Chemical fine structure of Franciscan jadeitic pyroxene from Ward Creek, Cazadero area, California

Jadeitic pyroxenes in two Franciscan samples, one, a possible segregation v ein in metabasite, and another, a metabasite, both from Ward Creek, Cazader o area, western California, were examined with the electron microprobe. The samples were collected from the area of Type III rocks, the highest-grade portion of the pumpellyite zone. Jadeitic pyroxenes from both samples are v ery heterogeneous and cannot be ascribed to crystallization under condition s of surface equilibrium with the coexisting phases. Two possibilities for their origin are suggested: one is as tectonic blocks, and the other as the highest-grade rocks from the zone that crystallized close to or within the P-T stability field of pure jadeite + quartz.