A WINDOW INTO PALEOPROTEROZOIC GRANULITE FACIES METAMORPHISM AND PRE-YAVAPAI TECTONISM, SOUTHERN HUALAPAI MOUNTAINS, NW ARIZONA

The southern Hualapai Mts., NW Arizona, lie within or at the western margin of the N-S trending isotopically mixed boundary zone (BZ) between the Mojave and Yavapai Paleoproterozoic crustal provinces. Recent mapping of Paleoproterozoic rocks in the southern Hualapai Mts. yields structural, metamorphic, and lithologic information that may bear on the location and tectonic evolution of the BZ. The southern Hualapai Mts. consists of granulite facies migmatitic psammitic and pelitic gneiss, amphibolite, and subordinate metaperidotite all intruded by a heterogeneous suite of granitoids.

The earliest of three recognized deformational fabrics, tentatively correlated with regional D₁ (1740 to <1715 Ma), is a dominantly NW-striking, gently east-dipping foliation containing an east-plunging lineation and is axial planar to mesoscopic recumbent folds. This fabric is cut, and locally reactivated, by normal sense shear bands that record E-W extension. NE-trending upright folds, typical of regional D₂ deformation (1700-1685 Ma), locally deform these fabrics and have a similar orientation to F₂ folds throughout the Mojave and Yavapai provinces.

Preliminary thermobarometry on migmatitic pelitic gneiss yields T>700°C and P>5.5 kbar. Textural and compositional evidence, including formation of incongruent garnet, suggests that partial melting occurred by muscovite- and biotite-dehydration reactions likely during tectonic burial associated with development of the D₁ structures. Normal sense shear bands may record post-burial exhumation. The lack of NE-trending fabrics in the southern Hualapai Mts. suggests regional D₂ strain partitioning; providing a valuable window into D₁ tectonism.

Metamorphic and structural data from this study correlate well with similar studies elsewhere in NW Arizona, but extend the recognized range of Paleoproterozoic granulite facies metamorphism farther south and east of previous studies. If the BZ's western margin coincides with the eastern limit of granulite facies metamorphism, that margin must lie east of the southern Hualapai Mts., forcing a reassessment of the BZ extent. This interpretation is consistent with recent work suggesting tectonic reworking of the Mojave-Yavapai margin during Paleoproterozoic assembly resulting in a complex distribution of crustal fragments.