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 2M1 white K-mica polytype, and phase assemblages for shales and shalematrix melange from various areas in the Diablo Range. Additional information concerning metamorphic conditions and fluid composition was derived from stable isotope work and fluid inclusion measurements. This study presents new constraints on the pressure-temperature evolution of the Franciscan units from the Diablo Range. Several shales indicate a lower pressure, possibly higher temperature overprint on the peak pressure assemblages. Peak pressure assemblages include jadeitic pyroxene, sodic amphibole, lawsonite, aragonite, and phengite. Reactions, which indicate a lower pressure, possibly higher temperature overprint, are the transition from aragonite to calcite, the formation of albite from jadeitic pyroxene + quartz, the compositional gradient from phengite to muscovite in white K-mica, and the transition from sodic to calcic amphibole. The assemblage albite, calcic amphibole, lawsonite, and chlorite found in a single shale from Pacheco Pass indicates the transition from blueschist to greenschist facies conditions. We suggest that this transition occurred at a pressure and temperature of 5-8 kilobars and 300-350°C. Fluid inclusion data from post-kinematic quartz veins from the Mount Hamilton area support the idea of a lower pressure, possibly higher temperature metamorphic event. Pressure-temperature conditions of trapping early H2O- and CH4-rich fluids were 4.5 kilobars and ≈230 °C. Geothermal gradients for the transition from blueschist to greenschist facies and the formation of early fluid inclusions are estimated to be ≈14 °C/km. Illite crystallinity for all areas indicates diagenetic to high-grade anchizonal conditions; coal ranks are between 1.6% and 2.5% mean Ro. These estimates are not consistent with metamorphic conditions deduced for two different shale assemblages from Pacheco Pass, suggesting that illite crystallinity and coal rank are retarded. Poor illite crystallinity is thought to result from small coherent scattering domain sizes along c* of white K-mica crystals rich in a phengite component (Dalla Torre et al., 1994a, in press). The exact cause of the retardation of coal rank is not clear. The high pressure in the Diablo Range may have affected 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.