DETERMINING PRE-ERUPTIVE COMPOSITIONS OF LATE PALEOZOIC MAGMA FROM KAOLINIZED VOLCANIC ASHES - ANALYSIS OF GLASS INCLUSIONS IN QUARTZ MICROPHENOCRYSTS FROM TONSTEINS

Glass inclusions in quartz microphenocrysts were analyzed for major an d minor elements by electron microprobe and H, Li, Be, B, Rb, Sr, Y, N b, Mo, Sn, Cs, Ce, Th, and U by ion microprobe. The phenocrysts and in clusions occur as fresh relicts in about eleven strongly kaolinized, a ir-fall volcanic ash units (tonsteins) that outcrop in five states loc ated in the central Appalachian basin; the ashes were erupted during t he Pennsylvanian. Even though the whole-rock tonstein samples are extr emely altered, the glass trapped in quartz microphenocrysts preserves pre-eruptive melt compositions, and, consequently, the inclusions are useful for determining compositions of source magmas and identifying g eochemical trends indicative of magmatic evolution. Interpretation of inclusion compositions indicates the strongly altered tonsteins were d erived from potassium-enriched, metaluminous to mildly peraluminous ma gma(s). The tonsteins can be divided into two groups on the basis of t rapped melt compositions: older tonsteins that have inclusions with hi gh Sr and normative quartz contents and comparatively low concentratio ns of U, Th, Rb, Y, Cs, Nb, F, and Cl(+/-Be) and younger tonsteins who se inclusions contain low Sr and normative quartz and high concentrati ons of U, Th, Rb, Y, Cs, Nb, F, and Cl (+/-Be). In general, as concent rations of Sr decreased, the magmatic abundances of Rb, Y, Cs, Nb, U, Th, Cl, and F (+/-Be) increased. The associated magma or magmas were h ighly evolved, volatile enriched, and contained Rb, Nb, and Y abundanc es characteristic of continental within-plate granites; compositions r anged from high-silica rhyolite to topaz rhyolite. Pre-eruptive volati le abundances in the source magma(s) were generally high but also high ly variable. Chlorine contents of melt(s) ranged from 0.02-0.23 wt%, a nd F ranged from 0.01 -0.7 wt%. Concentrations of H2O in melt(s) range d from 1.6-6.5 wt%. The high pre-eruptive H2O contents are consistent with large eruptive volumes indicating the precursor rhyolites, which weathered to tonsteins, were a result of plinian eruptions. Even thoug h pre-eruptive water concentrations exhibit no recognizable trends wit h any elements studied, magmatic evolution appears to have been a stro ng function of F and H2O in melt(s); the thermal stabilities of quartz and feldspar were controlled by F and H2O activities at pressures of approximately 0.5-1 kbar.