HR: 0800h
AN: V41D-0798    [Abstracts]
TI: Geologic Evolution of the Sierra de Nanchititla Silicic Volcanic Center, Southern Mexico
AU: * Gonzalez, N
EM: norma@geociencias.unam.mx
AF: Centro de Geociencias UNAM, Campus Juriquilla, Queretaro, Qro 76230, Mexico
AU: Ferrari, L
EM: luca@geociencias.unam.mx
AF: Centro de Geociencias UNAM, Campus Juriquilla, Queretaro, Qro 76230, Mexico
AU: Lopez, M
EM: marlopez@cicese.mx
AF: Cepartamento de Geologia, CICESE, Km 107 Carretera Tijuana-Ensenada, Ensenada, BC 22860, Mexico
AU: Cerca, M
EM: mcerca@geociencias.unam.mx
AF: Centro de Geociencias UNAM, Campus Juriquilla, Queretaro, Qro 76230, Mexico
AU: Orozco, M
EM: torozco@geociencias.unam.mx
AF: Centro de Geociencias UNAM, Campus Juriquilla, Queretaro, Qro 76230, Mexico
AB: The Sierra de Nanchititla Volcanic Center (SNVC) is located at the borders of the States of Mexico, Michoacán and Guerrero, in the northern sector of Sierra Madre del Sur magmatic province (SMS) of México. Previous studies in the areas of Taxco and Tilzapotla recognized an Early Tertiary brittle deformation along WNW-ESE to NW-SE trending strike slip fault systems at the north of the SMS (Alaniz-Alvarez et al., 2002, JVGR; Morán-Zenteno et al., 2005, JVGR). This fault systems apparently favored the emplacement of several silicic volcanic centers aligned in a WNW-ESE direction during the Eocene and earliest Oligocene. These volcanic centers include Huatla, Tilzapotla, Taxco, La Goleta and Sierra de Nanchititla. We carried out a detailed geologic study of the SNVC backed by Ar/Ar geochronology and trace element geochemistry. Our results indicate that the SNVC is part of the Eocene magmatic episode recorded in the SMS. The first volcanic rocks in the area are basaltic-andesitic lavas dated at ~42 Ma. However, the activity of the SNVC proper is concentrated between 37.8±0.2 and 35.6±0.5 Ma and was characterized by the emplacement of rhyolitic sub-volcanic bodies (cryptodomes), voluminous pyroclastic deposits and endogenous domes. The oldest magmatic event is the intrusion of WNW-ESE-trending mafic aphyric dikes dated 35.6±0.5 Ma that fed the Puerto El Salitre andesite. Magmatic activity continued with the emplacement of cryptodomes with sub-volcanic texture. The main phase of the SNVC took place in late Eocene with the emplacement of three units: (1) WNW-ESE-trending felsic pyroclastic dikes feeding the large Nanchititla Ignimbrite (35.6±0.5 Ma), (2) rhyolitic domes with vents aligned in a WNW-ESE direction (35.6±0.1 Ma), the and (3) a pyroclastic flow (Tequezquite Ignimbrite) likely vented from a dome. Geochemical data are consistent with a differentiation process, starting with primary mafic magmas and originating both basaltic- andesitic and andesitic rocks. These magmas present typical arc-like features, and the oldest rhyolitic rocks tend to have a peraluminous character. Since the distribution of volcanic bodies and centers in the SNVC coincides with WNW-ESE-trending regional lineaments, and considering the significant amount of material emplaced during a relatively short time span (~2.2 Ma), it can be concluded that the formation of the SNVC was produced by the trapping and differentiation of magma along a pre-existing, and inactive, regional structure. The absence of a clear sub circular depression, intracaldera deposits, ring or radial dikes, and ring domes suggest that the magma chamber feeding the ignimbrite was likely very deep and that the ignimbrite was emplaced through WNW-ESE trending fissure with limited or negligible collapse of the venting area.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
DE: 8428 Explosive volcanism
DE: 8440 Calderas
DE: 8486 Field relationships (1090, 3690)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting