HR: 1340h
AN: V53A-0606    [Abstracts]
TI: Eruptive History of Volc n Tepetiltic, Mexico: Evidence for Remelting of Silicic Ashflows Revealed by 40Ar/39Ar Geochronology
AU: * Frey, H M
EM: hfrey@umich.edu
AF: University of Michigan, 425 E. University, Ann Arbor, MI 48109-1063
AU: Lange, R A
EM: becky@umich.edu
AF: University of Michigan, 425 E. University, Ann Arbor, MI 48109-1063
AU: Hall, C M
EM: cmhall@umich.edu
AF: University of Michigan, 425 E. University, Ann Arbor, MI 48109-1063
AU: Nelson, S A
EM: snelson@tulane.edu
AF: Tulane University, Dinwiddie Hall, New Orleans, LA 70118
AU: Granados, H D
EM: hugo@tonatiuh.igeofcu.unam.mx
AF: Instituto de Geofisica, UNAM, Circuito Exterior, C.U., Coyocan, D.F 04510 Mexico
AB: Volc n Tepetiltic (VT) is located in the northwestern part of the Trans-Mexican Volcanic Belt and features an elliptical caldera (5 x 2.5 km). Previous detailed $^{40}$Ar/$^{39}$Ar geochronology studies at V. Tequila and V. Ceboruco, showed that cone-building events occur within narrow time intervals ($<$ 25 kyrs) and eruptive phases may be separated by hiatuses of more than 100 kyrs. At those volcanoes, our studies were restricted to surface flows. However, at VT, a rhyolitic Plinian eruption created a caldera which exposed $\sim$600 m of stratified andesitic and dacitic lava flows and thus, allowed the opportunity to study the cone-building events and compositional evolution of this arc stratocone. A suite of samples from a stratigraphic section of the southern caldera wall were dated to determine the eruptive history during construction of the stratovolcano. Samples from andesite flows at the base of the caldera wall (516 $\pm$ 11 and 529 $\pm$ 18 ka), mid-way up the wall (533 $\pm$ 32 ka) and the second highest flow of the wall (528 $\pm$ 15 ka) yielded indistinguishable ages. Similar ages were obtained from the northern rim of the caldera wall (502 $\pm$ 26 ka), an andesite flank flow north of the stratocone (552 $\pm$ 18 ka) and a vertical dike cutting flows in the southern caldera wall (505 $\pm$ 10 ka). Therefore the bulk of the edifice was built within a 20 kyr interval (given 1 sigma errors) at $\sim$524 ka. The stratigraphically highest sample in the caldera wall yielded an age of 231 $\pm$ 36 ka. Thus, there may have been a hiatus of $>$200 kyr between cone building episodes along the southern flank of the volcano. Following construction of the main stratocone (dominated by andesitic effusive activity), there was a clustering of rhyolite eruptions, including the caldera forming event which produced rhyolitic ashflow and airfall deposits. The age of the eruption is constrained by stratigraphic and cross-cutting relations. The eruption must be younger than 231 $\pm$ 36 kyr, based on the uppermost andesite dated from the caldera rim. The minimum age of the eruption is constrained by the occurrence of airfall from VT found underneath lava flows from the Amado Nervo shield volcano (220 $\pm$ 30 ka). Given these results, the Plinian eruption likely occurred $\sim$225 $\pm$ 40 kyrs ago. The two rhyolite obsidian domes on the eastern flanks of VT were dated at 307 $\pm$ 34 ka and 274 $\pm$ 52 ka and are thus similar in age or slightly older than the Plinian eruption. The $>$200 kyr hiatus between the construction of the andesite/dacite stratocone and the rhyolitic activity argues against crystal fractionation of the andesite to form the rhyolite. The upper crustal magma chamber feeding the main edifice $\sim$524 kyrs ago was likely short-lived (tens of kyrs), and may have crystallized to a granitoid. We suggest that the rhyolites formed $\sim$230 kyrs ago by partial melting of older shallow basement rhyolitic ashflows prevalent in the area. Evidence for this process is found in an unconsolidated pyroclastic flow from VT with pumiceous rhyolitic blocks and andesitic lithic fragments. Anorthoclase crystals in this deposit yield ages of 4.5 $\pm$ 0.34 Ma and 25.9 $\pm$ 0.24 Ma. Thus the rhyolites were likely produced by refusing of 4-5 Ma silicic ashflows and ashflows from the Sierra Madre Occidental province (20-35 Ma) and not crystal fractionation.
DE: 8400 VOLCANOLOGY
DE: 3640 Igneous petrology
DE: 1035 Geochronology
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting