Petrography, chemistry, and mineral compositions for subunits of the Tshirege Member, Bandelier Tuff within the Valles Caldera and Pajarito Plateau
Richard G. Warren, Fraser Goff, Emily C. Kluk, and James R. Budahn

Abstract:

The chemistry, petrography, and mineral compositions for subunits of the Tshirege Member, Bandelier Tuff, are well determined on the Pajarito Plateau within numerous publications by the Los Alamos National Laboratory. We summarize these characteristics to determine that these same subunits occur within the Valles caldera. Most trace oxides and a few major oxides determined by X-ray fluorescence and neutron activation individually discriminate among subunits, and taken together confidently discriminate among all except the uppermost two subunits. Such oxides are generally immobile, except for a few under the conditions of extensive, strong alteration within VC2A and VC2B (collectively VC2), holes drilled on the western side of the resurgent dome of the Valles caldera. Analytical errors may also obscure chemical correlations. Phenocrysts of quartz, immune to alteration, provide an effective petrographic discriminator between the uppermost two subunits. Feldspar phenocryst compositions by electron microprobe show maximum abundances of anorthoclase, equal to those of sanidine, near the middle of Smith and Bailey’s (1966) uppermost subunit V of Tshirege Member; elsewhere within their subunit V sanidine strongly dominates over anorthoclase. Most subunits of the Tshirege Member as well as the underlying Otowi Member occur in VC2. VC2A bottoms within the Otowi Member, which directly overlies the Santa Fe Group within VC2B. Pre-Bandelier tuff, characterized from outcrop on the southwest margin of Valles caldera, does not occur within VC2B, undermining evidence for a precursor caldera to the Toledo caldera. The thicknesses and absences of particular subunits of the Tshirege Member within VC2 provide structural data that we explain by explosion and collapse of the Valles caldera rather than by subsequent postcaldera structural activity such as resurgence.


Citation:

  1. Warren, Richard G.; Goff, Fraser; Kluk, Emily C.; Budahn, James R., 2007, Petrography, chemistry, and mineral compositions for subunits of the Tshirege Member, Bandelier Tuff within the Valles Caldera and Pajarito Plateau, in: Geology of the Jemez Region II, Kues, Barry S.; Kelley, Shari A.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 58th Field Conference, pp. 316-332.

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