ORIGIN OF GREEN COLOR IN LAPILLI TUFF FROM THE PINNACLES VOLCANIC FORMATION, CALIFORNIA

Green pumice lapilli tuff from the Pinnacles Volcanic Formation was studied in order to determine the origin of the green color. Together with its offset sibling, the Neenach Volcanic Formation, this 23- to 24-Ma formation located in the Gabilan Range of western central California provides robust evidence for 315 km offset on the San Andreas fault. The Pinnacles Volcanic Formation is composed of seven members totaling about 2 km in thickness, The pumice lapilli tuff member, of probable overall rhyolite composition, is composed of ca 30% quartz, 30% secondary analcime, 20% plagioclase, 10% pyroxene, <5% K-feldspar, and 5% phyllosilicates. The mineral(s) responsible for the green color is extremely fine grained and irregularly disseminated; many lapilli are completely green. Microprobe and XRD results rule out chlorite, illite, biotite, and mixed phyllosilicates; both sets of data permit celadonite and glauconite. Celadonite is rarely green, and our analyses are distinctly lower in Mg and off the Al-Fe trend compared to literature data for celadonite. Glauconite is characteristically green in color and is a more likely candidate, despite slight differences in Al and Fe between our analyses and those in the literature. However, glauconite usually forms rounded, fine grained aggregates of ill formed platelets or rarely as better formed crystals in a quiet marine environment in the presence of organic material; the Pinnacles tuff was deposited in a dynamic environment with little organic material likely present. We suggest that the origin of glauconite is less restricted than previously thought.