HR: 0800h
AN: V41B-1446    [Abstracts]
TI: Late Pleistocene magmatic evolution of Tacana volcano, Mexico-Guatemala.
AU: * Arce, J L
EM: jlarce@geologia.unam.mx
AF: Instituto de Geologia, UNAM, Cd. Universitaria, Coyoacan, Mexico, DF 04510 Mexico
AU: Macias, J L
EM: macias@geofisica.unam.mx
AF: Instituto de Geofisica, UNAM, Cd. Universitaria, Coyoacan, Mexico, DF 04510 Mexico
AU: Gardner, J E
EM: gardner@mail.utexas.edu
AF: Department of Geological Sciences, University of Texas, 1 University Station C1100, University of Texas at Austin, Austin, TX 78712-0254 United States
AB: Tacana volcano, one of the most active volcanoes in Mexico, erupting as recently as 1950 and 1986, has been the source of major Plinian eruptions and dome destruction events. The stratigraphic record of Tacana volcano shows that during the last 40 ka at least three Plinian eruptions have occurred at <32, <29, and <14 ka. Compositions of the magmas erupted in those three Plinian events range from 56 to 61 SiO2 wt% (silicic andesite), and all have similar mineral assemblages of plagioclase, two pyroxenes, ilmenite, titanomagnetite, and rare hornblende, set in a vesicular rhyolitic glass (73-74 wt% SiO2). It is noteworthy that the <29 ka pumice, in contrast to the others, is highly crystalline with large phenocrysts embedded in a matrix rich in plagioclase microlites. From the compositions of the two pyroxenes and two oxides we estimate pre-eruptive temperatures (QUILF); the two younger magmas had temperatures of ~840-860 ° C, whereas the oldest one ranged from 878-938 ° C. All three silicic andesites had oxygen fugacities of NNO + 1.5. The large range in temperature in the oldest magma may record a magma mixing event, whereas the more restricted temperatures for the younger magmas suggest that the system has cooled substantially, and was more homogeneous. Likely, lower temperatures provoked crystal growth in the <29 ka pumice followed by decompression that produced the microlitic groundmass.
DE: 1090 Field relationships (3690, 8486)
DE: 1145 Tephrochronology (8455)
DE: 3690 Field relationships (1090, 8486)
DE: 8428 Explosive volcanism
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005