CONSTRAINING THE TECTONIC HISTORY OF THE PROTEROZOIC MOJAVE-YAVAPAI PROVINCE BOUNDARY: INSIGHTS FROM DETAILED METAMORPHIC PETROLOGY AND MONAZITE GEOCHRONOLOGY ALONG A TRANSECT ACROSS THE 96-MILE AND CRYSTAL SHEAR ZONES OF THE UPPER GRANITE GORGE, GRAND CANYON, ARIZONA

A challenge regarding Proterozoic geology of the southwestern U.S. involves the recognition of tectonic boundaries and understanding their role in the evolution of Proterozoic lithosphere. Based on structural, isotopic, and geochronological data and the presence of metamorphosed “mélange-like” lithologies, it has been suggested that a major Proterozoic accretionary boundary coincides with the Crystal shear zone at mile 98 in the Upper Granite Gorge of the Grand Canyon.

Rocks on the west side of the near-vertical, NE-striking shear zone consist of gt + st + bt + crd + pl + qtz + ms schists, metamorphosed sulfide deposits, cherts, relict pillow structures, mafic dikes, and metavolcanic rocks. East of the zone, rocks contain gt + bt + cl + qtz + pl + qtz  ± ksp assemblages. Rocks west of the zone record peak pressures of  ~ 0.6 GPa, and preliminary data suggest rocks to the east may record pressures as high as 0.8 GPa before juxtaposition. We suggest that this boundary was primarily active prior to and during the ~1.7 Ga Yavapai orogeny.

A second boundary, the 96-Mile shear zone, juxtaposes upper amphibolite grade rocks to the east with upper greenschist grade rocks to the west. Rocks east of the ultramylonitic zone consist of sill + gt + bt + pl + qtz pelitic schists, gt + hb + pl mafic rocks, pegmatitic pods, and granitic dikes; they record peak conditions of  ~0.6-0.7 GPa and 625^o-725^oC. Rocks to the west record similar pressures but with temperatures < 600^oC. Ar⁴⁰/Ar³⁹ data reveal a major thermochronologic break near the 96-Mile shear zone, separating ~1.4 Ga muscovite cooling ages on the east from ~1.6 Ga cooling ages on the west. No effects of  ~1.4 Ga tectonism have been documented. Thus, this boundary seems to juxtapose distinct peak and post-peak thermal regimes.

The Crystal shear zone appears to represent a prograde to peak metamorphic boundary whereas the 96-Mile shear zone represents a peak to post-peak boundary. The two zones do not correspond spatially at the surface, but they may reflect a persistent crustal or lithospheric weakness at depth. Precisely calibrated P-T-D-time paths across these boundaries are critical. We are currently using detailed geothermobarometery and in situ monazite geochronology to characterize the evolution of the three blocks and the significance of the boundaries for the Proterozoic lithosphere in general.