Petrology of franciscan metabasites along the jadeite-glaucophane type facies series, Cazadero, California

The Cazadero blueschist allochthon lies within the Central Melange Belt of the Franciscan assemblage in the northern Coast Range of California. Mineral compositions and assemblages of more than 200 blueschists from Ward Creek were investigated. The results delineate lawsonite-, pumpellyite-, and epidote-zones. The lawsonite and pumpellyite zones are equivalent to the Type II metabasites of Coleman & Lee (1963) and are characterized by well-preserved igneous textures, relict augite, and pillow structures, whereas epidote zone rocks are equivalent to the Type III strongly deformed and schistose metabasites. Chlorite, phengite, aragonite, sphene, and minor quartz and albite are ubiquitous.The lawsonite zone metabasites contain lawsonite ( < 3 wt. per cent Fe₂O₃), riebeckite-crossite, chlorite, and Ca-Na-pyroxene; some rocks have two distinct clinopyroxenes separated by a compositional gap. The clinopyroxene of the lowest grade metabasites contains very low X_jd. In pumpellyite zone metabasites, the most common assemblages contain Pm + Cpx + Gl + Chl and some samples with higher Al₂O₃ and/or Fe₂O₃ have Pm + Lw + Cpx + Chl, Actinolite joins the above assemblage in the upper pumpellyite zone where the actinolite-glaucophane compositional gap is well defined. The epidote zone metabasites are characterized by the assemblages Ep + Cpx + two amphiboles + Chl, Lw + Pm + Act + Chl, and Ep + Pm + two amphiboles + Chl depending on the Fe₂O₃ content of the rock. In the upper epidote zone, winchite appears, Fe-free lawsonite is stable, pumpellyite disappears and omphacite contains very low Ac component. Therefore, the common assemblages are Ep + winchite + Lw, and Lw + Omp + winchite. With further increase in metamorphic grade, epidote becomes Al-rich and lawsonite is no longer stable. Hence Ep + winchite + omphacite ± garnet is characteristic.Mineral assemblages and paragenetic sequences delineate three discontinuous reactions: (1) pumpellyite-in; (2) actinolite-in; and (3) epidote-in reactions. Using the temperatures estimated by Taylor & Coleman (1968) and phase equilibria for Ca-Na-pyroxenes, the P-T positions of these reactions and the metamorphic gradient are located. All three metabasite zones occur within the aragonite stability field and are bounded by the maximum pressure curve of Ab = Jd + Qz and the maximum stabilities of pumpellyite and lawsonite. The lawsonite zone appears to be stable at T below 200°C with a pressure range of 4-6·5 kb; the pumpellyite zone between 200 and 290°C and the epidote zone above 290°C with pressure variation between 6·5 and 9 kb. The metamorphic field gradient appears to have a convex nature towards higher pressure. A speculative model of underplating seamounts is used to explain such feature.