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Mantle derived crystal-poor rhyolitic ignimbrites:Eruptive mechanism from geochemical and geochronological data of the Piedra Parada caldera,Southern Argentina

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Geoscience Frontiers 2018年第5期9卷 1529-1553页

作者： Eugenio Aragon Antonio Castro Juan Diaz-Alvarado Lucio Pinotti Fernando D'eramo Manuel Demartis jorge coniglio Irene Hernando Carmen Rodriguez Centro de Investigaciones Geológicas(UNLP-CONICET) 1 N^o 644(1900)La PlataBuenos AiresArgentina Departamento de Geología Universidad de Huelva21071 HuelvaSpain Departamento de Geología Universidad de AtacamaCopayapu 485CopiapóChile Departamento de Geología Universidad Nacional de Río Cuarto(UNRC-CONICET)Ruta 36 km 601Río CuartoCórdobaArgentina

Trace elements, isotopic modeling and U-Th-Pb SHRIMP zircon age constraints are used to reconstruct the eruption history and magmatic processes of the Piedra Parada Caldera. In the early Eocene, the crystal-poor Barda Colorada ignimbrite(BCI), having >>15% micro-porphyritic crystals with respect to magmatic components, erupted a volume estimated in more than 300 km^3. The Piedra Parada caldera is located in the Patagonian Andes foreland, at the southern end of the calderas field of the Pilcaniyeu Volcanic Belt(PVB). This belt is related to an extensional tectonic setting as a result of the collision of the Farallon-Aluk ridge with South America, which enabled the development of a transform ocean/continental plate margin followed by the detachment of the Aluk plate and the opening of a slab window. The BCI extra-caldera Plateau is a >100 m thick deposit, having a lower unit with high silica(Si O_2> 76 wt.%),potassium poor rhyolitic composition(trondhjemitic like magma), and an upper unit with normal to high potassium rhyolitic composition(granitic like magma). A trace elements modeling of the BCI units shows that the BCI lower and upper units did not evolve from fractionation or immiscibility in the shallow magma reservoir. The BCI also have a primitive isotopic signature(initial87 Sr/86 Sr =0.7031-0.7049 and ε_(Nd)= +3.4 to +3.65). Thus, tectonic, compositional and isotopic constraints suggest the fast ascent of high silica magmas to a shallow reservoir, and point to an upper mantle origin for these rhyolitic magmas in a transitional(Orogenic-Anorogenic) tectono-magmatic setting. U-Th-Pb SHRIMP zircon crystallization ages of the Syn-caldera stage BCI units(56 -51.5 Ma) show a protracted life of 5 Ma for this caldera reservoir. The age of 52.9 ± 0.3 Ma is considered the best fit for the possible maximum age for the caldera collapse. The Late-caldera magmatism has trachyandesitic and rhyolitic *** trace element modeling suggests that these rhyolites evolv

关键词： Mantle derived rhyolite Crystal-poor ignimbrite Bimodal volcanism Slab window volcanism K poor-rhyolite

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Magma chamber growth models in the upper crust: A review of the hydraulic and inertial constraints

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Geoscience Frontiers 2019年第3期10卷 1211-1218页

作者： Eugenio Aragón Fernando J.D'Eramo Lucio P.Pinotti Manuel Demartis José María Tubía Roberto F.Weinberg jorge E.coniglio Centro de Investigaciones Geologicas (CONICET-Universidad Nacional de La Plata).Diagonal 113 N° 275 Universidad Nacional de Río Cuarto CONICET Departamento de Geología (Ruta Nac.N° 36 Km 601) Departamento de Geodinámica Universidad del País Vasco (UPV/EHU) School of Earth Atmosphere and Environment Monash University

Finite volumes of magma moving in confinement, store hydraulic potential energy for the generation,control and transmission of power. The Pascal's principle in a hydraulic jack arrangement is used to model the vertical and lateral growth of sills. The small input piston of the hydraulic jack is equivalent to the feeder dike, the upper large expansible piston equivalent to the magmatic chamber and the inertial force of the magma in the dike is the input force. This arrangement is particularly relevant to the case of sills expanding with blunt tips, for which rapid fracture propagation is inhibited. Hydraulic models concur with experimental data that show that lateral expansion of magma into a sill is promoted when the vertical ascent of magma through a feeder dike reaches the bottom contact with an overlying, flat rigid-layer. At this point, the magma is forced to decelerate, triggering a pressure wave through the conduit caused by the continued ascent of magma further down(fluid-hammer effect). This pressure wave can provide overpressure enough to trigger the initial hydraulic lateral expansion of magma into an incipient sill, and still have enough input inertial force left to continue feeding the hydraulic system. The lateral expansion underneath the strong impeding layer, causes an area increase and thus, further hydraulic amplification of the input inertial force on the sides and roof of the incipient sill, triggering further expansion in a self-reinforcing process. Initially, the lateral pressure increase is larger than that in the roof allowing the sill to expand. However, expansion eventually increases the total integrated force on the roof allowing its uplift into either a laccolith, if the roof preserves continuity, or into a piston bounded by a circular set of fractures. Hydraulic models for shallow magmatic chambers, also suggest that laccolith-like intrusions require the existence of a self-supported chamber roof. In contrast, if the roof of magmatic cham

关键词： Pascal’s principle Geologic hydraulic jack Emplacement Sills growth Fluid hammer

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The Farallon-Aluk ridge collision with South America:Implications for the geochemical changes of slab window magmas from fore- to back-arc

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Geoscience Frontiers 2013年第4期4卷 377-388页

作者： Eugenio Aragón Lucio Pinotti Fernando D'Eramo Antonio Castro Osvaldo Rabbia jorge coniglio Manuel Demartis Irene Hernando Claudia E.Cavarozzi Yolanda E.Aguilera Centro de Investigaciones Geologicas(UNLP-CONICET) 1 N°644(1900) La Plata.Buenos Aires.Argentina Facultad de Ciencias Naturales y Museo Universidad Nacional de la Plata122 y 60s/n.(1900).La PlataBuenos AiresArgentina Departamento de Geologia.Universidad Nacional de Rio Cuarto.(UNRC-CONICET).Ruta 36 km 601.Rio Cuarto.Cordoba.Argentina Departamento de Geologia.Universidad de Huelva.21071 Huelva.Spain Instituto de Geologia Economica Aplicada.Universidad de Concepcion.Concepcion.Chile Departamento de Geologia.Universidad Nacional de Rio Cuarto.(UNRC-CONICET) Ruta 36 km 601.Rio Cuarto.CordobaArgentina Centro de Investigaciones Geologicas(UNLP-CONICET).1 N°644.(1900) La Plata.Buenos Aires.Argentina Facultad de Ciencias Naturales y Museo.Universidad Nacional de la Plata.122 y 60 s/n.(1900).La PlataBuenos Aires.Argentina Facultad de Ciencias Naturales y Museo.Universidad National dela Plata.122 y 60 s/n(1900)La Plata.Buenos Aires.Argentina Direccion de Aplicacion de Imagenes Satelitarias(M.O.S.P.).58 e/ 7 y 8.piso 2.1900 La Plata.Buenos Aires.Argentina

The collision of a divergent ocean ridge may evolve into two end cases： in the continuity of ocean-floor subduction, or in the detachment of the subducted plate. The northern Patagonia active plate margin has the unique situation that in Cenozoic time it has been subjected to two divergent ridge collisions, each one representing one of the end members. The Neogene Antarctica-Nazca divergent ridge collision evolved as a continuous ocean-floor subduction system, promoting a magmatic hiatus at the arc axis, the obduction of part of the ridge ocean-floor in the fore-arc, and basaltic volcanism in the back-arc. In contrast, the Paleogene Farallon-Aluk divergent ridge collision evolved into a transform margin, with the detachment and sinking of the Aluk plate and the development of a large slab window. As in the previous case, this collision promoted a magmatic hiatus at the arc axis, but the tectono-magmatic scenario changed to postorogenic synextensional volcanism that spread to the former fore-arc （basalt, andesite, rhyolite） and former back-arc （bimodal ignimbrite flare-up, basalt）. Geochemistry of this slab window synextensional volcanism shows more MORB-like basalts towards the former fore-arc, and MORB-OIB-Iike basalts towards the former back-arc. Instead, an isolated undeformable crustal block in the former back-arc, with an ＂epeirogenic＂ response to the slab window and extensional regime, was covered by OIB-type basalts after uplift. Major elements show that slab window basalts reach TiO2 values up to 3 wt%, as compared with the top value of 1.5 wt% of arc magmas. Besides, the MgO with respect to （FeOt ＋ A1203） ratio helps to distinguish slab window magma changes from the former fore-arc to the former back-arc and also with respect to the ＂epeirogenic＂ block. Higher contents of HFS elements such as Nb and Ta also help to distinguish this slab window from arc magmas and also, to distinguish slab window magma changes from the former fore-arc to the former back-arc

关键词： AndesActive ridge collisionSubduction-transform transitionSlab window volcanismSynextensional calc-alkalineBimodal volcanism

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