EVOLUTION OF MOUNT TAYLOR COMPOSITE VOLCANO, NEW MEXICO,

GOFF, F., candf@swcp.com, Department of Earth & Planetary Sciences, University of New Mexico,
Albuquerque, New Mexico 87131; KELLEY, S.A., New Mexico Bureau of Geology & Mineral
Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New
Mexico 87801; OSBURN, G.R., Earth & Planetary Sciences, Washington University, St. Louis,
Missouri 63130; LAWRENCE, J.R., Lawrence GeoServices Ltd. Co., 2321 Elizabeth St. NE,
Albuquerque, New Mexico 87112; GOFF, C.J., Geologic Consultant, 5515 Quemazon, Los
Alamos, New Mexico 87544; FERGUSON, C., Professional Geologist, 119 North Fork Rd.,
Centennial, Wyoming 82055; McINTOSH, W.C., New Mexico Bureau of Geology & Mineral
Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New
Mexico 87801; FELLAH, K., School of Earth and Environmental Sciences, Washington State
University, Pullman, Washington 99164; DUNBAR, N.W., New Mexico Bureau of Geology &
Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro,
New Mexico 87801; and WOLFF, J.A., School of Earth and Environmental Sciences,
Washington State University, Pullman, Washington 99164

Detailed mapping of four 1:24,000 quadrangles augmented with >40 40Ar/39Ar dates, >100 major
and trace element analyses, and previously published data provide new insights on the evolution
of Plio-Pleistocene Mount Taylor (MT) composite volcano. MT is one of a string of Miocene–
Quaternary volcanoes erupted along the NE-trending Jemez lineament and overlies Jurassic–
Cretaceous sedimentary rocks of the Colorado Plateau. The 20-km3 edifice is constructed of
mildly alkaline volcanic rocks of basanite to alkali rhyolite composition. Most mafic rocks
contain phenocrysts of olivine and plagioclase whereas intermediate to silicic rocks contain
phenocrysts of plagioclase, hornblende and/or biotite. Rare trachybasalts contain phlogopite.
Trachydacites and rhyolites may contain alkali feldspar and quartz. Virtually all volcanics
contain clinopyroxene.
Volcanism began with eruption of basanite lavas (3.6 Ma) and a distinctive fine-grained
trachyte (3.3 Ma) followed by multiple eruptions of alkali rhyolite to trachyandesite lavas, domes
and pyroclastic deposits (3.1–2.8 Ma). During the first period, a heretofore unrecognized
sequence, up to 110 m thick, of rhyolitic fall, flow and surge deposits interlayered with volcanic
sediments was emplaced beneath the NW volcano flank. The early phase culminated with
widespread eruption of coarse-grained, plagioclase-phyric, trachybasalt to basaltic trachyandesite
flows (2.86–2.76 Ma, n = 3). The final phase (2.8–2.5 Ma) produced intermingled trachyandesite
to trachydacite domes, flows and minor pyroclastic beds cut by a radial swarm of trachydacite
dikes. These units are intruded by a small composite stock of trachydacite to alkali rhyolite. As
the final phase developed, volcanic debris flows accumulated on the volcano flanks.
More than 60 porphyritic to aphyric trachybasalt lavas and scoria cone deposits (3.3–<1.7 Ma)
are interlayered with MT eruptions and flank MT on Horace Mesa, Mesa La Jara, and southern
Mesa Chivato. Contemporaneous gabbroic intrusions of similar composition (3.1–<2.8 Ma)
created San Fidel Dome and other small uplifts. Debris flows and tuffs deposited on mesas
adjacent to MT define a possible hiatus in mafic activity that occurred 2.7–2.2 Ma. The youngest
trachybasalt complex (Cerro Pelón) is about 1.5 Ma and sits on the north shoulder of MT.