TECTONIC SIGNIFICANCE OF MYLONITES IN THE LITTLE BUCKSKIN MOUNTAINS, WEST-CENTRAL ARIZONA: INSIGHTS FROM QUARTZ MICROSTRUCTURAL DATA

Quartz microstructural data suggest the Little Buckskin Mountains in west-central Arizona underwent amphibolite-facies, top-to-the-NE directed shear in the Late Cretaceous to early Tertiary. The Little Buckskin Mountains consist primarily of mylonites exposed for ~10 km in the NE-SW shearing direction. These mylonites have previously been interpreted to record Miocene extensional shear associated with exhumation of mid-crustal rocks exposed in the Buckskin-Rawhide metamorphic core complex. We determined quartz recrystallization mechanisms and sizes of dynamically recrystallized quartz grains for 22 samples to evaluate the deformation conditions under which mylonites in the Little Buckskin Mountains formed. These samples primarily record a top-to-the-NE sense of shear and exhibit two distinct quartz grain size populations: 7 samples have mean grains sizes ≤~70 μm, and 15 samples have mean grain sizes from ~130-400 um. The larger grain sizes are incompatible with Miocene mylonitization in the region, which is typically characterized by ~20-60 um recrystallized quartz grains. Quartz dynamic recrystallization in 16/22 samples is dominated by grain boundary migration, suggesting amphibolite-facies conditions, which is also incompatible with the <500˚C deformation temperatures present in the Miocene. Amphibolite-facies, top-NE shearing likely occurred in the Late Cretaceous to early Tertiary, as Late Cretaceous granites are strongly deformed, but the range had cooled to <550°C by 63-45 Ma based on 40Ar/39Ar hornblende ages. Quartz grain sizes in amphibolite-facies mylonites increase modestly to the NE, suggesting that the NE part of the range was deeper that the SW during Late Cretaceous/early Tertiary shearing, and that these mylonites record a normal sense of shear. Greenschist-facies fabrics that most likely formed in the Miocene appear to be primarily restricted to metasedimentary rocks along the flanks of the range.