HR: 17:00h
AN: V44B-04    [Abstracts]
TI: Detailed ⁴⁰Ar/³⁹Ar chronology of the Tancítaro Volcanic Field, Michoacán, Mexico
AU: * Ownby, S
EM: sownby@umich.edu
AF: University of Michigan, Dept. of Geological Sciences, Ann Arbor, MI 48109 United States
AU: Delgado-Granados, H
EM: hugo@tonatiuh.igeofcu.unam.mx
AF: Universidad Nacional Autonoma de Mexico, Instituto de Geofisica, Coyoacan, D.F 04510 Mexico
AU: Lange, R
EM: becky@umich.edu
AF: University of Michigan, Dept. of Geological Sciences, Ann Arbor, MI 48109 United States
AU: Hall, C M
EM: cmhall@umich.edu
AF: University of Michigan, Dept. of Geological Sciences, Ann Arbor, MI 48109 United States
AB: The Tancítaro volcanic field (TVF) is characterized by over 300 cinder cones and fissure fed lava flows, in addition to the ~10 shield volcanoes and 1 large andesite stratovolcano, ~60 km³ Volcán Tancítaro. The TVF is part of the larger Michoacán Guanajuato volcanic field (MGVF) in the Trans Mexican Volcanic Belt, related to subduction of the Cocos plate. We report new ⁴⁰Ar/³⁹Ar age constraints on the most recent activity from Volcán Tancítaro. Previously, there was only one K-Ar date 530±60 ka for this volcano; Ban et al. (1992). It has been the site of at least two debris avalanche deposits (Capra et al., 2002; Garduno-Monroy, 1999; Ownby et al., 2004), and continues to threaten the cities of Uruapan and Apatzingán in the state of Michoacán, which have a combined population of >300,000. The most recent activity produced a thin blanket of ash (~1-5 m thick along the flanks of the volcano); this ash is tightly bracketed by dates on two nearby shield volcanoes, one underneath (268±14 ka) and the other on top (267±12 ka) of this ash layer. It appears to have triggered a large debris avalanche deposit off the steep slopes of V. Tancítaro (the distinctive ash is mixed in with this avalanche deposit), which itself is bracketed by two cinder cones, one underneath (425±45 ka) and the other on top (179±77 ka). The timing of this ash eruption is close to that for four different andesite lavas from near the summit of V. Tancítaro, which yielded ages of 251±25 ka, 241±25 ka, 228±16 ka, 223±23 ka, respectively. Other dates from the main edifice of V. Tancítaro reveal two earlier episode of activity at ~450 and ~700 ka. We also report an additional set of ~50 ⁴⁰Ar/³⁹Ar ages on various cinder cones, shields, and fissure-fed flows that are peripheral to V. Tancítaro. The samples range in age from ~980 ka to the present, with no obvious breaks in time. They range continuously from 51-62 wt% SiO₂, with no breaks in composition. It appears that neither dacite nor rhyolite has erupted within this 4200 km² region over the last 1 Myr, despite a crustal thickness of ~40 km. There is no age progression to the composition of the erupted lavas. Both temporal and petrological evidence suggests that the compositional diversity of the peripherally erupted magmas was determined in the lower crust and not within long-lived upper crustal magma chambers.
DE: 1100 GEOCHRONOLOGY
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 8040 Remote sensing
DE: 8400 VOLCANOLOGY
DE: 8488 Volcanic hazards and risks
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005