ZR-IN-RUTILE THERMOMETRY OF THE CATALINA SCHIST MÉLANGE, ESTIMATING ASIO2 IN THE ABSENCE OF QUARTZ, AND IMPLICATIONS FOR SUBDUCTION INTERFACE RHEOLOGY

The Catalina Schist contains a spectacular km-scale amphibolite facies mélange zone, thought to be part of a Cretaceous convergent margin plate interface. In the mélange, mafic and ultramafic blocks ranging from cms up to 100s of m in diameter are surrounded by finer-grained matrix that was derived from the blocks. All blocks contain assemblages consistent with upper amphibolite-facies conditions, suggesting fomation over a relatively restricted range of depths and temperatures. This apparent uniformity contrasts with other mélanges, such as the Franciscan Complex, where closely proximal rocks with highly variable peak metamorphic grade suggest extensive mixing of materials along the subduction interface. This mixing has been ascribed to flow of material within relatively low viscosity matrix. In this study, we measured the Zr content of rutile in block and matrix samples by EPMA and LA-ICP-MS to determine peak metamorphic temperatures, identify whether these temperatures differed among blocks, and whether the spatial distribution of temperatures throughout the mélange varied systematically or randomly. The equilibrium assemblage rutile + quartz + zircon occurs in all but four samples, which lack quartz. Following Ferry and Watson (2007), we estimated a_SiO2 (ranged from 0.94 to 1.07) using a calibrated equilibrium among amphibole, rutile, and quartz, resulting in relatively small changes in calculated temperatures, up to 6ºC lower. Resolvably different Zr contents, between 290 and 720 (±10-40) ppm (2σ), are found among the blocks, corresponding to different peak metamorphic temperatures of 650 to 730 (±3-15) °C at an assumed pressure of 1 GPa. No systematic distribution of temperatures was found, however. Material flow within the Catalina Schist mélange was likely chaotic, but appears to have occurred on a relatively restricted scale. Progressive metamorphism should progressively stiffen mélange matrix by forming stronger amphiboles and pyroxenes at the expense of weaker sheet silicates. Such stiffening would affect the overall rheological behavior of the mélange and dictate the scale of flow. The Catalina Schist amphibolite facies provides a snapshot of hot stiff portions of a subduction interface, likely representative of rheological behavior at depths approaching the subarc.