Glaucophane-schists and associated rocks near Valley Ford, California

Large residual masses of Franciscan glaucophane-schist, eclogite, and associated metamorphic rocks rest upon a basement of Franciscan graywackes and metagraywackes. The residual metamorphic blocks are probably relict masses from a former cover. Various stages in the transformation of graywackes and cherts into glaucophane-rocks have been observed in both basement rocks and residual masses. Progressive glaucophanization of residual blocks of eclogite also occurs, the final mineralogical and chemical composition approaching that of glaucophane- rocks derived from graywackes. Chemical changes accompanying the metamorphism of the gray-wackes involve an addition of Fe, Mg, and Ca, together with a decrease in Si and Na. These changes contrast with those which have produced glaucophane-rocks from the eclogites. The mineralogical character of the glaucophane-schists suggests affinities with the greenschist or epidote-amphibolite facies, but the instability of albite and the presence of hydrous minerals with high density indicates certain conditions peculiar to the glaucophane-schist facies; possibly high water-vapor pressure.