DIFFERENCE BETWEEN ELEMENTAL CONCENTRATIONS IN PSEUDOTACHYLYTE, CONTACT WHITE NATURAL GLASS AND PARENT RHYOLITE TUFF FROM THE HEALDSBURG - RODGERS CREEK FAULT ZONE, TAYLOR MOUNTAIN, SONOMA COUNTY, CALIFORNIA

On Taylor Mountain the Rodgers Creek fault zone is composed of bands of dark gray pseudotachylyte, parallel bands of light colored contact metamorphic glass, and a few scattered knots of parent high silica rhyolite tuff. Microdikes of pseudotachylyte intrude the white glass. The pseudotachylyte contains a few scattered spherical vesicles. Microprobe, XRF, mass spectroscopy, and wet geochemical analyses show that all three glasses have somewhat similar but systematically different bulk chemical compositions. The chemical variability of the light colored glass is much greater than that of the pseudotachylyte or parent rhyolite. The pseudotachylyte has about 4 percent more silica, but it has one third as much iron, one fifth as much magnesium, and slightly lower calcium than the parent rock. The other major cations are similar in concentration in the parent and the pseudotachylyte. The pseudotachylyte is distinctly lower in the minor elements sulfur, phosphorous, strontium, zircon, barium, and higher in rubidium and chlorine. Total iron in the tuff is about 2%, of which about one seventh is FeO. The pseudotachylyte has one third as much total iron as the parent rock, but nearly one third of that iron is in the reduced form. The concentrations of lanthanide elements in the parent and fault glasses is nearly identical except for europium, Eu, which is 0.60 ppm in the parent, 0.34 in the white glass and 0.21 in the pseudotachylyte. Except for thulium, Tm and ytterbium, Yb the two fault glasses are identical. The tuff is systematically a little richer than the fault glasses in the light lanthanides, lanthanum, La, cerium, Ce, praeseodymium, Pr, niobium, Nb, and samarium, Sm, and a little poorer in the heavy lanthanides, gadolinium, Gd, terbium, Tb, dysprosium, Dy, holmium, Ho, erbium, Er, thulium, Tm and lutetium, Lu.

Differences between pseudotachylyte and its parent are usually attributed to differential melting of the parent minerals but the Rodgers Creek fault glasses are clearly derived from tuff which is almost entirely glass. This strongly suggests that some kinetic process, during melting, favors greater incorporation of certain elements in the short lived pseudotachylyte melt. If so, we may be over emphasizing the importance of differential parent mineral response in other regions.