Large-scale propylitic alteration of two texturally distinct, Oligocene ash-flow tuffs has been investigated: the crystal-poor, one-feldspar, rhyolitic A-L Peak Tuff and the crystal-rich, two-feldspar, rhyolitic to quartz latitic Hells Mesa Tuff. Initial development of a petrographic criterion with which to separate samples into groups experiencing varying degrees of alteration was followed by univariate and multivariate statistical analyses of major and trace element data, including U and Th, to identify chemical trends accompanying alteration. The extent of alteration a sample displays can be petrographically characterized by several variables including the amount of replacement of the groundmass and feldspar phenocrysts with secondary minerals, the degree of bleaching f the groundmass and the appearance of the mafic minerals.

Both the A-L Peak Tuff and the Hells Mesa Tuff showed increases in K₂O, Rb, and FeO, increases in the Fe⁺²/Fe⁺³ ratio, decreases in Th, and no systematic trend in U, Nb, or Zr with alteration. The two units displayed opposite behavior with alteration for MnO, Y, and Sr. Total iron, Al₂O₃, Fe₂O₃, TiO₂, MgO, CaO, and Na₂O content showed a trend in one unit but not in the other. The mobility with alteration of Th, considered to be immobile in most geologic processes, may have been caused by the reducing environment of the prophylitic process. Such a reducing system might also explain the immobility of U during propylitization.

End_of_Article - Last_Page 765------------