Mineral paragenesis in Franciscan metagraywackes of the Nacimiento Block, a subduction complex of the Southern California Coast Ranges

Abstract

Geologic‐petrographic reconnaissance of Franciscan metagraywackes from the Nacimiento block of the southern California Coast Ranges has demonstrated a perceptible eastward increase in metamorphic grade. Most feebly recrystallized zone I metaclastic rocks bordering the Pacific Ocean are rich in calcite veins and patches and contain no new Ca‐Al hydrous silicates; abundant unstable detritus includes lithic fragments, biotite flakes, and epidote granules. Metasedimentary rocks of medial zone II are characterized by fine‐grained intergrowths of pumpellyite ±5 minor regenerated epidote‐clinozoisite. Easternmost zone III metasandstones contain lawsonite, rarely aragonite and/or jadeitic pyroxene. In general, proportions of calcite, rock fragments, epidote, and biotite decrease eastward, and white mica, chlorite, and stilpnomelane increase. On the basis of experimental phase equilibrium data, temperatures of 150±50°C are estimated to have attended recrystallization, with lithostatic pressures increasing eastward from about 2–3 kbar on the west to approximately 5 kbar, and locally approaching 8 kbar, on the east. Absence of zeolites in the lowest grade zone I metagraywackes may indicate dilution of the aqueous fluid phase by CO2 during the metamorphism. In combination with the northwest tectonic ‘grain’ of the Nacimiento block and its inferred imbricate structure, the assemblage of rock types, the general north‐eastward dip of bedding, and predominantly right‐side‐up nature of the section, this continental margin deposit is thought to have been recrystallized during a period of relatively rapid (and perhaps brief) Late Cretaceous plate convergence. The lack of eclogite and amphibolite as tectonic blocks may indicate less profound depths of recovered subduction compared to Franciscan basement lying northeast of the San Andreas fault. Metamorphosed Nacimiento lithic belts evidently were assembled over a brief period of subduction offscraping at the western edge of the American lithospheric plate and seem to mirror events which unfolded over a longer time interval in Franciscan rocks of the northern California Coast Ranges.