METAMORPHIC HISTORY OF THE UPPER GRANITE GORGE, GRAND CANYON, ARIZONA, AND IMPLICATIONS FOR THE SIGNIFICANCE OF DOMAIN BOUNDARIES IN THE YAVAPAI/MAZATZAL OROGEN

The Upper Granite Gorge of the Grand Canyon exposes an extraordinary cross-section of the Proterozoic orogen between the central Arizona Transition Zone and the Rocky Mountain region. The gorge consists of several lithotectonic blocks bounded by major high strain zones that were primarily active during the 1.7-1.6 Ga accretion and stabilization of the Yavapai and Mojave crustal provinces. However, a longstanding problem with the characteristic “block” architecture of this region concerns the nature and tectonic significance of the block-bounding structures. The metamorphic history of rocks in the Upper Granite Gorge is broadly characterized by a clockwise P-T path with maximum pressures of 0.6-0.7 GPa followed by high temperature decompression and nearly isobaric cooling at ca. 0.3 GPa. Here we focus on detailed metamorphic analysis within and among the blocks. Our methods include observations of microstructures, major and minor phase relationships, quantitative thermo barometry and modeling, X-ray compositional mapping, and microprobe dating of monazite. Some important new features of the metamorphic history are revealed. Similarities among blocks include a clockwise geometry for the initial P-T path followed by isobaric cooling at lower pressure. The main differences concern the degree of heating early in the prograde history (i.e. early andalusite versus kyanite), the peak temperatures reached (upper greenschist- to lower granulite-facies) and consequently the equilibrium pressures recorded at peak temperature, late-stage and perhaps localized thermal spikes, and the relative pressure at which the isobaric cooling portion of the path occurred. Therefore, significant differences in metamorphic history across block boundaries appear to be primarily thermal in nature. In cases where a steep thermal gradient closely corresponds to a block-bounding structure (e.g., Vishnu shear zone, mile 80), subsequent lateral displacement may in part be responsible. Any significant vertical displacement (i.e., greater than several km’s) across the Crystal shear zone (mile 98), which has been proposed as marking a major crustal boundary based on isotopic variations, likely predates peak metamorphism or has been complicated by opposite-sense displacement on the adjacent and younger 96 mile shear zone.