MICROSTRUCTURAL ANALYSIS OF QUARTZ MYLONITES AND NEW U-PB ZIRCON AGES OF CRETACEOUS INTRUSIONS IN THE CENTRAL PART OF THE NORTHERN SNAKE RANGE METAMORPHIC CORE COMPLEX, NEVADA

The northern Snake Range (NSR) in eastern Nevada is a classic metamorphic core complex with a footwall composed largely of mylonitic quartzite, schist, marble and assorted intrusive rocks. The timing of mylonitization is poorly constrained but was previously attributed to the same extensional event that produced rapid cooling of the footwall and large-scale slip on the northern Snake Range Decollement (NSRD) at 15-20Ma. Intrusions in the central part of the NSR include: the Horse Canyon orthogneiss (Khg) and abundant leucogranite dikes and sills. New LA-ICPMS U-Pb zircon ages of 101Ma for Khg and 85Ma, 85Ma and 84Ma for 3 different leucogranite bodies generally confirm previous dating by Miller et al. (1989-Rubey Vol VII). A fine-grained 2-mica granite that cuts some of the leucogranite sills yielded a distinctly younger age of 76.1 ± 1.5 Ma signaling a protracted late K magmatic history.

Mylonitic quartzites from Smith Creek, Horse Canyon, and Deadman Creek possess a conspicuous L-S fabric with a consistent ESE trending stretching lineation and gently E-dipping foliation. Thinning of stratigraphic units and aspect ratios on stretched pebbles indicate high strains (X/Z > 10). Ubiquitous kinematic indicators (S-C fabric, mica fish, sigma clasts, oblique grain shape foliations, CPOs) consistently indicate top-to-SE shear sense. . EBSD analyses of quartz CPOs indicate predominantly prism<a> slip with variable amounts of rhomb<a> slip and alternating dominance of regime I and regime II. Recrystallization textures are dominated by subgrain rotation, with lessor grain boundary migration and bulging recrystallization, Together, these observations suggest temperatures of deformation between 400-500°C. New and previously published argon thermochronologic data indicates that most of the footwall had cooled below 350°C by 20Ma, suggesting mylonitic deformation occurred prior to the Miocene unroofing event. We suggest that the mylonitic deformation of the footwall occurred during an earlier (Eocene) unroofing evnt following late Cretaceous thickening.