Mineral chemistry of some Franciscan blueschist facies metasedimentary rocks from the Diablo Range, California: Summary

Abstract

In recent years it has been demonstrated that much of the Franciscan Complex of California has undergone zeolite or blueschist facies metamorphism (McKee, 1962a, 1962b; Coleman and Lee, 1962, 1963; Ernst, 1965, 1971a, 1971b, 1971c; Bloxam, 1956, 1960, among others). Observed mineral assemblages are consistent with experimentally derived mineral stabilities and are suggestive of progressive metamorphism in a subduction-zone environment (Ernst, 1971a, 1971b). The mineral chemistry of some of these Franciscan metamorphic rocks has been studied; most investigations have centered on individual minerals-sodic amphibole (Coleman and Papike, 1968), sodic pyroxene (Essene and Fyfe, 1967; Coleman and Clark, 1968), and garnet (Lee and others, 1963)-with particular emphasis on high-grade exotic blocks. A few studies have been conducted to characterize specific metamorphic terrances within the Franciscan (Crawford, 1975) or to compare. Franciscan metamorphism with that of other subduction-zone complexes (Ernst and Seki, 1967; Ernst and others, 1970). Thus far, however, little work has been done to compare the composition of minerals crystallized at different grades of metamorphism in Franciscan metasedimentary terrances.