HIGH-PRECISION U-PB DATING OF NEOBLASTIC SPHENE FROM MYLONITIC ROCKS OF THE COUGAR CREEK COMPLEX, BLUE MOUNTAINS PROVINCE, OREGON – IDAHO: IMPLICATIONS FOR THE INTERPLAY BETWEEN DEFORMAITON AND ARC MAGMATISM

The Cougar Creek Complex (CCC) is interpreted as the mid-crustal level of the Wallowa-Seven Devils (WSD) arc and is constructed of numerous, variably deformed dikes and plutons that record two distinct cycles of magmatism. ²⁰⁶Pb/²³⁸U crystallization ages of magmatic zircon and sphene define older Middle Permian to Early Triassic silicic magmatism and younger Late Triassic mafic to intermediate intrusive activity. These two separate episodes of magmatic activity are substantiated by abundant field evidence observed throughout the complex where relatively younger mafic to intermediate intrusive units crosscut older silicic dikes and plutons. Petrographic and microstructural analysis of two plastically deformed Middle Permian tonalite screens identified neoblastic sphene within dynamically recrystallized quartzo-feldspathic shear bands which show a well-developed left-lateral, moderately oblique to strike-slip sense of displacement. Multi-grain and single-grain U-Pb sphene analyses from one sample gave ²⁰⁶Pb/²³⁸U ages from 229.50 ± 0.33 Ma to 224.80 ± 0.39 Ma indicating Late Triassic crystallization and deformation. Multi-grain and single-grain ages of sphene from the second dike vary from 260.04 ± 0.33 Ma to 220.17 ± 0.49 Ma indicating growth of new grains and the crystallization of younger material over existing grains. These new high-precision age constraints indicate that left-lateral mylonitization of Middle Permian tonalitic rocks is synchronous with Late Triassic magmatism and provide valuable insight pertaining to: 1) the tectonic evolution of the WSD and the Blue Mountains Province (BMP) as a whole, and 2) the interplay between deformation and magmatism at the mid-crustal level of an actively shearing arc axis environment. Our data suggests that the WSD underwent left-lateral, oblique subduction in the Late Triassic which would in turn point to a southerly transport direction of the WSD at that time, relative to the subducting plate. Late Triassic deformation is also contemporaneous with Late Triassic hornblende cooling ages indicating rapid uplift likely associated with the amalgamation of the BMP. Finally, the synchrony of magmatism and deformation help describe how the strike-slip component of oblique subduction is partitioned into the thermally weakened axis of an active arc system.