APATITE U-PB PETROCHRONOLOGY TO DATE DEFORMATION AND METASOMATISM ALONG THE SUBDUCTION PLATE INTERFACE IN THE CATALINA SCHIST, SANTA CATALINA ISLAND, CA

The timing and nature of deformation and metasomatic processes in subduction zones are challenging to date due to the lack of viable in-situ geochronometers, particularly in blueschist and greenschist facies rocks. Apatite is present in most subduction zone lithologies and has a U-Pb temperature sensitivity of 350–550°C that overlaps with blueschist-greenschist facies P-T conditions, making apatite potentially well suited to place absolute time constraints on deformation and metasomatism recorded in these rocks. We are investigating the Catalina Schist, CA—an exhumed subduction complex from paleodepths corresponding to the base of the seismogenic zone. We collected samples across lawsonite-blueschist to upper-amphibolite facies units, targeting outcrops with evidence of deformation and fluid flow, for EBSD and U-Pb analyses to understand how apatite behaves across a range of P-T conditions in subduction zones. Lawsonite-blueschist facies mica schists show apatites aligned with the c-axis parallel to the main foliation and evidence of subgrains suggesting deformation by dislocation creep. Quartz microstructures in the schist show bulging recrystallization and incipient subgrain rotation with evidence of pressure solution in the mica matrix, consistent with temperatures around 400°C. An intermediate facies metachert contains glomerocrysts of randomly oriented apatite grains likely reprecipitated during fluid flow. Quartz microstructures in the metachert show evidence of bulging to subgrain rotation recrystallization regimes consistent with blueschist-greenschist facies temperatures. High-temperature samples from the coherent amphibolite facies unit contain apatites with little to no intragranular misorientations, likely due to high-temperature annealing processes. We will continue characterizing apatite grains with additional EBSD from the blueschist-intermediate facies units to investigate temperature-dependent deformation of apatite in comparison to quartz deformation mechanisms. U-Pb dating of dynamically recrystallized and reprecipitated apatite will provide constraints on the timing of (re)crystallization during deformation and fluid-rock interactions at blueschist-greenschist facies conditions (350–550°C).