hawkins et al., 1996); the age spectra contain surprisingly few zircons of this age, which is difficult to reconcile with models that the Vishnu Schist is part of a 1.75 Ga juvenile oceanic arc complex. The ~1.84 Ga detrital age is identical to the 1.84 Ga Elves Chasm orthogneiss of western Grand Canyon, which has been interpreted to be a Paleoproterozoic arc pluton. Therefore, these data support the interpretation that Vishnu turbidite successions were deposited on this older 1.84 Ga arc basement. The equally abundant distribution of Archean (~2.5 Ga and older) detrital zircons is puzzling given the complete lack of proximal Archean outcrop in the Grand Canyon region. As has been previously proposed for the Mojave province (Wooden and Miller, 1990), the Archean detritus may have been derived from the Wyoming craton, similar to Himalayan detritus appearing in distant Indonesian arc systems. The Archean detrital age distribution is roughly consistent with age probability plots of Wyoming province zircon ages. Alternatively, inherited Archean zircons are also present in some plutons in the Grand Canyon raising the possibility that Archean crust may have been available in nearby microplates for erosion into Vishnu turbidites. The presence of this large amount of Archean detritus highlights the need for further isotopic studies of the extent and character of pre-1.75 Ga zircon grains in the Paleoproterozoic rocks of the southwestern U.S.