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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 4. (April)

First Page: 765

Last Page: 765

Title: Chemical Changes Associated with Propylitic Alteration of Two Ash-Flow Tuffs, Datil-Mogollon Volcanic Field, New Mexico: ABSTRACT

Author(s): J. I. Lindley, P. C. Ragland, C. E. Chapin

Article Type: Meeting abstract


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.

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Copyright 1997 American Association of Petroleum Geologists