Search

In this contribution we present new U-Pb SHRIMP ages and in situ LA-ICP-MS trace element geochemistry of zircon crystals from the Río de los Sauces anatectic granite, Córdoba, Argentina. Notable difference in texture and composition allowed us to identify two zircon populations in a single granite sample that are interpreted as reflecting metamorphic and igneous origins. Zircons regarded as restitic crystals entrained during the melt segregation yielded a slightly older concordia age of 537.1 ± 4.8 Ma (2σ) than those interpreted as igneous, dated at 529 ± 6 (2σ) Ma. Inherited metamorphic zircons are interpreted to represent solid-state growth during high temperature metamorphism of the Pampean orogeny at the onset of the anatexis or metamorphic peak. By contrast, igneous zircons would record the crystallization age of Zr within the Río de los Sauces granite. The textural, compositional and geochronological data of both zircon populations suggest that the inception of the anatexis, the melt segregation and crystallization occurred during a short period of time of 8 my En este trabajo se aportan nuevos datos de edades U-Pb SHRIMP y análisis in situ LA-ICP-MS de elementos traza de circones provenientes del granito Río de los Sauces, Córdoba, Argentina. A partir de marcadas diferencias texturales y composicionales se pudieron identificar dos poblaciones de circones en una misma muestra del granito, las cuales sugieren orígenes metamórficos e ígneos. Las edades concordia obtenidas en los circones metamórficos e ígneos fueron de 537,1 ± 4,8 Ma (2σ) y 529 ± 6 (2σ) Ma, respectivamente. Se interpreta que los circones metamórficos representan el crecimiento en estado sólido durante el metamorfismo de alta temperatura de la orogenia Pampeana, durante el inicio o el clímax de la anatexia. Por su parte, las edades de los circones ígneos representan la edad de cristalización del granito Río de los Sauces. Los datos texturales, composicionales y geocronológicos de ambas poblaciones de circones sugieren que el inicio de la anatexia, la segregación del fundido y la cristalización ocurrieron durante un periodo breve de tiempo de 8 ma.

In this contribution we present new U-Pb SHRIMP ages and in situ LA-ICP-MS trace element geochemistry of zircon crystals from the Río de los Sauces anatectic granite, Córdoba, Argentina. Notable difference in texture and composition allowed us to identify two zircon populations in a single granite sample that are interpreted as reflecting metamorphic and igneous origins. Zircons regarded as restitic crystals entrained during the melt segregation yielded a slightly older concordia age of 537.1 ± 4.8 Ma (2σ) than those interpreted as igneous, dated at 529 ± 6 (2σ) Ma. Inherited metamorphic zircons are interpreted to represent solid-state growth during high temperature metamorphism of the Pampean orogeny at the onset of the anatexis or metamorphic peak. By contrast, igneous zircons would record the crystallization age of Zr within the Río de los Sauces granite. The textural, compositional and geochronological data of both zircon populations suggest that the inception of the anatexis, the melt segregation and crystallization occurred during a short period of time of 8 my, En este trabajo se aportan nuevos datos de edades U-Pb SHRIMP y análisis in situ LA-ICP-MS de elementos traza de circones provenientes del granito Río de los Sauces, Córdoba, Argentina. A partir de marcadas diferencias texturales y composicionales se pudieron identificar dos poblaciones de circones en una misma muestra del granito, las cuales sugieren orígenes metamórficos e ígneos. Las edades concordia obtenidas en los circones metamórficos e ígneos fueron de 537,1 ± 4,8 Ma (2σ) y 529 ± 6 (2σ) Ma, respectivamente. Se interpreta que los circones metamórficos representan el crecimiento en estado sólido durante el metamorfismo de alta temperatura de la orogenia Pampeana, durante el inicio o el clímax de la anatexia. Por su parte, las edades de los circones ígneos representan la edad de cristalización del granito Río de los Sauces. Los datos texturales, composicionales y geocronológicos de ambas poblaciones de circones sugieren que el inicio de la anatexia, la segregación del fundido y la cristalización ocurrieron durante un periodo breve de tiempo de 8 ma.

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 chambers loses the self-supporting capacity, lopoliths and calderas should be expected for more or less dense magmas, respectively, owing to the growing influence of the density contrast between the host rock and the magma. (C) 2018, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. We are particularly indebted to Eduardo Llambias, for his useful advice and stimulating discussions. Ideas and data presented in this manuscript have benefited from an anonymous reviewer. This study is the result of research projects UNLP 11N/689 and 790, CONICET PID 00688: Agencia Nacional PICT 0910/13 and PICT 1754/16. J.M. Tubia thanks the GIU17/033 (UPV/EHU) and CGL2017-82976-P (AEI/FEDER, UE) projects.

El distrito pegmatítico Comechingones, ubicado en el faldeo oriental de la sierra homónima, en la provincia de Córdoba, involucra pegmatitas graníticas correspondientes a la clase de Elementos Raros, tipo berilo, subtipo berilo-columbita-fosfatos, algunas en transición a la clase muscovítica, con mineralizaciones de Be-Nb- Ta-U y minerales industriales. Dos tipos de pegmatitas graníticas han sido descriptas en el sector sur del distrito: pegmatitas tipo I, con tamaños que en total pueden alcanzan los 1000 metros de longitud y superar los 50 de ancho, internamente zonadas y portadoras de Be, Nb-Ta y U; y pegmatitas tipo II, de menores dimensiones, no zonadas, ricas en cuarzo de alta pureza, carentes de mineralizaciones metalíferas, y asociadas espacial y genéticamente con leucogranitos aplíticos. En este trabajo se presentan y discuten los datos geoquímicos preliminares de ambos tipos de pegmatitas y granitos asociados. Los datos geoquímicos obtenidos, apoyados con descripciones de campo y petrográficas, permiten establecer que las dos tipologías de pegmatitas corresponden a dos eventos magmáticos diferentes (muy probablemente diacrónicos). El primero generó las pegmatitas tipo I, las cuales de sur a norte presentan un aumento en el grado de fraccionamiento desde pegmatitas poco evolucionadas y sin mineralizaciones de elementos raros, hasta pegmatitas evolucionadas con depósitos metalíferos de interés económico. El segundo evento dio origen a las pegmatitas tipo II y a los granitos aplíticos, y carece de especialización metalogenética, evidenciado por los indicadores de diferenciación magmática sistemáticamente inferiores a los de las pegmatitas tipo I y a la carencia de mineralizaciones metalíferas., The Comechingones pegmatitic field (CPF) is located in the eastern flank of the Sierra de Comechingones, Córdoba province. It is composed of granite pegmatites belonging to the Rare-Element class, beryl type, beryl-columbite-phosphate subtype; some of them are transitional into the Muscovite class. Beryllium, Nb, Ta and U deposits, as well as high-quality industrial mineral deposits, are frequently associated with these pegmatites. In the southern part of the CPF two different pegmatite types have been described. Type I pegmatites constitute large zoned bodies with up to 1000 m long and 50 m thick, and may constitute rare element deposits, whereas type II pegmatites occur as small, unzoned quartz-rich dykes, without metalliferous mineralizations, spatial and genetically associated with aplitic leucogranites. Preliminary geochemical data from both pegmatites types and granites are presented and discussed in this contribution. Geochemical evidences, supported by field and petrographic observations, suggest that the two types of pegmatites identified in the study area represent two different, probably diachronic, magmatic stages. Type I pegmatites display a geochemical gradation in a S-N direction, from barren pegmatites in the south to fractionated pegmatites in the northern part of the study area, and are the lithological product of the first magmatic stage. The second stage lead to the crystallization of aplitic granites and barren type II pegmatites, geochemically less fractionated than type II pegmatites., Centro de Investigaciones Geológicas

Magmatic structures related to the mechanical interaction between mafic magmas and granitoids have been studied in the Valle Fértil calc-alkaline igneous complex, Argentina. Excepcional outcrops with vertical walls of more than 300 m high allow us the study of three-dimensional geometries of individual blobs of mafic magma as well as the geometry of pipe-like structures in which mafic microgranular enclaves are concentrated in more than 50 times the normal abundance in the granodiorite mass. The shape of enclaves and pipe-like structures are interpreted as the ressult of top-to-down intrusions of a mafic magma into a granodiorite-tonalite mass. These sinking structures are the result of a reverselly stratified magma chamber with gabbros and diorites at the top and granodiorite-tonalite at the bottom. They may account for most of the structures found in microgranular enclaves and magma mingling zones that characterize calc-alkaline batholiths. Synplutonic intrusions from the top is the only plausible mechanism to account for the observed structures. The model may be of general application to calc-alkaline batholiths characterized by the presence of mafic microgranular enclaves. An implication of these reverselly stratified magma chambers is the presence of a petrological inversion which may be the consequence of cold diapirs emplaced below the mantle wedge in a suprasubduction setting.

From structural and magnetic fabric patterns, this work reports a complex folded structure in the La Fronda pluton, a small trondhjemite massif located in the Sierras de Córdoba which form the eastern part of the Sierras Pampeanas in Argentina. Isoclinal folds with no thickening at the hinge zones deform the compositional banding of these igneous rocks. Both the orientation of axial surfaces and fold axes vary from place to place and do not concur with the orientation of folds from the metamorphic country rocks. Microstructural data point to a fabric formed by grain-supported flow in a crystal-rich magma during the imprint of a vanishing regional stress field