Rocky Mountain Section - 64th Annual Meeting (9–11 May 2012)

Paper No. 8
Presentation Time: 11:00 AM


DUNBAR, Nelia W.1, KELLEY, Shari A.1, GOFF, Fraser2, MCINTOSH, William1 and HEIZLER, Lynn L.1, (1)New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, NM 87801, (2)Earth and Planetary Sciences Dept, University of New Mexico, Albuquerque, NM 87131,

Rhyolitic to trachyandesitic pyroclastic deposits exposed in three canyons (La Mosca, San Mateo, and Water) around the Mt. Taylor edifice provide insight into early eruption processes at this composite volcano. Measured sections coupled with glass geochemistry allow correlations between the canyon sections and some probable vent areas. A 4 m thick pyroclastic surge and fall deposit at the base of La Mosca Canyon section (3.26±0.04 Ma), correlates with the 3.3 Ma Grants Ridge high silica rhyolitic dome complex, emplaced 15 km southwest of Mt. Taylor. Overlying this deposit is a 1 m thick rhyolitic pyroclastic surge. Both deposits are absent in nearby San Mateo Canyon, where instead a >28 m thick, complex sequence of rhyolite ignimbrites and pyroclastic surges is found at the base of the stratigraphic section. A rhyolitic ignimbrite in Water Canyon (2.87±0.04 Ma) chemically resembles the upper part of the basal San Mateo pyroclastic sequence. Above these pyroclastic surge and flow deposits in San Mateo Canyon, a series of small, chemically correlated, pyroclastic fall deposits are found in San Mateo and La Mosca canyons. Two rhyolitic falls (a lower one dated to 3.08±0.2 Ma), and an overlying thin, dark-colored trachyandesite fall deposit are present. The upper of the two rhyolitic falls is also found in Water Canyon (2.76±0.05 and 2.83±0.07 Ma). Above this sequence of ashfalls, a thick (~22 m.) rhyolitic pyroclastic flow sequence is present in San Mateo canyon, which chemically correlates with a thin (<1 m.) pyroclastic fall in La Mosca canyon. All these pyroclastic beds are probably sourced from vents buried within Mt. Taylor.

The chemical trend in this rapidly erupted pyroclastic sequence in the canyons range from high silica to low silica rhyolite, consistent with derivation from a single, normally zoned magma chamber. However, the presence of the trachyandesitic tephra layer in the middle of the sequence may instead argue instead for multiple small magma batches. Individual pyroclastic flow/surge deposits seem to have restricted ranges, consistent with small volume, high aspect ratio events, possibly related to dome collapse in early Mt. Taylor. In contrast, pyroclastic fall deposits, although small, are more widespread, and are deposited in all three canyons, allowing correlation between the stratigraphic sections.