ELUCIDATING THE EVOLUTION OF FUNDMENTAL TECTONIC BOUNDARIES USING "RELATIVE" THERMOBAROMETRY AND MONAZITE GEOCHRONOLOGY: A CASE STUDY FROM THE UPPER GRANITE GORGE, GRAND CANYON, ARIZONA

Several potentially significant tectonic boundaries, associated with the ca. 1.7 Ga collision of the Yavapai and Mojave crustal provinces, are recognized in basement rocks of the Grand Canyon, Arizona. In general, quantitative thermobarometric data indicate that peak P-T conditions throughout the Upper Granite Gorge were on the order of 0.6-0.8 GPa and 500-750°C. However, the data also suggest abrupt changes in T or P across some boundaries. For example, a temperature difference of as much as 100-150°C is suggested across the Vishnu, 96-Mile, and Crystal shear zones and a pressure difference of up to 0.2 GPa is suggested across the Crystal shear zone. Such differences have important implications for interpreting the kinematics and magnitude of displacement across the shear zones, the timing of metamorphic events, and the overall significance of these boundaries. However, given the current level of uncertainties associated with thermodynamic data for mineral phases and activity-composition models, uncertainties on calculated absolute P-T conditions are commonly too large (± 100°C, > 0.1 GPa) to recognize statistically significant differences. Recent work by others suggests that, in samples with identical or nearly identical mineral assemblages, the uncertainties on the differences in P-T conditions (i.e. comparisons of P or T) may be substantially reduced (<±50°C, <±0.1 GPa) because much of the uncertainty associated with the common mineral phases is effectively cancelled (using the DPT approach). The Upper Granite Gorge may be ideally suited for this technique because much of the pelitic rocks contain a common sub-assemblage of Grt-Ms-Bt-Sil-Pl-Qtz. Preliminary data suggest that temperature changes across the Vishnu and 96-Mile shear zones are significant, but pressure changes are not supported. A significant P-change across the Crystal shear zone remains possible. Current work involves using monazite geochronology to constrain the timing of deformation in order to understand the significance of the juxtaposed “block-type” structure of the Upper Granite Gorge. The five major shear zones that help define this architecture provide a natural laboratory for resolving the accretionary and subsequent intra-continental tectonic history during growth, stabilization, and reworking of the continental lithosphere in this region.