Characterization of ultramafic, mafic and metasedimentary rocks of the Cordón del Peñasco, western Precordillera, Mendoza

En el cordón del Peñasco, ubicado en el sector norte de la provincia de Mendoza, dentro del ámbito de la Precordillera occidental, afloran metasedimentitas en contacto tectónico con una asociación máfica-ultramáfica. Dentro de las metasedimentitas en facies esquitos verdes se han reconocido dos asociaciones: metalimolitas carbonáticas y metaareniscas y metapelitas. Las rocas máficas-ultramáficas están representadas por serpentinitas derivadas de harzburgitas y dunitas, gabros bandeados metamorfizados en facies granulita; diabasas constituyendo diques o filones capa, metabasaltos amigdaloides y metahialoclastitas con bajo grado de metamorfismo. Se realizaron estimaciones termométricas en una granulita máfica (gabro bandeado) utilizando el par granate-clinopiroxeno, obteniéndose temperaturas de 884°C. Las mismas permiten estimar que estos cuerpos experimentaron metamorfismo de alta temperatura, para posteriormente retrogradar a facies de esquistos verdes, sobre la base de aspectos mineralógicos y texturales. Con respecto a las unidades metasedimentarias, el paleoambiente inferido para la unidad de metaareniscas y metapelitas corresponde a pie de talud continental con predominio de sedimentación turbidítica sobre la base de estructuras sedimentarias reconocidas. Mientras que la unidad de metalimolitas carbonáticas pertenecerían a un ambiente marino profundo. Estas unidades han sido asignadas al Eopaleozoico a partir de criterios estratigráficos y deformadas y metamorfizadas hacia fines del Devónico como consecuencia de la Orogenia Chánica.In Cordón del Peñasco area, located in northern Mendoza province, western Precordillera, metasedimentites are in tectonic contact with a mafic-ultramafic association. The former have been under greenschist facies metamorphism and two associations have been recognized: carbonate metasiltstones and metasandstones and metapelites. The mafic-ultramafic association is represented by serpentinized peridotites derived from dunites and harzburgites, layered gabbros in granulite facies; diabase dikes or sills, amigdaloid metabasalts and metahialoclastic rocks with low grade metamorphism. In this contribution, we have estimated metamorphic temperatures on a mafic granulite (layered gabbro) based on garnet-clinopyroxene, obtaining a temperature of 884ºC. This suggests that these bodies have been under high temperature metamorphism and were later retrogressed to greenschist facies, evidenced by mineralogical and textural aspects. Regarding the metasedimentary units, the paleoenvironment of the metasandstones and metapelites association corresponds to a continental slope with turbiditic sedimentation, as suggested by sedimentary structures. The carbonate metasiltstone unit belongs to a deep marine environment. These units have been assigned to Eopaleozoic times based on stratigraphic criteria and deformed and metamorphosed in late Devonian as a consequence of Chanica orogeny.Fil: Boedo, Florencia Lucila. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Vujovich, Graciela Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Barredo, Silvia. Universidad de Buenos Aires; Argentin

Seismic velocities in outcropping units in the western flank of the la Invernada range, San Juan Precordillera

En este trabajo se muestran determinaciones de velocidades de ondas sísmicas P (Vp) en un sector (Reserva Don Carmelo) del flanco occidental de la sierra de la Invernada, Departamento Ullúm, Provincia de San Juan. Las velocidades obtenidas con el método de refracción sísmica indican valores de Vp de 550; 1.670 y 4.134 m/s y espesores de 1 m y 7 m aproximadamente, para un modelo de dos capas apoyadas sobre un substrato correspondiente a la mayor velocidad. Las unidades aflorantes reconocidas abarcan grauvacas y areniscas muy bien compactadas que se correlacionan con los valores más altos de Vp determinados en el experimento geofísico para los niveles interpretados a mayor profundidad. Asimismo los valores más bajos de Vp son consistentes con material de tipo aluvional situado en las capas sísmicas más superficiales (< 7m de profundidad). Otras unidades relevadas en el terreno indican afloramientos de diabasas y gabros, los cuales no han sido explorados geofísicamente. Sin embargo, a partir de la comparación de las litologías y velocidades sísmicas Vp observadas con bases de datos globales, es posible predecir la continuación en profundidad de las litologías máficas con velocidades sísmicas de Vp aún mayores que aquellas determinadas en superficie. Estos resultados son de interés debido a que ayudan a identificar una posible faja ofiolítica en Precordillera, en un sector donde el espesor cortical actual estimado en 64 km aproximadamente, estaría engrosado.P-wave seismic velocity (Vp) determinations are shown in this work for a region in the western flank (Don Carmelo nature reserve) of the la Invernada range in the Ullúm Department of the San Juan Province. The velocities obtained using the seismic refraction method indicate Vp values of 550; 1.670 and 4.134 m/s and thicknesses of 1 m and 7 m approximately, for a two-layer over a higher Vp half space model. The exposed units show greywacke and well compacted sandstones, which have a good correlation with the deepest and higher Vp values of the seismic model. Also the lowest Vp values are consistent with alluvial material located in the shallower seismic layers (< 7m depth). Other units observed on the surface exhibit outcrops of diabases and gabros, which were not geophysically explored. However, a comparison between the observed lithologies and Vp values with a global database predicts the continuation at depth of mafic rocks associated with even higher Vp values than those determined at the surface. These results are of interest because they help us to identify an ophiolite belt in the Precordillera, a region presently characterized by an overthickened crust of 64 km.Fil: Perez Lujan, Sofia Beatriz. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geofísica y Astronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Juan. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Alvarado, Patricia Monica. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geofísica y Astronomía; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico San Juan. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Güel, Arturo. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geofísica y Astronomía; ArgentinaFil: Saez, Mauro. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geofísica y Astronomía; ArgentinaFil: Vujovich, Graciela Irene. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología. Laboratorio de Tectónica Andina; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos; Argentin

Geochemistry of Precordillera serpentinites, western Argentina : evidence for multistage hydrothermal alteration and tectonic implications for the Neoproterozoic-early Paleozoic

Serpentinites are a powerful tool to evaluate mantle composition and subsequent alteration processes during their tectonic emplacement. Exposures of this type of rocks can be found in the Argentine Precordillera (Cuyania terrane) and Frontal Cordillera, both located in central-western Argentina, within the Central Andes. In these regions a Neoproterozoic to Devonian mafic-ultramafic belt composed of serpentinites, metabasaltic dikes/sills, pillow lavas (with an Enriched to Normal Mid-Ocean Ridge Basalts (E- to N-MORB) geochemical signature) and mafic granulites crop out, spatially associated with marine metasedimentary rocks. The serpentinite bodies consist of lizardite/chrysotile+brucite+magnetite, with scarce pentlandite and anhedral reddish-brown Cr-spinel (picotite, pleonaste and spinel sensu stricto) as relict magmatic phases. The original peridotites were moderately-depleted harzburgites (ultramafic cumulates) with an intermediate chemical signature between a mid-ocean ridge and an arc-related ophiolite. Whole-rock Rare Earth Elements (REE) patterns of serpentinites exhibit enriched REE patterns ((La/Yb)CN=13-59) regarding CI chondrite with positive Eu anomalies. These features are the result of an interaction between hydrothermal fluid and serpentinites, in which moderate temperature (350º-400ºC), CO2-rich, mildly basic hydrothermal fluid was involved and was responsible for the addition of Ca, Sr and REE to serpentinites. The presence of listvenites (silica-carbonate rocks) in the serpentinite margins allow us to infer another fluid metasomatism, where lowtemperatures (<250ºC), highly-oxidized, highly-acid fluid lead to the precipitation of silica. The association of these metasomatized serpentinite bodies with neoproterozoic continental margin sucessions and MORB magmatism at the suture zone of the Cuyania and Chilenia terranes suggests the development of an oceanic basin between them during the Neoproterozoic-early Paleozoic

Sedimentary provenance analysis of the Early Paleozoic units of the arroyo El Leoncito area, Southern Precordillera, province of San Juan

En el área del arroyo El Leoncito, en el ámbito de la Precordillera Sur, afloran la Formación Cabeceras y el Grupo Ciénaga del Medio. Ambas unidades consisten en sucesiones silicoclásticas marinas asociadas espacialmente a rocas máficas toleíticas depositadas en un margen continental sometido a extensión, el cual se desarrolló durante el Ordovícico-Devónico a lo largo del margen occidental del terreno Cuyania previo a la colisión del terreno Chilenia. A fines de caracterizar este ambiente y sus áreas de aporte, este trabajo presenta un análisis de procedencia sedimentaria mediante conteo modal de componentes de metareniscas de la Formación Cabeceras, Grupo Ciénaga del Medio y otras unidades de la Precordillera Sur (formaciones Portezuelo del Tontal, Peñasco, Garganta del León y Sandalio). Las modas analizadas sugieren que las rocas que actuaron como fuente fueron principalmente metamorfitas de bajo grado y sedimentitas. En menor proporción, participaron rocas ígneas de composición intermedia-ácida y/o rocas metamórficas de grado medio a alto. El aporte de rocas volcánicas fue subordinado. Los diagramas de discriminación señalan a un orógeno reciclado como la principal zona de aporte, con participación de una zona de arco disectado. Estos resultados pueden explicarse en un contexto donde el orógeno Oclóyico habría actuado como área de aporte y, a la vez, como barrera orográfica ante el transporte de material desde el este (arco Famatiniano y áreas cratónicas). La proveniencia de detritos desde el oeste no es totalmente descartada. La similitud de las modas analizadas sugiere que no hubo cambios significativos en las áreas de aporte para el segmento de la Precordillera analizado (31°45´S-32°15´S) durante el período Ordovícico-Devónico.In the Arroyo El Leoncito area, northern sector of the Southern Precordillera, the Cabeceras Formation and Ciénaga del Medio Group crop out. These units consist of marine siliciclastic successions and tholeiitic mafic rocks developed on a continental margin during the Ordovician-Devonian. In order to characterize this setting and its source areas, this work presents a sedimentary provenance analysis using modal composition of metasandstones of the Cabeceras Formation and Ciénaga del Medio Group, as well as other units of the Southern Precordillera (Portezuelo del Tontal, Peñasco, Garganta del León and Sandalio formations). Analyzed detrital modes suggest that mainly low-grade metamorphic and sedimentary rocks acted as source. To a lesser extent, intermediate-acid igneous rocks and/or medium- to high-grade metamorphic rocks of similar composition also participated. The contribution of volcanic rocks was subordinated. Discrimination diagrams indicate a recycled orogen as the main provenance zone, with the participation of a dissected arc zone. These results can be explained in a context where the Ocloyic orogen would have acted as source and, at the same time, as a barrier that prevented the transport of detritus from regions located further east (i.e. Famatinian arc and/or cratonic areas). Detritus provenance from the west is not totally ruled out. The similarity of the detrital modes of the formations involved in the analyzed segment of the Southern Precordillera (31°45'-32°15'S) suggests that there were no significant changes in source areas during the Ordovician-Devonian.Fil: Giunta, Agostina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias Geológicas; ArgentinaFil: Schmidt, Daniela Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias Geológicas; ArgentinaFil: Boedo, Florencia Lucila. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Vujovich, Graciela Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; Argentina. Secretaría de Industria y Minería. Servicio Geológico Minero Argentino; Argentin

Detrital zircon analysis from the Neoproterozoic-Cambrian sedimentary cover (Cuyania terrane), Sierra de Pie de Palo, Argentina: Evidence of a rift and passive margin system?

Metamorphic basement and its Neoproterozoic to Cambrian cover exposed in the Sierra de Pie de Palo, a basement block of the Sierras Pampeanas in Argentina, lie within the Cuyania terrane. Detrital zircon analysis of the cover sequence which includes, in ascending order, the El Quemado, La Paz, El Desecho, and Angacos Formations of the Caucete Group indicate a Laurentian origin for the Cuyania terrane. The lower section represented by the El Quemado and La Paz Formations is interpreted as having an igneous source related to a rift setting similar to that envisioned for the southern and eastern margins of Laurentia at approximately 550 Ma. The younger strata of the El Desecho Formation are correlative with the Cerro Totora Formation of the Precordillera, and both are products of rift sedimentation. Finally, the Angacos Formation and the correlative La Laja Formation of the Precordillera were deposited on the passive margin developed on the Cuyania terrane. The maximum depositional ages for the Caucete Group include ca. 550 Ma for the El Quemado Formation and ca. 531 Ma for the El Desecho Formation. Four different sediment sources areas were interpreted in the provenance analysis. The main source is crystalline basement dominated by early Mesoproterozoic igneous rocks related to the Granite-Rhyolite province of central and eastern Laurentia. Possible source areas for 1600 Ma metamorphic detrital zircons of the Caucete Group include the Yavapai-Mazatzal province (ca. 1800–1600 Ma) of south-central to southwestern Laurentia. Younger Mesoproterozoic zircon is likely derived from Grenville-age medium- to high-grade metamorphic rocks and subordinate igneous rocks that form the basement of Cuyania as well as the southern Grenville province of Laurentia itself. Finally, Neoproterozoic igneous zircon in the Caucete Group records different magmatic pulses along the southern Laurentian margin during opening of Iapetus and break-up of Rodinia. Northwestern Cuyania terrane includes a small basement component derived from the Granite- Rhyolite province of Laurentia, which was the source for detrital zircons found in the middle Cambrian passive margin sediments of Cuyania.Centro de Investigaciones GeológicasUniversidad de Buenos AiresConsejo Nacional de Investigaciones Científicas y Técnica

Complejo La Resina: evidencias de una suite AMCG en la sierra de La Huerta, provincia de San Juan

La Resina Complex: evidences for an AMCG suite in the sierra de La Huerta, San Juan province. This contribution presents a geological characterization of the proterozoic anorthosites, granites and amphibolites comprised in the La Resina Complex, located in western Sierra de la Huerta. The presence of massif-type anorthosite associated to granite suggests an AMCG (Anorthosite-Mangerite-Charnockite-Granite) suite. Particularly, the anorthosite bodies exhibit comparable characteristics to the ones from the sierra de Maz and Espinal, which also show a mesoproterozoic age. This would suggest a correlation with the anorthosites from the Grenville province and a common origin related to the Rodinia break-up.Fil: Boedo, Florencia Lucila. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Vujovich, Graciela Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Martino, Roberto Donato. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaFil: Guereschi, Alina Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; ArgentinaXIII Congreso de Mineralogía, Petrología Ígnea y Metamórfica, y MetalogénesisCórdobaArgentinaAsociacion de Mineralogia y MetalogeniaComisión de PetrologíaAsociación Geología Argentin

The Late Neoproterozoic-Early Paleozoic basin of the western Argentine Precordillera: Insights from zircon U-Pb geochronology

In central-western Argentina, a belt including marine metasedimentary rocks and mafic-ultramafic bodies occurs throughout the western margin of the Precordillera. The belt is considered as the suture zone between the poorly known Chilenia terrane and the Cuyania terrane, part of the composite West Gondwana margin. It is assigned to the Late Neoproterozoic-Early Devonian based on fossil fauna and radiometric ages. In the southern sector of this belt, in the Peñasco area, two units crop out. The Peñasco Formation comprises metasandstone and metapelite spatially associated with mafic metavolcanic and metahyaloclastic rocks. Metagabbro bodies intrude the succession. The Garganta del León Formation consists of metasandstone and scarce metapelite where tractive and deformational sedimentary structures are preserved. Both units are affected by low-grade metamorphism, but the main foliation S1 and crenulation cleavage S2 are better developed in the Peñasco Formation rocks. U-Pb data on detrital zircon of two metasandstone samples from these units show a dominant detrital input from sources with 1.0–1.3 and 0.65–0.53 Ga ages. Detritus may come from reworked sedimentary units or from igneous/metamorphic complexes from the Cuyania terrane basement that was possibly exhumed in the Ocloyic orogen. A Gondwanan provenance for the Late Neoproterozoic-Cambrian population would also be plausible. A ca. 460 Ma zircon population in the Garganta del León Formation is interpreted to be derived from the Famatinian Arc. This would imply that the deposition of the sediment occurred after the collision of the Cuyania terrane against West Gondwana, and that the Ocloyic orogen acted as a barrier for detritus from the Famatinian Arc and other rocks further east.Fil: Boedo, Florencia Lucila. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Perez Lujan, Sofia Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Naipauer, Maximiliano. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotópica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; ArgentinaFil: Vujovich, Graciela Irene. Secretaría de Industria y Minería. Servicio Geológico Minero Argentino; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: Martins Pimentel, Márcio. Universidade do Brasília; BrasilFil: Ariza, Juan Pablo. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto Geofísico Sismológico Volponi; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan; ArgentinaFil: Barredo, Silvia Patricia. Universidad de Buenos Aires. Facultad de Ingeniería; Argentin

U–Pb (LA-ICP-MS) ages on detrital zircon grains from Angacos limestone siliciclastic levels (Caucete Group), San Juan province, Argentina: provenance implications for the Cuyania terrane

In this work, we present isotopic results on detrital zircon grains in siliciclastic levels from Angacos Limestone. This unit, together with the El Quemado, La Paz and El Desecho Formations, constitute the Caucete Group (Borrello, 1969; emend. Vujovich, 2003), which occurs on the western side of the Pie de Palo range in northwestern Argentina (Fig. 1). The Pie de Palo Complex is bordered by the Las Pirquitas Thrust and is characterized by Grenville-age basement with medium- to high-grade metamorphic and minor igneous rocks (Vujovich et al., 2004, and references therein). The Pie de Palo range is part of the Cuyania composite terrane (Ramos et al., 1998) and has been recognized as an allochthonous microcontinent derived from Laurentia or as paraautochthonous unit from Gondwana (Finney et al., 2005). The Angacos Limestone is mainly composed of limestone, calcareous schist, dolostone, and calcitic marble. The metamorphism indicates greenschist facies (Ramos and Vujovich, 2000). Moreover, mylonitic products of the high deformation that affected the entire zone are present. The protolith of these carbonates is interpreted as a succession of limestone and dolostone with minor sandstone interbeds. The age of the Angacos Limestone is uncertain because of the absence of diagnostic fossils. Isotopic studies of Sr, C and O has led to the correlation with the Early Paleozoic platform limestone sequence from the Precordillera (Linares et al., 1982; Galindo et al., 2004; Naipauer et al., 2005). It is important to note that the El Quemado Formation as a part of the Caucete Group has a maximum depositional age of 550 Ma as defined by detrital zircon data (Naipauer et al., 2005). The main purpose of this work is to present isotopic data of the detrital zircons separated from siliciclastic levels interlayered within the Angacos Limestone. We compare the obtained results with detrital zircon data from the El Quemado Formation (Naipauer et al., 2005; Ellis, 2005), and discuss some provenance implications in the tectonic terrane.Centro de Investigaciones GeológicasConsejo Nacional de Investigaciones Científicas y Técnica

Top-down structures of mafic enclaves within the Valle Fértil magmatic complex (Early Ordovician, San Juan, Argentina)

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

Crustal shortening, exhumation, and strain localization in a collisional orogen: the Bajo Pequeño Shear Zone, Sierra de Pie de Palo, Argentina

The Bajo Pequeño Shear Zone (BPSZ) is a lower-crustal shear zone that records shortening and exhumation associated with the establishment of a new plate boundary, and its placement in a regional structural context suggests that local- to regional-scale strain localization occurred with progressive deformation. A kilometer-scale field and analytical cross section through the ~80 m thick BPSZ and its adjacent rocks indicates an early Devonian (405–400 Ma) phase of deformation on the western margin of Gondwanan continental crust. The earliest stages of the BPSZ, recorded by metamorphic and microstructural data, involved thrusting of a hotter orthogneiss over a relatively cool pelitic unit, which resulted in footwall garnet growth and reset footwall white mica 40Ar/39Ar ages in proximity to the shear zone. Later stages of BPSZ activity, as recorded by additional microstructures and quartz c-axis opening angles, were characterized by strain localization to the center of the shear zone coincident with cooling and exhumation. These and other data suggest that significant regional tectonism persisted in the Famatinian orogenic system for 60–70 million years after one microplate collision (the Precordillera) but ceased 5–10 million years prior to another (Chilenia). A survey of other synchronous structures shows that strain was accommodated on progressively narrower structures with time, indicating a regional pattern of strain localization and broad thermal relaxation as the Precordillera collision evolved.Fil: Garber, Joshua M.. University of California at Davis; Estados UnidosFil: Roeske, Sarah M.. University of California at Davis; Estados UnidosFil: Warren, Jessica. University of Stanford; Estados UnidosFil: Mulcahy, Sean R.. University of California at Berkeley; Estados UnidosFil: McClelland, William C.. University of Iowa; Estados UnidosFil: Austin, Lauren J.. University of Oregon; Estados UnidosFil: Renne, Paul R.. University of California at Berkeley; Estados UnidosFil: Vujovich, Graciela Irene. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología. Laboratorio de Tectónica Andina; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin