New Mexico Geological Society Annual Spring Meeting
April 7, 2017

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The Onset of Rhyolite Volcanism and Subsequent Collapse in the Schoolhouse Mountain Caldera, Mogollon-Datil Volcanic Field, Southwest New Mexico

Vanessa M. Swenton1, Jeffrey M. Amato1, Tara Jonell2 and William C. McIntosh3

1New Mexico State University, Las Cruces, NM, 88003,
2Louisiana State University, Baton Rouge, LA, 70803
3New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, Socorro, NM, 87801

Megabreccia deposits in calderas are diagnostic of a collapse event where hot ash and gasses interact with pre-caldera rocks and basement rocks. The late-Eocene Schoolhouse Mountain caldera (SMC) in the Mogollon-Datil volcanic field (MDVF) of southwest New Mexico is a relatively unstudied caldera, where little is known regarding the caldera stratigraphy and the timespan of magmatism. We undertook field mapping, petrology, and 40Ar/39Ar geochronology to develop a model for the evolution of the caldera and its relationship to MDVF volcanism. Volcanism began with the pre-collapse dacite lava flows and dikes of the Saddlerock Canyon sequence, dated at 35.2 ± 0.2 Ma (U-Pb zircon). The top of this lowermost sequence has interlayered dacite flows and rhyolite tuffs overlain by lithic-rich rhyolite tuffs, flows, and breccias at the base of the overlying Kerr Canyon sequence. Tuffs are separated by volcaniclastic sandstone or flow-banded rhyolite. Clasts within breccia include rhyolite with abundant phenocrysts and pumice, Cretaceous sandstone, and sparse Proterozoic granite. Clasts range from up to 15 cm in the lithic breccia, and up to 4 m in the megabreccia. Total thickness of the Kerr Canyon sequence is ~800–2000 m but may be thicker in the central area and tapering to the edges, suggesting infilling of a topographic low. Several clasts in the breccia have apparent injection of rhyolite into the clasts in a wispy, flame-like structure, discolored thermally altered rinds, and vesiculation of the rhyolite near the clasts from degassing. Thus, we suggest this is a collapse breccia associated with an eruption, rather than a lahar deposit consisting of cold, reworked volcanic material. Sanidine crystals from two boulder clasts and the rhyolite matrix were dated using 40Ar/39Ar single-crystal laser fusion. The clasts yielded identical ages of 35.34±0.03 Ma and the matrix had ages of 35.17±0.11 Ma and 35.35±0.05 Ma. Some clasts have abundant quartz and sanidine phenocrysts similar to the Kneeling Nun tuff, which is similar in age (McIntosh et al., 1992), raising the possibility that these were derived from this unit. Two tuffs at similar stratigraphic levels in this sequence had 40Ar/39Ar biotite dates of 34.71±0.14 Ma and 34.33±0.08 Ma. This may indicate that the Kerr Canyon sequence was erupting at 34.7–34.3 Ma and that the breccia contains clasts with “inherited” sanidine that were not reset during emplacement. The higher units in this system have an uncertain relationship with respect to the history of the SMC. The next highest unit, Mangas Creek, consists of quartz-latitic tuffs and andesite lava flows, volcaniclastic sandstones, and volcanic breccias up to ~2000 m. Overlying this is the McCauley Ranch tuff, with a sanidine 40Ar/39Ar date of 33.99±0.04 Ma, and the Cherokee Canyon tuff (600 m), which has a sanidine 40Ar/39Ar date of 33.84±0.02 Ma. Thus, the eruptive history of this caldera ranges from at least as old as 34.71 Ma to 33.84 Ma, or approximately 1.1 m.y.


  1. McIntosh, W.C., Chapin, C.E., RattÉ, J.C., and Sutter, J.F., 1992, Time-stratigraphic framework for the Eocene-Oligocene Mogollon-Datil volcanic field, southwest New Mexico: GSA Bulletin, v.104, p.851-871.
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2017 New Mexico Geological Society Annual Spring Meeting
April 7, 2017, Macey Center, New Mexico Tech campus, Socorro, NM