Textural evolution and metamorphism of pillow basalts from the Franciscan Complex,western Marin County,California

Petrogenesis of Franciscan pillow basalts from the Franciscan Complex of western Marin County California entails both dynamic crystallization of tholeiitic magma and subsequent low-temperature metamorphism. Brittle deformation during tectonic emplacement of pillow basalts into a chert greywacke terrain is manifested by the shearing of interpillow matrix and polishing of pillow rims, but the igneous textures within pillows are well preserved.The cooling history of pillow basalts can be understood through analysis of morphologic variations of primary olivine and plagioclase from rim to core of the pillow. Crystal sizes and plagioclase dendrite spacings are consisted with a cooling rate which generally decreases inward. Some pillows show a marked asymmetry in plagioclase and olivine morphology suggesting lower cooling rates caused by asymmetric cooling of the pillows. Olivine morphologies, primarily hopper and chain forms, are consistent with cooling rates of 2–10 °C/h for pillow cores and 50–75 °C/h for pillow rims.Low temperature hydrothermal alteration has produced secondary minerals indicative of zeolite facies conditions. Pillow matrix is either chloritic or zeolitic (in part laumontized). Pillow rims display incomplete replacement of calcic palagonite by pumpellyite (Fe₂O₃=9–21 wt%), prehnite (Fe₂O₃=5–7 wt%), sphene and quartz. Metamorphism of pillow interiors, manifested by: (1) veins of quartz, pumpellyite, calcite, or harmotome (BaO=15 wt%); (2) amygdules containing analcime, chlorite or quartz; and (3) replacement of olivine by pumpellyite or smectite/illite, of plagioclase by albite (An3)+sericite, and of glassy groundmass by fine-grained chlorite. Primary augite (Wo3₃₉En₁₃Fs₄₈) was not altered. The described paragenesis may be attributed to oceanfloor and/or Franciscan-type metamorphism.