Contribución al alcance estratigráfico de la Formación Agueguexquite (Mioceno), Veracruz, México

La Formación Agueguexquite aflora en el sureste de México, en el sur de Veracruz y norte de Chiapas. Su edad aún no ha sido determinada formalmente, estudios previos han considerado su depósito entre el Mioceno y el Plioceno. Moluscos y muestras de roca de tres localidades de la Formación Agueguexquite en el estado de Veracruz, fueron recolectados a fin de establecer el alcance estratigráfico de la formación por medio de edades relativas utilizando foraminíferos bentónicos y nanoplancton calcáreo, así como estratigrafía isotópica (Sr87/Sr86). La datación con foraminíferos bentónicos mostró un alcance estratigráfico amplio (Mioceno-Plioceno). La nanofauna calcárea muestra una edad de depósito del Mioceno medio-tardío, que fue corroborada con los resultados de la estratigrafía isotópica con (Sr87/ Sr86). Con base a los resultados obtenidos se sugiere que la Formación Agueguexquite fue depositada entre el Mioceno medio y tardío.The Agueguexquite Formation crops out in southeastern Mexico, in southern Veracruz and northern Chiapas; its age has not been formally determined, but previous studies consider that it was deposited between the Miocene and Pliocene. Mollusks and rock samples from three localities of the Agueguexquite Formation in Veracruz were collected in order to define the stratigraphic range of this formation by means of relative ages using benthic forams and calcareous nannoplankton, as well as isotopic stratigraphy (87Sr/86Sr). Dating with benthic forams indicates a broad stratigraphic range (Miocene-Pliocene); calcareous nannoplankton yielded a middle-late Miocene age, which is supported by the results of isotopic stratigraphy (87Sr/86Sr). Based on our stratigraphic results, it is suggested that the Agueguexquite Formation was deposited between the middle and late Miocene.Facultad de Ciencias Naturales y Muse

Contribución al alcance estratigráfico de la Formación Agueguexquite (Mioceno), Veracruz, México

La Formación Agueguexquite aflora en el sureste de México, en el sur de Veracruz y norte de Chiapas. Su edad aún no ha sido determinada formalmente, estudios previos han considerado su depósito entre el Mioceno y el Plioceno. Moluscos y muestras de roca de tres localidades de la Formación Agueguexquite en el estado de Veracruz, fueron recolectados a fin de establecer el alcance estratigráfico de la formación por medio de edades relativas utilizando foraminíferos bentónicos y nanoplancton calcáreo, así como estratigrafía isotópica (Sr87/Sr86). La datación con foraminíferos bentónicos mostró un alcance estratigráfico amplio (Mioceno-Plioceno). La nanofauna calcárea muestra una edad de depósito del Mioceno medio-tardío, que fue corroborada con los resultados de la estratigrafía isotópica con (Sr87/ Sr86). Con base a los resultados obtenidos se sugiere que la Formación Agueguexquite fue depositada entre el Mioceno medio y tardío.The Agueguexquite Formation crops out in southeastern Mexico, in southern Veracruz and northern Chiapas; its age has not been formally determined, but previous studies consider that it was deposited between the Miocene and Pliocene. Mollusks and rock samples from three localities of the Agueguexquite Formation in Veracruz were collected in order to define the stratigraphic range of this formation by means of relative ages using benthic forams and calcareous nannoplankton, as well as isotopic stratigraphy (87Sr/86Sr). Dating with benthic forams indicates a broad stratigraphic range (Miocene-Pliocene); calcareous nannoplankton yielded a middle-late Miocene age, which is supported by the results of isotopic stratigraphy (87Sr/86Sr). Based on our stratigraphic results, it is suggested that the Agueguexquite Formation was deposited between the middle and late Miocene.Facultad de Ciencias Naturales y Muse

Geochemical and Sr - Nd isotopic characterization of the Miocene volcanic events in the Sierra Madre del Sur, central and southeastern Oaxaca, Mexico

Las regiones volcánicas de Etla, Mitla-Tlacolula y Nejapa, en la parte central y sureste de Oaxaca, conforman la porción oriental de un amplio arco magmático cenozoico dentro de la Sierra Madre del Sur. La mayoría de los eventos volcánicos ocurrieron entre los 22 y 15 Ma, casi de manera contemporánea con los primeros eventos volcánicos de la Faja Volcánica Transmexicana. Se determinaron las características petrográfi cas, geoquímicas e isotópicas de muestras volcánicas obtenidas de las tres regiones, donde existen ignimbritas, depósitos volcaniclásticos y epiclásticos, fl ujos de lava, y algunos depósitos lacustres. La clasifi cación química de las rocas en un diagrama SiO2 vs. álcalis varía entre andesitas y riolitas, siguiendo un patrón subalcalino bimodal. Al parecer no existen rocas con contenidos de SiO2 de entre 58 y 67 % en peso. Los patrones de comportamiento de elementos traza de las andesitas y riolitas son muy similares, con un enriquecimiento en los elementos litófi los respecto a los elementos de alto potencial iónico, mientras que los patrones de los elementos de tierras raras, normalizados con respecto a condrita, muestran un enriquecimiento de las tierras raras ligeras (La-Sm) respecto a las tierras raras pesadas, con un comportamiento casi plano para estas últimas (Eu-Lu). Este tipo de patrones se ha asociado con rocas de arco volcánico. Las relaciones isotópicas iniciales de Sr y Nd muestran ciertas diferencias entre las regiones de Etla, Mitla-Tlacolula (87Sr/86Sr: 0.7047 a 0.7066 y eNd: -1.15 a 1.75) y Nejapa (87Sr/86Sr: 0.7035 a 0.7048 y eNd: +0.52 a +1.42). Con base en estos datos isotópicos, más los existentes para rocas del basamento de estas regiones, se sugiere que los magmas de las dos primeras regiones sufrieron una mayor interacción con la corteza continental en comparación con los magmas de la región de Nejapa. Las composiciones isotópicas de las rocas del área de estudio son similares a las observadas en otras regiones volcánicas cenozoicas de la Sierra Madre del Sur. Los eventos volcánicos del Mioceno temprano a medio de la parte central y sureste de Oaxaca, junto con los primeros eventos magmáticos de la Faja Volcánica Transmexicana podrían conformar un arco magmático continuo con una orientación anómala, antes de alcanzar su posición actual

Rocas graníticas pérmicas en la Sierra Pinta, NW de Sonora, México: Magmatismo de subducción asociado al inicio del margen continental activo del SW de Norteamérica

Multidisciplinary studies in the Sierra Pinta area in NW Sonora, including U-Pb zircon geochronology, mayor and trace element geochemistry as well as Sm-Nd isotopic studies have allowed a precise characterization of Permian granitic rocks. The results suggest the existence of a suite of Permian granitoids with U-Pb zircon ages between ~275 y ~258 Ma. Initial epsilon Nd values between -19.2 and -10.4 and corresponding Nd model ages (TDM) between 2.5 and 1.6 Ga, as well as some geochemical characteristics, indicate melting of Proterozoic crustal basement associated to the early stages of subduction forming the continental magmatic arc of SW North America. It is likely that this Permian magmatic pulse in NW Sonora was part of a larger event in the American Cordillera extending from western-southwestern USA, passing through northern Mexico and up to central and southern Mexico and, perhaps, to northern South America (i.e., Colombia). We suggest that the occurrence of the Permian granitoids in Sierra Pinta is associated with a Paleoproterozoic crustal weakness zone formed by a narrow stretch of the Yavapai crustal province. This weakness zone could have acted as a channel-way that facilitated the ascent towards higher crustal levels of the first magmas generated by subduction.Estudios geológicos multidisciplinarios realizados en rocas cartografiadas en el área de Sierra Pinta, en el NW de Sonora, que incluyeron geocronología U-Pb en zircones, estudios isotópicos de Sm- Nd en roca total y geoquímica de elementos mayores, traza y tierras raras, han permitido un precisa caracterización de rocas graníticas pérmicas. Los datos muestran la existencia de rocas pérmicas con edades U-Pb en zircones entre ~275 y ~258 Ma y valores iniciales de épsilon Nd de -19.2 a -10.4 (edades modelo Nd TDM entre 2.5 y 1.6 Ga). Estos datos, junto con los estudios geoquímicos, indican fusión de basamento cortical proterozoico relacionada a los estadios iniciales de la subducción que propició el establecimiento de un arco magmático continental en el SW de Norteamérica. Probablemente este pulso magmático es parte de un gran evento a nivel cordillerano que se extiende desde el SW de los EE. UU. pasando por Sonora, Chihuahua y Coahuila hasta el centro y sur de México, y quizás hasta el norte de Sudamérica (específicamente Colombia). La ocurrencia de estas rocas graníticas pérmicas en la Sierra Pinta podría asociarse a una zona de debilidad cortical formada por una franja angosta de basamento paleoproterozoico Yavapai, como el encontrado en algunos lugares del NW de Sonora, que sirvió para que los primeros magmas generados por subducción ascendieran con mayor facilidad hacia la superficie

Miocene andesitic lavas of Sierra de Angangueo: a petrological, geochemical, and geochronological approach to arc magmatism in Central Mexico

<p>The early and middle Miocene andesitic lavas of the Sierra de Angangueo (MALSA) represent one of the most prominent landforms of intermediate magmatism in Central Mexico. In this paper, we present new petrological, geochemical, thermobarometric, and geochronological data in order to discuss the conditions of MALSA’s emplacement in the lithosphere.</p> <p>MALSA comprises a voluminous eroded early Miocene andesitic structure, emplaced on a Mesozoic basement. MALSA shows a wide variety of textures, from glassy and aphanitic to porphyritic, mainly composed of plagioclase (An_10-55), clinopyroxene (Wo_60-65; En_35-40), amphibole (Mg-hornblende/edenite), and rarely olivine (Fo_75–86) or orthopyroxene (En_72-80). Major and trace elements plots follow a typical calc-alkaline trend with relatively increasing fractionation.</p> <p>The ⁸⁷Sr/⁸⁶Sr isotopic signatures range between 0.703343 and 0.704459 and εNd values from +1.37 to +4.84; apparently without a significant contribution of an old, thick, or highly radiogenic continental crust. Lead isotope values ²⁰⁶Pb/²⁰⁴Pb vary between 18.68 and 18.83, ²⁰⁷Pb/²⁰⁴Pb from 15.57 to 15.65, and ²⁰⁸Pb/²⁰⁴Pb from 38.39 to 38.67, suggesting contamination of magmas by juvenile crust. Thermodynamic calculations indicate equilibrium conditions at ≤900°C and ~2 kbar and an average log ƒO₂ ≈ −10.</p> <p>Ar–Ar and K–Ar dating carried out on whole rock, matrix, amphibole, and plagioclase phenocrysts yielded ages between 13.0 ± 0.5 and 23.9 ± 0.3 Ma. These dates indicate a series of pulses with maximum magmatic activity between 24 and 18 Ma.</p> <p>The geochemical and petrologic data suggest partial melting processes in the lower or middle crust associated with possible magma mixing during its ascent; such a mechanism could explain a hybrid mantle-lower crust origin of this large volume of andesites. The MALSA, as well as the early to middle Miocene magmatism from Western Mexico, could represent two coeval and independent magmatic arcs prior to the present Trans-Mexican Volcanic Belt (TMVB) in Central Mexico.</p

The life story of a gomphothere from east-central Mexico: A multidisciplinary approach

The Gomphotheriidae family belongs to the Proboscidea order. Gomphotheres were elephant-like mammals whose representatives inhabited North America from the Middle Miocene to the late Pleistocene. In this work, a gomphothere remains from Santiago Tepeticpac (Tlaxcala, Mexico) are described. The comparative study with other Proboscidea, mainly from different localities of Mexico and South America, mostly Argentina and Brazil, allowed to identify them as Cuvieronius genus. The microwear signature together with carbon and oxygen stable isotopes showed that this animal was a mixer feeder and inhabited open zones with patches of wooded flora. These analyses also indicated the presence of high abrasiveness components and C4 plants (grasslands) and medium to low wearing elements as well as C3 vegetation (forest) in the site. The strontium isotopic signature implies that this gomphothere lived in the Puebla-Tlaxcala Valley. The paleoenvironmental inferences would show regional conditions.We thank Project 11824 of the National Coordination of Cultural Heritage Conservation-INAH for facilitating the collaboration of Dr. María Teresa Alberdi Alonso during the month of November 2016 in the workshop/laboratory of Conservation of Organic Materials of the CNCPC-INAH. To PAPIIT-UNAM-México (#IN404714, IA#104017 and IA#102719), support for this project to Edith Cienfuegos-Alvarado and Víctor Adrián Pérez Crespo. DGICYT CGL2016-79334-P from Spain granted to Maria Teresa Alberdi contributed to funding this investigation.Coordinación Nacional De Conservación Del Patrimonio Cultural (México)Universidad Autónoma Nacional de MéxicoPeer reviewe

Sm-Nd isotope whole rock and garnet from the Southwestern Grenvillian Oaxacan Complex, Mexico: a review of garnet closure temperature and structural implications

The Oaxacan Complex represents the largest outcrop of late Mesoproterozoic age, granulite-facies metamorphic rocks in Mexico. The southwestern Oaxacan Complex was studied using the Sm-Nd isotopic system in whole-rocks and garnets. The use of garnet for dating granulites is a good approach because garnet is usually a rock-forming mineral in this facies, it strongly prefers heavy over light rare earth elements, and its closure temperature is close to the range of the granulite facies. Since the studied garnets display resorbed borders, they are interpreted as pre- or syn- Grenvillian-age granulitic peak. Their closure temperatures, calculated using the Dodson equation and the Nd diffusion coefficients, are 720–770ºC (5– 30ºC/my). The whole-rock Sm-Nd evolution lines intersect the depleted mantle model at ca. 1.4–1.7 Ga, although two paraderivate samples intersect at 2.1–2.2 Ga, suggesting a protolith from an older continental crust. In nearby outcrops, Sm-Nd isochron garnet-whole rock ages follow a certain NW-SE trend, displaying two hypothetical age groups: old ages (954–976 Ma) and young ages (907–920 Ma). The younger age garnet's group display Ti and LREE-MREE rim enrichments with respect to their cores that imply diffusional resetting processes. Folding and faulting structures suggest that these two age groups correspond to different structural levels during the early cooling history of the Oaxacan Complex.The Oaxacan Complex represents the largest outcrop of late Mesoproterozoic age, granulite-facies metamorphic rocks in Mexico. The southwestern Oaxacan Complex was studied using the Sm-Nd isotopic system in whole-rocks and garnets. The use of garnet for dating granulites is a good approach because garnet is usually a rock-forming mineral in this facies, it strongly prefers heavy over light rare earth elements, and its closure temperature is close to the range of the granulite facies. Since the studied garnets display resorbed borders, they are interpreted as pre- or syn- Grenvillian-age granulitic peak. Their closure temperatures, calculated using the Dodson equation and the Nd diffusion coefficients, are 720–770ºC (5– 30ºC/my). The whole-rock Sm-Nd evolution lines intersect the depleted mantle model at ca. 1.4–1.7 Ga, although two paraderivate samples intersect at 2.1–2.2 Ga, suggesting a protolith from an older continental crust. In nearby outcrops, Sm-Nd isochron garnet-whole rock ages follow a certain NW-SE trend, displaying two hypothetical age groups: old ages (954–976 Ma) and young ages (907–920 Ma). The younger age garnet's group display Ti and LREE-MREE rim enrichments with respect to their cores that imply diffusional resetting processes. Folding and faulting structures suggest that these two age groups correspond to different structural levels during the early cooling history of the Oaxacan Complex.The Oaxacan Complex represents the largest outcrop of late Mesoproterozoic age, granulite-facies metamorphic rocks in Mexico. The southwestern Oaxacan Complex was studied using the Sm-Nd isotopic system in whole-rocks and garnets. The use of garnet for dating granulites is a good approach because garnet is usually a rock-forming mineral in this facies, it strongly prefers heavy over light rare earth elements, and its closure temperature is close to the range of the granulite facies. Since the studied garnets display resorbed borders, they are interpreted as pre- or syn- Grenvillian-age granulitic peak. Their closure temperatures, calculated using the Dodson equation and the Nd diffusion coefficients, are 720–770ºC (5– 30ºC/my). The whole-rock Sm-Nd evolution lines intersect the depleted mantle model at ca. 1.4–1.7 Ga, although two paraderivate samples intersect at 2.1–2.2 Ga, suggesting a protolith from an older continental crust. In nearby outcrops, Sm-Nd isochron garnet-whole rock ages follow a certain NW-SE trend, displaying two hypothetical age groups: old ages (954–976 Ma) and young ages (907–920 Ma). The younger age garnet's group display Ti and LREE-MREE rim enrichments with respect to their cores that imply diffusional resetting processes. Folding and faulting structures suggest that these two age groups correspond to different structural levels during the early cooling history of the Oaxacan Complex