New Mexico Geological Society Annual Spring Meeting — Abstracts

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.