MICROSTRUCTURAL AND 40AR/39AR THERMOCHRONOLOGIC EVIDENCE FOR MESOPROTEROZOIC DISPLACEMENT WITHIN THE GNEISS CANYON SHEAR ZONE: LOWER GRANITE GORGE, GRAND CANYON, ARIZONA

The Proterozoic Gneiss Canyon shear zone is a roughly 1.5 km wide shear zone located in the Lower Granite Gorge of Grand Canyon, AZ. Pb isotopic data as well as change in metamorphic conditions, which vary from ca. 550^oC(east) to ca. 750^oC (west) across the shear zone, have been used to propose that this zone may represent part of a wide transitional boundary between the Yavapai province to the east and the Mojave province to the west.

Evidence from previous studies indicates west-side-up and dextral shear during, and soon after, peak metamorphic conditions , which occurred ~1.69 Ga. Microstructural evidence, presented here, shows several, 1 to 5-meter-wide, discrete lower-temperature shear zones within the wider, high T, Gneiss Canyon shear zone. Shear sense indicators, viewed on kinematic sections cut parallel to the subvertical stretching lineation and perpendicular to the NE trending S₂ foliation, show west-side-up shear based on S-C fabrics, sigma porphyroclasts, and offset grains. When lineations are SW plunging, movement has a dextral component. Assuming normal deformation rates, observed microstructures indicate deformation at temperatures ranging from > 450^oC, indicated by core and mantle structure in feldspar, to < 300^oC indicated by highly undulose and fractured quartz grains. This wide range of inferred temperatures may represent a single progressive event, or lower temperature reactivation of older shear zones.

⁴⁰Ar /³⁹Ar data from the Lower Granite Gorge of Grand Canyon and previously published data from the tilted Gold Butte block of the adjacent Basin and Range province are presented here in order to help constrain when movement on the retrograde shear zones took place. Lower Gorge data show muscovite ages ranging from 1562-1552 Ma and biotite ages from 1572-1401 Ma. These data suggest either that: 1) rocks were slowly cooled to below 375^oC by about 1550 Ma and observed microstructures formed pre-1550 Ma, or 2) the 1550 Ma ages represent incomplete resetting by a 1.4 Ga thermal event, in which case the microstrucutres could have formed during 1.4 Ga tectonism. The resolution of this issue requires additional work using Ar dating of mica fish and feldspar, and U-Pb dating of monazite in the shear zones.