VERY LOW-TEMPERATURE METAMORPHISM OF SHALES FROM THE DIABLO RANGE, FRANCISCAN COMPLEX, CALIFORNIA - NEW CONSTRAINTS ON THE EXHUMATION PATH

We have determined illite crystallinity, coal rank, percent 2M(1) whit e K-mica polytype, and phase assemblages for shales and shale-matrix m elange from various areas in the Diablo Range, Additional information concerning metamorphic conditions and fluid composition was derived fr om stable isotope work and fluid inclusion measurements. This study pr esents new constraints on the pressure-temperature evolution of the Fr anciscan units from the Diablo Range, Several shales indicate a lower pressure, possibly higher temperature overprint on the peak pressure a ssemblages, Peak pressure assemblages include jadeitic pyroxene, sodic amphibole, lawsonite, aragonite, and phengite. Reactions, which indic ate a lower pressure, possibly higher temperature overprint, are the t ransition from aragonite to calcite, the formation of albite from jade itic pyroxene + quartz, the compositional gradient from phengite to mu scovite in white K-mica, and the transition from sodic to calcic amphi bole, The assemblage albite, calcic amphibole, lawsonite, and chlorite found in a single shale from Pacheco Pass indicates the transition fr om blueschist to greenschist facies conditions, We suggest that this t ransition occurred at a pressure and temperature of 5-8 kilobars and 3 00-350 degrees C. Fluid inclusion data from post-kinematic quartz vein s from the Mount Hamilton area support the idea of a lower pressure, p ossibly higher temperature metamorphic event, Pressure-temperature con ditions of trapping early H2O- and CH4-rich fluids were 4.5 kilobars a nd approximate to 230 degrees C. Geothermal gradients for the transiti on from blueschist to greenschist facies and the formation of early fl uid inclusions are estimated to be approximate to 14 degrees C/km. Ill ite crystallinity for all areas indicates diagenetic to high-grade anc hizonal conditions; coal ranks are between 1.6% and 2.5% mean Ro. Thes e estimates are not consistent with metamorphic conditions deduced for two different shale assemblages from Pacheco Pass, suggesting that il lite crystallinity and coal rank are retarded. Poor illite crystallini ty is thought to result from small coherent scattering domain sizes al ong c of white K-mica crystals rich in a phengite component (Dalla To rre et al., 1994a, in press), The exact cause of the retardation of co al rank is not clear, The high pressure in the Diablo Range may have a ffected the maturation of organic matter, Our results imply that coal rank data from high-pressure terranes must be interpreted with caution if used as an absolute paleothermometer.