PALEOPROTEROZOIC DEFORMATION AND METAMORPHISM IN THE CENTRAL AND SOUTHERN PEACOCK MOUNTAINS, NW ARIZONA: IMPLICATIONS FOR THE LOCATION OF THE MOJAVE-YAVAPAI BOUNDARY ZONE

The Paleoproterozoic crust of southwest Laurentia has been divided into three provinces, the Mojave, Yavapai, and Mazatzal, based largely on Nd and Pb isotopic characteristics. The boundary between the Mojave and Yavapai crustal provinces has been described as a 75+ km-wide zone of mixed Pb isotopic signatures (MYBZ). The eastern margin of this zone is generally agreed to coincide with a sharp break in Pb isotopic character approximately coincident with the Crystal shear zone in the Upper Granite Gorge of the Grand Canyon. The location of the western margin is more controversial and the Peacock Mountains, northwestern Arizona, are located between two proposed locations for this margin. In the Lower Granite Gorge of the Grand Canyon, the Gneiss Canyon shear zone (GCSZ), commonly considered the western margin of the MYBZ, coincides with a transition from granulite facies metamorphism (to west) to amphibolite facies metamorphism (to east). An along-strike, southwestward projection of the GCSZ lies west of the Peacock Mts. and would continue between the Hualapai and Cerbat Mts. In the central and southern Peacock Mts. (Mojave Pb isotopic character), mineral assemblages in pelites (Grt+Sil+Bi+Kfs+Pl+Qtz) and preliminary P-T determinations suggest granulite facies metamorphism at T>600°C and P=5.4 kbar in contrast to greenschist facies metamorphism (T~ 450°C, P~2.5-3.0 kbar) in the Cottonwood Cliffs (Yavapai-like Pb isotopic character) <12 km to the east. This abrupt metamorphic and Pb isotope transition suggests the presence of a buried north-trending structure between the two ranges that may mark the western margin of the MYBZ. This location is supported by spatially coincident Pb isotopic, geochemical, and geophysical boundaries. Furthermore, granulite facies metamorphism (T~650-800°C, P~5.1-8.0 kbar) has been documented throughout the Hualapai and Cerbat Mts. These data and observations suggest that the western margin of the MYBZ either (1) lies east of the Peacock and Hualapai Mts., (2) does not everywhere coincide with the eastern limit of granulite facies metamorphism as it appears to in the Lower Granite Gorge, or (3) coincides with the GCSZ, but that the GCSZ must change to a more north-south orientation between the Peacock Mts. and the Cottonwood Cliffs.