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New time-constraints on provenance, metamorphism and exhumation of the Bahía Mansa Metamorphic Complex on the Main Chiloé Island, south-central Chile

dc.contributoren-US
dc.contributores-ES
dc.creatorDuhart, Paul
dc.creatorAdriasola, Alberto C.
dc.date2010-06-21
dc.identifierhttp://www.andeangeology.cl/index.php/revista1/article/view/V35n1-a04
dc.identifier10.5027/andgeoV35n1-a04
dc.descriptionLa Cordillera de la Costa, en la parte occidental del Archipielago de Chiloe, representa un alto emergido de antearco en el frente de subduccion del centro-sur de Chile. Previo al Cenozoico, periodo en el cual la region ha sido dominada por un sistema de subduccion de la placas Farallon y Nazca por debajo de la placa Sudamericana, la historia del basamento metamorfico de la Cordillera de la Costa involucro episodios de subduccion y/o acrecion de material oceanico y ensialico a lo largo del margen proto-Pacifico de Gondwana. El registro termal del basamento metamorfico incluye metamorfismo regional, exhumacion y finalmente magmatismo para los distintos segmentos de la Isla Grande de Chiloe. Edades convencionales U-Pb en circones detriticos de un esquisto pelitico del Segmento Central de la Isla Grande de Chiloe, restringen la posible edad maxima de sedimentacion al Carbonifero (310 Ma). Otras fracciones de circones concordantes (360, 390 y 412 Ma) sugieren fuentes primarias del Ordovicico. La proveniencia sedimentaria de los circones podria corresponder a rocas magmaticas continentales de edades similares actualmente expuestas en areas adyacentes, tales como en las cordilleras deNahuelbutay Principal y, adicionalmente, desde areas distales en los macizos Nor-Patagonico y Deseado. Un extenso metamorfismo en la facies de esquistos verdes esta bien representado en las rocas del basamento metamorfico. Edades de enfriamiento K-Ar y 40Ar/39Ar en mica blanca se situan en el rango de entre 245 y 220 Ma, y son interpretadas como cercanas al maximo metamorfico ocurrido durante el Triasico Medio-Triasico Tardio temprano. Edades centrales por medio de trazas de fision (TF) en circon y apatita se situan en el rango Jurasico Tardio (ca. 156 Ma) al Eoceno (ca. 38 Ma) y estan acompañadas por una relativa gran dispersion. El modelamiento de las distribuciones de las TF en un grano unico de circon, muestra un maximo en el Cretacico Tardio (ca. 80 Ma) y, el modelamiento en apatita, muestra dos poblaciones, una antigua con un maximo en el Cretacico Tardio (ca. 64-91 Ma), y una joven, con un maximo en el Eoceno (ca. 38-53 Ma). Los maximos de circon y apatita en el Cretacico Tardio, junto con la presencia de abundantes sedimentos de presumible edad cretacica tardia alta, que se encuentran en la cuenca de antearco, al oeste de la Isla Grande de Chiloe, sugieren que la exhumacion de la unidad metamorfica ocurrio durante el Cretacico Tardio temprano. Circones magmaticos de un cuerpo granodioritico (Pluton Met alqui) emplazado en esquistos peliticos dentro del Segmento Central de la Isla Grande de Chiloe, un bloque diferencialmente alzado en relacion a los segmentos norte-sur, dieron una edad de cristalizacion U-Pb eocena de 39,6±0,3 Ma. Diques y 'sills' daciticos que contienen biotita (Dacita Gamboa) del Eoceno Superior (ca. 37 Ma) tambien ocurren dentro del Segmento Central. Edades concordantes de TF en circon y apatita (ca. 36 Ma) indican un rapido enfriamiento de las rocas subvolcanicas durante el Eoceno. De acuerdo con estos datos, las edades centrales y modeladas de TF en apatita del Eoceno, detectadas en rocas metamorficas, probablemente representen un reajuste termal debido a magmatismo poco profundo, contemporaneo con la Dacita Gamboa y, posiblemente, con el Pluton Met alqui.ABSTRACT:The Coastal Ranges in the western part of the Chiloe Archipelago represent an emerged forearc high at the subduction front of south-central Chile. Prior to the Cenozoic framework of the subducting Farallon and Nazca plates beneath the South American plate, the history of the metamorphic basement in the Coastal Ranges involves episodes of subduction and/or accretion of oceanic and ensialic material along the proto-Pacific margin of Gondwana. Along different segments of the Main Chiloe Island the thermal record of the metamorphic basement rocks includes regional metamorphism, exhumation and finally magmatism. Detrital zircons conventional U-Pb ages from a pelitic schist of the Central Segment of the Main Chiloe Island constrain the maximum possible sedimentation age to the Carboniferous (310 Ma). Other concordant zircon fractions (360, 390 and 412 Ma) suggest Ordovician primary sources. The sedimentary provenance of zircons could correspond to continental magmatic rocks of similar ages presently exposed in adjacent areas such as Nahuelbuta Mountains and Principal Cordillera and, additionally, from distal areas in the North-Patagonian and Deseado massifs. Widespread metamorphism in greenschist facies is well represented in the basement unit of the Main Chiloe Island. K-Ar and 40Ar/39Ar cooling ages in white mica range between 245 to 220 Ma, interpreted as near the peak of metamorphism during Middle to early Late Triassic times. Zircon and apatite fission-track (FT) central ages in metamorphic rocks range from Late Jurassic (ca. 156 Ma) to Eocene (ca. 38 Ma) and are accompanied by relatively large dispersions. The modelling of the zircon FT single-grain age distributions depicts younger deconvoluted Late Cretaceous peak (ca. 80 Ma) and the apatite modelling shows two populations, an older Late Cretaceous peak (ca. 64-91 Ma) and a younger Eocene peak (ca. 3 8-5 3 Ma). The zircon and apatite Late Cretaceous peaks together with the presence of a marine sedimentary succession of presumably later Late Cretaceous age found at the forearc slope of the Main Chiloe Island, suggest exhumation of the metamorphic unit during the early Late Cretaceous. Magmatic zircons from a granodioritic body (Met alqui Pluton) emplaced in pelitic schists within the Central Segment of the Main Chiloe Island, a differentially uplifted block in relation to the northern and southern segments, gave an Eocene U-Pb crystallization age (39.6±0.3 Ma). Upper Eocene (ca. 37 Ma) biotite-bearing porphyric dacitic sills and dikes (Gamboa Dacite) also occur within the Central Segment. Zircon and apatite FT concordant ages (ca. 36 Ma) indicated rapid cooling for these subvolcanic rocks during Eocene times. According to this data, Eocene apatite central and modelling single-grain FT ages detected in the metamorphic rocks probably represent thermal resetting by shallow magmatism, coeval with the Gamboa Dacite and possibly the Met alqui Pluton.es-ES
dc.descriptionThe Coastal Ranges in the western part of the Chiloé Archipelago represent an emerged forearc high at the subduction front of south-central Chile. Prior to the Cenozoic framework of the subducting Farallón and Nazca plates beneath the South American plate, the history of the metamorphic basement in the Coastal Ranges involves episodes of subduction and/or accretion of oceanic and ensialic material along the proto-Pacific margin of Gondwana. Along different segments of the Main Chiloé Island the thermal record of the metamorphic basement rocks includes regional metamorphism, exhumation and finally magmatism. Detrital zircons conventional U-Pb ages from a pelitic schist of the Central Segment of the Main Chiloé Island constrain the maximum possible sedimentation age to the Carboniferous (310 Ma). Other concordant zircon fractions (360, 390 and 412 Ma) suggest Ordovician primary sources. The sedimentary provenance of zircons could correspond to continental magmatic rocks of similar ages presently exposed in adjacent areas such as Nahuelbuta Mountains and Principal Cordillera and, additionally, from distal areas in the North-Patagonian and Deseado massifs. Widespread metamorphism in greenschist facies is well represented in the basement unit of the Main Chiloé Island. K-Ar and 40Ar/39Ar cooling ages in white mica range between 245 to 220 Ma, interpreted as near the peak of metamorphism during Middle to early Late Triassic times. Zircon and apatite fission-track (FT) central ages in metamorphic rocks range from Late Jurassic (ca. 156 Ma) to Eocene (ca. 38 Ma) and are accompanied by relatively large dispersions. The modelling of the zircon FT single-grain age distributions depicts younger deconvoluted Late Cretaceous peak (ca. 80 Ma) and the apatite modelling shows two populations, an older Late Cretaceous peak (ca. 64-91 Ma) and a younger Eocene peak (ca. 3 8-5 3 Ma). The zircon and apatite Late Cretaceous peaks together with the presence of a marine sedimentary succession of presumably later Late Cretaceous age found at the forearc slope of the Main Chiloé Island, suggest exhumation of the metamorphic unit during the early Late Cretaceous. Magmatic zircons from a granodioritic body (Met alqui Pluton) emplaced in pelitic schists within the Central Segment of the Main Chiloé Island, a differentially uplifted block in relation to the northern and southern segments, gave an Eocene U-Pb crystallization age (39.6±0.3 Ma). Upper Eocene (ca. 37 Ma) biotite-bearing porphyric dacitic sills and dikes (Gamboa Dacite) also occur within the Central Segment. Zircon and apatite FT concordant ages (ca. 36 Ma) indicated rapid cooling for these subvolcanic rocks during Eocene times. According to this data, Eocene apatite central and modelling single-grain FT ages detected in the metamorphic rocks probably represent thermal resetting by shallow magmatism, coeval with the Gamboa Dacite and possibly the Met alqui Pluton.en-US
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dc.languageeng
dc.publisherServicio Nacional de Geología y Mineríaen-US
dc.relationhttp://www.andeangeology.cl/index.php/revista1/article/view/V35n1-a04/405
dc.relationhttp://www.andeangeology.cl/index.php/revista1/article/view/V35n1-a04/html
dc.sourceAndean Geology; Vol 35, No 1 (2008)es-ES
dc.sourceAndean Geology; Vol 35, No 1 (2008)en-US
dc.source0718-7106
dc.source0718-7092
dc.titleNuevas restricciones temporales sobre proveniencia, metamorfismo y exhumacion del Complejo Metamorfico Bahia Mansa en la Isla Grande de Chiloe, centro-sur de Chile.New time-constraints on provenance, metamorphism and exhumation of the Bahia Mansa Metamorphic Complex on the Main Chiloe Island, south-central Chile.es-ES
dc.titleNew time-constraints on provenance, metamorphism and exhumation of the Bahía Mansa Metamorphic Complex on the Main Chiloé Island, south-central Chileen-US
dc.typeinfo:eu-repo/semantics/article
dc.typeinfo:eu-repo/semantics/publishedVersion
dc.typees-ES
dc.typeen-US


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