Pliocene volcanic rocks of the Tshicoma Formation, east-central Jemez volcanic field: chemistry, petrography, and age constraints
David Broxton, Giday Woldegabriel, Lisa Peters, James Budahn, and Gary Luedemann


The Tschicoma Formation in the central and southern Sierra de los Valles is composed of thick sequences of silicic to intermediate lavas and subordinate tuffs that erupted from overlapping dome and flow complexes over a period of about 2.5 m.y. during the Pliocene. The chemical and petrographic characteristics of the individual dome complexes are discussed in detail, and are sufficiently distinct that the lavas can be readily distinguished from one another. A south to north description of the dome complexes is as follows. The Dacite of Sawyer Dome contains 32-34% phenocrysts of plagioclase, hornblende, and orthopyroxene, and two 40Ar/39Ar ages range from 3.18 to 3.67 Ma. The Dacite of Cerro Grande contains about 21% phenocrysts of plagioclase, hornblende, orthopyroxene, and subordinate clinopyroxene; three 40Ar/39Ar ages range from 2.88 to 3.35 Ma. The Dacite of Pajarito Mountain is divided into a lower and upper series of lavas. The lower series is a two-pyroxene dacite characterized by 10.3-16.4% phenocrysts of plagioclase and subequal orthopyroxene and clinopyroxene, whereas the upper series contains 23-24% phenocrysts of plagioclase, orthopyroxene, and subordinate clinopyroxene, and the lava chemistry is slightly more silicic. Two samples from the upper series lavas produced 40Ar/39Ar ages of 2.93 and 3.09 Ma. The Rhyodacite of Rendija Canyon contains 11-16% phenocrysts consisting of quartz and plagioclase, subordinate sanidine and anorthoclase, and low (<0.5%) amounts of biotite ± clinopyroxene ± hornblende. It is predominantly a low-silica rhyolite with 40Ar/39Ar ages of 4.98 and 5.04 Ma. A sample of high-silica rhyolite from a small ash-flow tuff near the base of the sequence yielded an 40Ar/39Ar age of 5.36 Ma. The Dacite of Caballo Mountain consists of two main parts. The summit area of Caballo Mountain is underlain by dacite containing about 2% phenocrysts consisting of subequal plagioclase and hornblende, subordinate clinopyroxene, and trace quartz and orthopyroxene. These relatively crystal-poor lavas overlie lavas that contain up to 24% phenocrysts consisting of plagioclase, hornblende, subordinate quartz and biotite, and minor to trace sanidine, orthopyroxene, and clinopyroxene. The crystal-poor summit dacite yielded an 40Ar/39Ar age of 3.06 Ma and the crystal-rich dacite returned an 40Ar/39Ar age of 4.66 Ma. After about 2.88 Ma, Tschicoma volcanism in the Sierra de los Valles died out and intermediate volcanism shifted eastwards to the western Española basin where several small-volume dacite lavas erupted between 2.36 and 2.74 Ma.


  1. Broxton, David; Woldegabriel, Giday; Peters, Lisa; Budahn, James; Luedemann, Gary, 2007, Pliocene volcanic rocks of the Tshicoma Formation, east-central Jemez volcanic field: chemistry, petrography, and age constraints, 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. 284-295.

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