HR: 11:05h
AN: V42C-04    [Abstracts]
TI: Evidence From a Crystal-Poor, Zoned (Rhyolite-Andesite) Pyroclastic Deposit From Volcan Tepetiltic, Western Mexico for Rapid Generation of Silicic Melt by Partial Melting of Granitoid and not by Segregation From a Long-Lived Crystal-Rich Mush
AU: * Lange, R A
EM: becky@umich.edu
AF: University of Michigan, Dept. of Geologial Sciences 1100 N. University Ave., Ann Arbor, MI 48109-1005, United States
AU: Frey, H
EM: freyh@union.edu
AF: University of Michigan, Dept. of Geologial Sciences 1100 N. University Ave., Ann Arbor, MI 48109-1005, United States
AU: Hall, C
EM: cmhall@umich.edu
AF: University of Michigan, Dept. of Geologial Sciences 1100 N. University Ave., Ann Arbor, MI 48109-1005, United States
AU: Delgado-Granados, H
EM: hugo@geofisica.unam.mx
AF: UNAM, Instituto de Geofisica, Mexico City, 04510 DF, Mexico
AB: Volcan Tepetiltic is an intermediate (56-66 wt % SiO₂) arc stratovolcano (~42 km³) that is composed primarily of crystal-rich (25-40 vol%), hornblende-absent andesite flows, with an elliptical caldera (5 km x 2.5 km). The Plinian, caldera-forming eruption produced a zoned (60-75 wt % SiO₂) pyroclastic deposit (6-9 km³) that is everywhere crystal-poor (0-3 vol%); where crystals are present, hornblende occurs. The caldera wall exposes ~600 m of stratified lava flows. A series of ⁴⁰Ar/³⁹Ar dates on these caldera-wall flows, along with numerous dates on flank flows, constrain the time scale for the construction of the andesitic, main edifice of V. Tepetiltic. The collection of Ar dates indicates that V. Tepetiltic was formed at ~525 ka, over an interval of ~90 ± 75 kyrs. In other words, within the 2 sigma error on all the Ar dates, the eruption interval for cone construction of V. Tepetiltic may have been as long as 165 kyrs or as short as 15 kyrs. At the 95% confidence interval, cone construction ceased by ~380 ka. Peripheral to the central vent of V. Tepetiltic are a series of basaltic andesite (56-58 wt%) flows, cones, and one shield (total volume is ~9 km³), which all erupted at ~214 ± 64 ka, over an interval that may have been as long as 128 kyrs or as short as 22 kyrs, within 2 sigma error of the Ar dates. The pyroclastic deposit is found both underneath and on top of these basaltic andesite units, which broadly constrains its eruption age and indicates a temporal association with the pulse of basaltic andesite eruptions. The age of the caldera-forming eruption is further constrained by (must be younger than) the age of a small dacite dome on the caldera floor (190 ± 22 ka) and the age of a lithic (113 ± 76 ka) within the pyroclastic deposit. In summary, the best estimate for the eruption age of the pyroclastic deposit is at ~190 ka. Thus, there was a hiatus of ~200 kyrs between the cone- building episode that produced the crystal-rich (25-40 vol%), intermediate (56-66 wt% SiO₂), hornblende- free lavas of V. Tepetiltic and the explosive, caldera-forming eruption that produced the crystal-poor (0-3 vol%), zoned (60-75 wt% SiO₂), hornblende-bearing pyroclastic deposit. Most models for the formation of high- silica rhyolite, especially when part of an eruption that zones to andesite/dacite, is that it forms by melt segregation from the andesite/dacite crystalline mush. In the case of the erupted products from V. Tepetiltic, however, this oft-cited model is not viable. The crystal-poor andesite cannot be the parent for the rhyolite as too few crystals have formed. Instead, the most plausible scenario is that the magma chamber that fed construction of the main edifice of V. Tepetiltic solidified below its solidus by ~350 ka. Then, at ~214 ± 64 ka, a pulse of basaltic andesite magma was emplaced into the upper crust, which drove partial melting of granitoid beneath V. Tepetiltic, by the transfer of heat and volatiles. The partial melt (ranging from voluminous rhyolite to minor andesite) segregated, ascended, and erupted quickly (at ~190 ka) before extensive degassing- induced crystallization could occur. This model for the formation of rhyolite (by partial melting of granitoid) may be more widespread than currently recognized.
DE: 1115 Radioisotope geochronology
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 3625 Petrography, microstructures, and textures
DE: 8414 Eruption mechanisms and flow emplacement
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting