Structural geology, Petrology, and Geochronology of metamorphic rocks of the La Sepultura unit as part of the Chiapas Massif/Southeast Mexico
Final Report Abstract
Field work revealed that the recently discovered metasediments, originally called the La Sepultura unit, are wide-spread in the Chiapas Massif of southern Mexico. They are mainly composed of metapelites, metagreywacke, metacarbonates, calc-silicate rocks and amphibolites, and probably represent former continental shelf deposits including volcano-sedimentary sequences. Geochemical and isotope data demonstrate that the protoliths of the metasediments derived from two different major source regions. The metapelite and metapsammite precursors derived from a Grenvillian Proterozoic basement, which also was a major crustal source for the Permian plutonic rocks that make up most of the Chiapas Massif. The Nd isotope data of the metagreywackes indicate a different and somehow younger provenance. Preliminary zircon isotope data of metagreywackes yield a dominantly Pan- African provenance but also various older Proterozoic and Archean sources are involved. The metasediments show medium to high-grade metamorphism with generally garnet + sillimanite ± cordierite. In amphibolites of the southern part of the Chiapas Massif high-pressure amphiboles were observed. High-grade metamorphism in the Chiapas Massif resulted in partial anatexis of metasediments, amphibolites and orthogneisses and intrusion of syntectonic granitoids. Geothermobarometry indicate peak metamorphic conditions ranging from about 650 to 800°C and 5 to 9 kb with a clock-wise PT-path. Furthermore, increasing peak temperatures and pressures from north to south suggest deeper burial in the southern part of the Chiapas Massif during regional metamorphism. Structural data and isotopic ages indicate that the orthogneisses and metasediments underwent the same medium to high-grade metamorphism and deformation at about 250 to 254 Ma, which is a unique orogenic event in southern Mexico. Retrograde metamorphism at temperatures below ?500°C is shown by retrograde Mn-rich garnet, biotite, andalusite (replacing sillimanite), muscovite and chlorite within metapelites and by actinolithic hornblende, epidote, and chlorite within amphibolites. Argon-argon dating of hornblende, biotite and pyrite resulted in 232 to 245 Ma for major retrograde metamorphism in the Chiapas Massif. Increasing Ar-Ar ages of retrograde hornblende from the southeast (232 ± 3) towards the northwest (245 ± 4) of the Chiapas Massif suggest later cooling of the south-eastern part in accordance with deeper burial of this area during peak metamorphism. Our new data indicate that during the Permotriassic the southern Chiapas massif was part of a continuous magmatic arc from northeastern Mexico to northern South America along the proto-Pacific coast of western Pangea. Triassic cooling ages, however, reflect the time of retrometamorphism at low-grade conditions, probably contemporaneous with dextral transtension. The medium- to highgrade Permian event with considerable shortening, which is unique along the proposed arc, is interpreted as either a phase of flat subduction along the same active continental margin or as the final amalgamation of Pangea within the inner orogenic zone during the latest stage of the Alleghenian orogeny. In both cases, Paleozoic sedimentary sequences of the Maya block of different age and provenance were involved in the Permian orogeny, whose front is the Chiapas Massif.
Publications
- (2003): La Historia Metamórfica del Macizo de Chiapas: Nuevos datos geocronológicos y petrológicos de “La Sepultura”. – XIII Congreso Nacional de Geoquímica, INAGEQ, Querétaro, México
Weber, B., López-Martínez, L., Cameron, K., Hecht, L., Hiller, R., Gruner, B. Köhler, H.
- (2003): U-Pb zircon dating of metamorphic rocks from the Chiapas Massif: evidence for Grenville crust and a late Permian orogeny in the southern Maya terrane. - GSA Cordilleran Meeting Pto. Vallarta, GSA Abstracts with Programs, 35, 4, Paper 27-2
Weber, B. and Cameron, K.L.
- (2004): New insights on the provenance of the southern Maya terrane and its implications on southern Mexico – South America connections. – IV Reunión Nacional de Ciencias de la Tierra, Querétaro, Libro de Resúmenes, p. 200
Weber, B., Schaaf, P., Premo, W.R., Iriondo, A., Ortega-Gutierrez, F.
- (2004): The Igneous and metamorphic history of the Chiapas Massif, SE Mexico: new insights from zircon geochronology by SHRIMP and TIMS analysis. – 32nd International Geological Congress, Florence, Abstracts, p. 799
Weber, B., Schaaf, P., Premo, W.R., Iriondo, A., Cameron, K.L., and Ortega-Gutierrez, F.
- (2004): The ´Sepultura´ unit – A médium to high grade metasedimentary sequence in the Chiapas Massif, SE Mexico. – IV Reunión Nacional de Ciencias de la Tierra, Querétaro, Libro de Resúmenes, p. 200
Hiller, R., Weber, B., Hecht, L., Ortega-Gutiérrez, F., Schaaf, P., López-Martínez, M.
- (2005): A late Permian tectonothermal event in Grenville crust of the Southern Maya terrane: U-Pb zircon ages from the Chiapas massif, Southeastern Mexico. – International Geology Reviews, Vol. 47, p. 509-529 (IF.0.95)
Weber, B., Cameron, K.L., Osorio, M., and Schaaf, P.
- (2005): Edad y cinemática de milonitas en el área de Pijijiapan, Chiapas: Evidencia para el movimiento lateral entre el bloque Maya y Chortis. Reunión Anual Unión Geofísica Mexicana, GEOS, 27, 1, 94
Tovar-Cortés, J., Weber, B., López-Martínez, M.
- (2005): Evaluación paleomagnética y fábrica magnética de pseudotaquilitas en el Macizo de Chiapas, México. – Reunión Anual Unión Geofísica Mexicana, GEOS, 27, 1, 93
Molina-Garza, R., Weber, B., López-Martínez, M., Geissman, J.
- (2005): Revealing the history of a complex amphibole from the Chiapas Massif. – Reunión Anual Unión Geofísica Mexicana, GEOS, 27, 1, 106
López-Martínez M., Weber, B., Hecht, L., Gradilla-Martínez, L.
- (2005): Zircones detríticos de la formación Santa Rosa y del Macizo de Chiapas: Implicaciones para la evolución tectónica del bloque Maya. – Reunión Anual Unión Geofísica Mexicana, GEOS, 27, 1, 93-94
Weber, B., Valencia, V., Iriondo, A., Schaaf Peter, Ortega-Gutiérrez, F., Ruiz, J.
- (2006): Geocronología de núcleos detríticos en zircones del Macizo de Chiapas: Implicaciones para la historia Paleozoica del bloque Maya. – XIV Congreso Nacional de Geoquímica, Mérida, Yucatán, Actas INAGEQ, 12, 25
Weber, B., Valencia, V., Iriondo, A.
- (2006): Provenance ages of detrital zircon cores from paragneissanatexites: unraveling the pre-metamorphic history of the Permian Chiapas massif, Mexico. – GSA annual meeting Philadelphia, Geol. Soc. Am. Abstracts with Programs, 38, 7, 409
Weber, B., Valencia, V., Iriondo, A.
- (2006): Provenance ages of Late Paleozoic sandstones (Santa Rosa Formation) from the Maya block, SE Mexico – implications on the tectonic evolution of western Pangea. – Revista Mexicana de Ciencias Geológicas, 23 (2), 262-276
Weber, B., Schaaf, P., Valencia, V., Iriondo, A., Ortega-Gutiérrez, F.
- (2007): New insights into the history and origin of the southern Maya Block, SE México: U-Pb-SHRIMP zircon geochronology from metamorphic rocks of the Chiapas Massif. – International Journal of Earth Sciences (Geologische Rundschau), Vol. 96, No. 2, 253-269
Weber, B., Iriondo, A., Premo, W.R., Hecht, L., Schaaf, P.
- (2007): Petrogenesis and P-T Conditions of Metamorphic Rocks from the Chiapas Massif Complex in the Custepec Area, Chiapas, Mexico. – AGU Joint Assembly, Acapulco, Guerrero, Eos Trans. AGU, 88 (23), Jt. Assem. Suppl.U51A-04
Estrada-Carmona, J., Weber, B., Hecht, L., Martens, U.
- (2007): Provenance Ages of Protoliths from the Chiapas Massif Complex and Adjacent Strata of the Southern Maya Block - Implications on the Paleozoic Reconstruction of Middle America. – AGU Joint Assembly, Acapulco, Guerrero, Eos Trans. AGU, 88 (23), Jt. Assem. Suppl. U54A-02
Weber, B., Schaaf, P., Valencia, V., Lopez-Martínez, M., Ortega-Gutierrez, F.
