NEW EVIDENCE FOR OLD SUBDUCTION IN THE CATALINA SCHIST, SANTA CATALINA ISLAND, CA

Recent geochronology has suggested that the Catalina Schist formed between 115 and 95 Ma – an age range that is difficult to reconcile with the onset of Middle - Late Jurassic (180 – 150 Ma) subduction recorded by the Franciscan complex and the Western Baja terrane. Tenuous ⁴⁰Ar/³⁹Ar and Rb-Sr ages from garnet-bearing blueschist blocks within the lawsonite-blueschist unit of the Catalina Schist, however, indicate that portions of the Catalina Schist were metamorphosed before 115 Ma. This study examined a ~25m garnet-blueschist tectonic block hosted by lawsonite-blueschist-facies mélange. The block contains an assemblage of glaucophane + lawsonite + phengite + titanite + garnet ± quartz that is typical of blueschist-facies metamorphism of oceanic basalt. Whole-rock geochemical analysis of the block shows notable variability of several elements (e.g. SiO₂, MgO, total Fe₂O₃, Th) suggesting the block has experienced considerable alteration. In particular, the ~20% variation of SiO₂ from core to rim of the block is evidence for alteration due to metasomatism. Major and trace-element data from the least altered portions suggest that the garnet-blueschist block is similar to N-MORB or island-arc tholeiite. Laser-ablation split stream analyses were conducted at the University of California Santa Barbara for geochronology and titanite trace-element geochemistry. Titanite from the garnet-blueschist block contains 4.77±2.5 ppm Zr. Zr-in-titanite thermometry suggests a temperature of 603±21 °C at an estimated pressure of 10 kbar. Titanite U-Pb analyses indicate a metamorphic age of 155 ± 8 Ma for the garnet-blueschist block. This age is coeval with Franciscan and Western Baja subduction. The presence of a thermally mature subduction zone near Catalina prior to 115 Ma further bolsters the recently revised tectonic model proposed by Grove et al. (2008).