HR: 0800h
AN: U51A-04    [Abstracts]
TI: Petrogenesis and P-T Conditions of Metamorphic Rocks From the Chiapas Massif Complex in the Custepec Area, Chiapas, Mexico
AU: Estrada-Carmona, J
EM: jestrada@cicese.mx
AF: Depto. de Geologia, CICESE, km. 107 carr. Tijuana-Ensenada, Ensenada, BC 22860, Mexico
AU: * Weber, B
EM: bweber@cicese.mx
AF: Depto. de Geologia, CICESE, km. 107 carr. Tijuana-Ensenada, Ensenada, BC 22860, Mexico
AU: Hecht, L
EM: Lutz.Hecht@museum.hu-berlin.de
AF: Mineralogie, Museum f. Naturkunde, Invalidenstr. 43, Berlin, 10115, Germany
AU: Martens, U
EM: umartens@stanford.edu
AF: Department of Geosciences, Stanford University, Stanford, CA 94309, United States
AB: The Chiapas Massif Complex (CMC), located in the southern Maya block, is primarily composed of igneous and metaigneous rocks of late Permian age. Within the CMC, two metasedimentary basement units have been described: (1) The ‘La Sepultura Unit' is located in the NW part of the CMC and is composed of metapsammites, metapelites and calcsilicate rocks metamorphosed under high-T/low-P conditions and (2) the ‘Custepec Unit' located in the SE part of the CMC. The Custepec Unit is mainly composed of anatectic amphibolites with or without garnet, intercalated with quartz-feldspar and pelitic gneisses, marbles, and calcsilicates. The main foliation trend in the Custepec area is E-W to NW-SE mostly dipping to the north. Stretching lineations and fold axes of varied orientations indicate that the D1 deformation was folded by a subsequent D2 event. We applied the garnet-biotite geothermometer, the GASP, and GRAIL geobarometers in metapelites and the garnet-plagioclase-hornblende-quartz geobarometer in amphibolites. The results are consistent, yielding peak metamorphic conditions above 800°C and 9 Kbar. These data, along with petrographic observations place the metamorphic peak in the high amphibolite facies to granulite facies transition at 25-30 km depth. Relics of bluish-green (low-T) amphiboles yielded similar pressures than the high-T brownish hornblende, indicating a clockwise P-T path with isobaric heating at the metamorphic pressure peak. The high-grade event was followed by greenschist facies retrogression, which is probably contemporaneous with the formation of E-W trending dextral mylonite zones. On the basis of our field observations, chemical composition, and the presence of detritic zircons in the amphibolites, we interpret the Custepec Unit as a volcanosedimentary sequence. Our data favor a model in which peak metamorphism in the CMC during the late Permian is the result of stacking in an orogenic wedge with the Sepultura Unit as the upper plate thrusted over the Custepec Unit from present day north to south, followed by dextral transtension during the early Triassic.
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3660 Metamorphic petrology
DE: 3690 Field relationships (1090, 8486)
DE: 8102 Continental contractional orogenic belts and inversion tectonics
SC: Union [U]
MN: 2007 Joint Assembly