LOW-GRADE BLUESCHIST FACIES METAMORPHISM OF METAGREYWACKES, FRANCISCAN COMPLEX, NORTHERN CALIFORNIA

Metagreywackes in the Eastern Belt of the Franciscan Complex contain t he assemblage: Qtz + Ab + Lws + Chl + Ph + Pmp + Fgl + Hem +/- Cal/Arg or compatible subassemblages. Blue amphibole first appears in the wes ternmost part of the belt and pumpellyite is absent in the eastern par t. The compositions of the coexisting minerals and the nature of the c ontinuous reactions in these low-grade blueschists suggest that the di stribution of blue amphibole and pumpellyite in the Eastern Belt of th e Franciscan Complex reflects differences of effective bulk compositio n rather than differences in physical conditions of metamorphism. In r ocks lacking pumpellyite, white mica may be essential to the growth of blue amphibole, but carbonate plays only a limited role. The continuo us reaction that limits the appearance of blue amphibole and the disap pearance of coexisting pumpellyite has the general form: Pmp + Chl + A b + Qtz + Hem + H2O + FeMg-1 = Fgl + Lws. This reaction requires signi ficant hydration as pressure increases in order to produce blue amphib ole. Most of the Eastern Belt of the Franciscan Complex formed in limi ted ranges of temperature and pressure, which are estimated to be 240- 280-degrees-C. 6.5-7.5 kbar. Pressures in the westernmost part of the area were about 1 kbar lower than in the east. Pressures of about 8.5- 10 kbar are estimated for tectonic blocks that contain sodic clinopyro xene.