POLYPHASE MYLONITIZATION IN THE HARCUVAR AND BUCKSKIN-RAWHIDE METAMORPHIC CORE COMPLEXES, WEST-CENTRAL ARIZONA

Mylonites in the lower plate of the Harcuvar and Buckskin-Rawhide metamorphic core complexes in west-central Arizona record multiple episodes of top-NE-directed shearing at conditions ranging from middle amphibolite facies to lower greenschist facies. We combine microstructural analysis from >150 samples and geo/thermochronology to document the distribution, deformation conditions, and timing of this mylonitization. Late Cretaceous leucogranite mylonites across the lower plate are typically characterized by quartz grain boundary migration recrystallization, 70-250 μm mean quartz grain sizes, and feldspar subgrain rotation recrystallization. Ti-in-quartz geothermometry indicate that these fabrics formed at ~520-600°C, mostly prior to cooling below hornblende Ar closure in the early Tertiary (~64-43 Ma). Late Cretaceous to early Tertiary U-Pb ages (~74-60 Ma) from synkinematic titanite in several samples directly record the timing of mylonitization, which overlapped with widespread leucogranite emplacement. These amphibolite-facies mylonites consistently record a top-NE sense of shear that parallels the Miocene detachment fault slip direction. Miocene mylonitic fabrics are most clearly recorded in ~22-21 Ma granitoids exposed across the central portion of the Buckskin-Rawhide core complex. These granitoids are characterized by quartz subgrain rotation recrystallization, 20–60 μm quartz grain sizes, and feldspar fracturing+bulging recrystallization, suggesting ~400-500°C deformation temperatures that were fairly uniform across the lower plate. Greenschist-facies mylonitic fabrics are pervasive across much of the Buckskin-Rawhide core complex, but appear to be largely restricted to a 150–250 m-thick carapace along the adjacent Harcuvar core complex. In many samples greenschist-facies fabrics overprint amphibolite-facies microstructures. Additionally, greenschist-facies mylonites have very a similar geometry and sense of shear as the structurally lower amphibolite-facies mylonites, suggesting that the Miocene shear zone reactivated the Late Cretaceous-early Tertiary shear zone. This study highlights the polyphase nature of the core complex development and the role of older tectonism in influencing the location and geometry of Miocene mid-crustal strain.