Glaucophane-bearing Metamorphic Rock Types of the Cazadero Area, California

A detailed field and laboratory study has been made of a well-exposedglaucophane schist sequence within the Jurassic and CretaceousFranciscan Formation of northern California. Three types ofglaucophane-bearing metamorphic rocks have been distinguishedin and around the area of the detailed study. Each is characterizedby distinctive textures and mineral assemblages that are interpretedto represent different grades of metamorphism within the glaucophaneschist facies. From the combination of small- and large-scale mapping in thearea described it is clear that coarsely schistose blocks, tensof feet in diameter, commonly rest directly upon and withinless intensely metamorphosed terrain. In the Cazadero area theseisolated blocks of coarsely crystalline rocks are concentratedin a band that is roughly concordant with some of the majorfaulting, and their metamorphic fabric shows no consistent relationto local or regional structures. It is tentatively suggestedthat these blocks have been transported upward tectonicallyand that they are not stratigraphically equivalent to the othertypes of glaucophane-bearing metamorphic rocks in the area. Chemical and petrographic evidence indicates that basalt andsediments have been converted to glaucophane-bearing rocks underconditions of metamorphism that were essentially isochemical,except for fugitive components and some minor elements.