Distribution of dacite lavas beneath the Pajarito Plateau, Jemez Mountains, New Mexico
K. E. Samuels, D. E. Broxton, D. T. Vaniman, G. Woldegabriel, J. A. Wolff, D. D. Hickmott, E. C. Kluk, and M. M. Fittipaldo


Boreholes drilled for groundwater characterization at Los Alamos National Laboratory (LANL) encountered four petrographically and chemically distinct dacite lavas beneath the Pajarito Plateau. Coarsely porphyritic lavas with 17-38% phenocrysts (42-74% plagioclase, up to 28% pyroxene, 15-43% amphibole) and 64.9-66.3% SiO2 occur in boreholes in the southwest part of LANL. Modal mineralogy and chemistry links these lavas to Cerro Grande, a Sierra de los Valles dacite dome west of the study area. Boreholes in the west-central part of LANL encountered a thin dacite lava with 22% phenocrysts (81-83% plagioclase, 12-15% pyroxene, and 1% relict amphibole) and relatively high SiO2 (67.4%) overlying equally thin basaltic lavas of the Cerros del Rio volcanic field. Boreholes in the northern part of LANL encountered thicker fine-grained dacitic lava with 2-5% phenocrysts (up to 35% plagioclase, 61-88% pyroxene, absent or minor amphibole) and relatively low SiO2 (~63.5%). One borehole in the east-central part of LANL encountered a sequence of three crystal-poor (4-6% phenocrysts) dacite lavas, with ~66% SiO2, 4-6% resorbed quartz and trace amounts of resorbed olivine, intercalated with basalt. Pajarito Plateau dacite lavas erupted over a relatively short time interval (2.3-3.6 Ma) during which both the Jemez Mountains and Cerros del Rio volcanic fields were active. The southwestern group of dacites probably represents a thick lobe of Cerro Grande lava that flowed ~3.5 km eastward into the subsiding Española Basin. Other plateau dacites, which do not correlate with dacites exposed in the Sierra de los Valles, probably represent local eruptions within the western part of the basin. Thickness variations and spatial distribution indicate the northern group of dacites probably erupted from a buried vent near the Pajarito fault zone. The dacites encountered in the west-central and eastern parts of LANL represent minor pulses of dacite volcanism in areas dominated by basaltic volcanism of the Cerros del Rio volcanic field. The Pajarito Plateau dacites overlap spatially and temporally with intermediate volcanic rocks in the eastern part of the Jemez volcanic field and with mafic volcanic rocks in the western part of the Cerros del Rio volcanic field. The distinctive compositional and petrographic characteristics of the Plateau dacites probably reflect a transitional style of magmatism that developed in the narrow region between these adjacent, concurrently active volcanic fields.


  1. Samuels, K. E.; Broxton, D. E.; Vaniman, D. T.; Woldegabriel, G.; Wolff, J. A.; Hickmott, D. D.; Kluk, E. C.; Fittipaldo, M. M., 2007, Distribution of dacite lavas beneath the Pajarito Plateau, Jemez Mountains, New Mexico, 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. 296-307.

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