Paper No. 5
Presentation Time: 10:00 AM


AWALT, Mitchell B.1, PAGE, F. Zeb2, WALSH, Emily O.3, KYLANDER-CLARK, Andrew4 and WIRTH, Karl R.1, (1)Department of Geology, Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105, (2)Department of Geology, Oberlin College, 52 West Lorain Street, Oberlin, OH 44074, (3)Department of Geology, Cornell College, 600 First St. SW, Mt. Vernon, IA 52134, (4)Department of Earth Science, University of California, Santa Barbara, CA 93106,

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).