Pliocene volcanic rocks of the Tshicoma Formation, east-central Jemez volcanic field: chemistry, petrography, and age constraints

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. 285 PLIOCENE ROCKS OF THE TSCHICOMA FORMATION about their field relationships, outcrop characteristics, and ages. The names for geologic units used in this paper are the same as those used on these maps. This paper also presents information relevant to understanding the depositional history of the Pliocene Puye Formation. The Puye Formation is a large apron of overlapping alluvial and pyroclastic fans shed eastward from the Jemez volcanic field into the western Española basin (Smith, 1938; Griggs, 1964; Bailey et al., 1969; Turbeville et al., 1989; Waresback and Turbeville, 1990). Because its primary source area was volcanic domes in the Sierra de los Valles, the Puye Formation overlaps and postdates the Tschicoma Formation in age. Turbeville et al. (1989) estimated its areal distribution at 518 km2 and its volume at ~15 km3. The formation reaches a maximum thickness of >330 m in well R-25 on the western side of the Pajarito Plateau (Broxton et al., 2002). Its thickness in the central and eastern parts of the plateau is ~200 m where the upper part contains interbedded basaltic lavas of the Cerros del Rio volcanic field. This paper describes the chemistry, petrography, mineral chemistry, and ages of eruptive centers of the Tschicoma Formation in the central and southern Sierra de los Valles between Sawyer Dome on the south and Caballo Mountain on the north (Fig. 1). The centers of Tschicoma Mountain, Polvadera Peak, and others in the northern Jemez Mountains are not discussed here. The data presented contribute to understanding the spatial and temporal trends in volcanism and tectonism in the Jemez volcanic field during the Pliocene, and they provide information about the timing, composition, and distribution of source areas that provided detritus to the Puye Formation. Ages and their supporting analytical data are also presented for tephras interbedded within the Puye Formation; these tephras are useful time-stratigraphic markers for understanding the internal structure and sediment accumulation rates for these fan deposits. Finally, ages are presented for small-volume dacites erupted in the western part of the Española basin to determine their temporal relationship to the Tschicoma Formation. FIGURE 1. Location map showing the distribution of the major Tschicoma volcanic centers in the central and southern Sierra de los Valles. Age dates are from this paper except for those for Cerro Rubio (from Heiken et al., 1986). 286 BROXTON, WOLDEGABRIEL, PETERS, BUDAHN, & LUEDEMANN