THE METAMORPHIC AND FLUID EVOLUTION OF THE ONLY MAINLAND EXPOSURE OF THE CATALINA SCHIST: GEORGE F CANYON, THE PALOS VERDES HILLS

The Catalina Schist at George F Canyon in the Palos Verdes Hills was metamorphosed to produce lawsonite-blueschist, epidote-blueschist, greenschist and possibly epidote-amphibolite rocks. Our samples contain no amphibolite facies rocks. Outcrops are highly deformed and reflect multiple episodes of tight folding, but foliation patterns define an antiform at the lower canyon. Discontinuities in foliation and lineation directions from N45°E to N135°E between the upper and lower canyon suggest the presence of a fault within the schist. Microtextures in thin section indicate recrystallization of quartz during metamorphism under ductile conditions. A mapped isograd reveals that the blueschist facies rocks are structurally below the greenschists, suggesting the preservation of an inverted thermal gradient. The nature of this isograd is poorly understood. Lawsonite blueschists (low-T blueschist, 250-400°C, > 8 kb) have quartz+white-mica+lawsonite+ chlorite assemblages. Epidote blueschists (high-T blueschist, 400-500°C, > 9 kb) have mineral assemblages of quartz+white-mica+glaucophane+epidote. Greenschists (350-500°C, 2-8 kbar) contain quartz+white-mica+feldspar+chlorite±epidote±actinolite. Most samples are quartz-rich and likely had siliceous sandstone or graywacke protoliths, but some mafic talc-bearing schists were found. Late calcite veins and alteration are common and most likely formed during Miocene exhumation. Chlorite (–65.3‰ to –63.5‰), muscovite (–57.4‰ to –52.2‰), amphibole (–55.4‰ to –54.3‰), glaucophane (–65.9‰ to –57.7‰), and talc (–60.5‰) δD values indicate equilibrium with high-D/H metamorphic waters (~ –20‰) at T ~ 450°C. These waters overlap those of Cretaceous seawater. Shear zones in the over-riding continental crustal plate (e.g. Eastern Peninsular Ranges Mylonite Zone) may have served as paths of fluid expulsion. These mainland exposures of Catalina Schist share a common subduction zone-related tectonometamorphic history with the better-studied exposures of Catalina Schist on Santa Catalina Island (e.g., Bebout and Barton, 1993). This study is the first known use of the PIMA portable infrared spectrometer to identify hydrous and carbonate minerals formed during metamorphism.