THE PALEOPROTEROZOIC GEOLOGY OF THE NORTHERN HUALAPAI MOUNTAINS, ARIZONA, AND ITS IMPLICATIONS FOR THE MOJAVE PROVINCE/TRANSITION ZONE BOUNDARY IN NORTHWESTERN ARIZONA

The Hualapai Mountains have traditionally been considered to be part of the isotopically mixed transition zone between the Mojave and Yavapai Proterozoic crustal provinces in Arizona. The western boundary of this transition zone has been proposed to trend southwest from the Gneiss Canyon shear zone in the Lower Granite Gorge of the Grand Canyon through the northern Hualapai Mountains. Mapping, structural analysis, and metamorphic studies in the northern Hualapai Mountains indicate that there are significant differences in the deformational style and metamorphic grade between these rocks and the rocks of the Lower Granite Gorge. The two primary Paleoproterozoic deformational events recorded in the Hualapai Mountains are the regional D₁, represented by a northwest-striking foliation dated elsewhere in Arizona at 1740-1720 Ma, and the regional D₂ (1700-1690 Ma), represented by a steeply dipping northwest-striking foliation that has reoriented and possibly reactivated the earlier fabric. These fabrics are identical to those preserved in the Cerbat Mountains, across the proposed southwest projection of the Gneiss Canyon shear zone. The Gneiss Canyon shear zone in the Lower Granite Gorge has been described as an 11-km wide, highly migmatitic, D₂ shear zone characterized by LS fabrics with strong northwest-side-up kinematic indicators. In contrast, in the northern Hualapai Mountains, domains of D₂ deformation are narrower, with significant areas of D₁ fabric preserved, very few rocks are migmatitic, S tectonite fabrics are dominant (suggesting flattening strains), and very few kinematic indicators are present. In the Lower Granite Gorge, the Gneiss Canyon shear zone separates granulite facies rocks (650-700^oC and 3.5-4.5 kb) to the northwest from amphibolite facies rocks (550-600^oC and 2.3-3.5 kb) to the southeast, whereas granulite facies assemblages are present across the projected trace of the Gneiss Canyon shear zone 100 km southwest of the Lower Granite Gorge. This observation is compatible with recent metamorphic work in the Peacock Mountains and Pb isotopic studies that place the western margin of the transition zone significantly east of the Hualapai Mountains. Therefore, we propose that the Gneiss Canyon shear zone may project south from its exposure in the Grand Canyon rather than southwest.