A MÉLANGE OF SUBDUCTION TEMPERATURES: ZR-IN-RUTILE AND ZR-IN-TITANITE THERMOMETRY OF THE CATALINA SCHIST, CA, AND ITS TECTONIC IMPLICATIONS

The Catalina Schist is a spectacular thermally inverted sequence of lower blueschist to upper amphibolite facies rocks. It consists of blocks in mélange interleaved with more coherent metasedimentary rocks, much of which attained the same peak P~1 GPa. The prevailing tectonic model (Grove et al., 2008) invokes earlier metamorphism of high-grade rocks in a forearc thrust system adjacent to hot arc plutons, with progressive underplating of younger, cooler metamorphic rocks derived from the main subduction channel over a period of ~20 Ma. Maximum Zr contents of rutile cluster in each sample and constrain maximum temperatures. In three metasedimentary and metamafic amphibolite-facies rocks, these are between 405 and 465 ppm (all uncertainties are ±35-40 ppm, 2σ), corresponding to an average T=680±10°C. Rutile in one amphibolite facies sample contains resolvably different maximum Zr of 555 ppm corresponding to 700±5 °C. Rutile from two epidote amphibolite facies metasedimentary rocks contains 100 and 155 ppm Zr or 570±20 and 600±15 °C. Rutile from a garnet blueschist block hosted within the lower blueschist facies unit contains 130 ppm Zr or 590±15 °C. These results are the first quantification of temperatures for epidote amphibolite and garnet blueschist facies rocks. Titanite Zr contents vary by a factor of 2-3 in epidote amphibolite facies samples, but at 1 GPa calculated Zr-in-titanite temperatures are consistently higher than Zr-in-rutile, with minimum T = 625-640 °C, and maximum T = 670 °C. Sector zoning may cause spuriously high temperatures; we infer a likely maximum Zr-in-titanite T = 640±10 ºC. Because Zr-in-titanite is moderately pressure-dependent, titanite could alternatively have formed during isothermal exhumation at T~ 600 °C and P~0.65 GPa. The published T-t history for each metamorphic zone is based on several chronologic systems applied to different samples within that zone. Variable trace element temperatures suggest that rocks within each unit experienced resolvably different P-T-t histories, e.g. the garnet blueschist block is older (Grove et al. 2008) and possibly 300 °C hotter than its hosting unit. Although ages may broadly decrease towards lower structural levels, block-by-block petrologic and chronologic analysis is needed to fully test tectonic models of subduction underplating.