Chemical composition of biotite and hornblende and cooling ages as indicators of the emplacement conditions of the La Esperanza Plutonic Complex (Upper Permian), North Patagonian Massif

El Complejo Plutónico La Esperanza está situado en el sector norte del Macizo Norpatagónico y es parte del cinturón magmático Paleozoico Superior del norte de la Patagonia. Dentro de este complejo se agrupan mayormente granodioritas y monzogranitos biotítico-anfibólicos (Granodiorita Prieto) o biotíticos (Granito Donosa), siendo 273 Ma la edad de cristalización de los primeros. En este trabajo se utilizan las composiciones minerales de biotitas y anfíboles para describir la naturaleza del magma y estimar la presión y temperatura del emplazamiento de estos cuerpos plutónicos. En base a la química de las biotitas, el Complejo Plutónico La Esperanza presenta una signatura magmática calcoalcalina con tendencia a la peraluminosidad, la cual puede ser asociada a ambientes transicionales orogénico- anorogénico. Las presiones de emplazamiento calculadas para la Granodiorita Prieto oscilan entre 0,6 y 2,0 kbar registrándose un aumento de la presión en la facies de granitoides inequigranulares aflorante al este de la localidad de La Esperanza. El termómetro hornblenda-plagioclasa indica que este cuerpo se emplazó a una temperatura de entre 630 y 790ºC en condiciones de alta fugacidad de oxígeno. Estos resultados, en combinación con edades de enfriamiento, indican un emplazamiento somero en un ambiente con un régimen termal elevado que habría permanecido en actividad desde el Pérmico medio al menos hasta el límite permotriásico.La Esperanza Plutonic Complex crops out in the northern part of the North Patagonian Massif in the Upper Paleozoic magmatic belt of northern Patagonia. This magmatic complex is mainly composed of 273 Ma biotite and amphibole bearing (Prieto Granodiorite) or biotite bearing (Donosa Granite) granitoids. In this study, biotite and amphibole mineral compositions are used to describe the nature of the granitic magma and to estimate the pressure and temperature at which these plutons were emplaced. Based on biotite chemistry, La Esperanza Plutonic Complex represents a calc-alkaline magma with a trend of increasing peraluminosity, which could be related to a transitional orogenic to anorogenic setting. Emplacement pressures for Prieto Granodiorite vary between 0,6 and 2,0 kbar, with higher values in an inequigranular facies which crops out to the east of La Esperanza locality. Hornblende-Plagioclase thermometer yields emplacement temperatures ranging from 630 to 790ºC at high oxygen fugacity conditions. These results, combined with good qualitycooling ages, propose a shallow emplacement in an elevated thermal regime that might have lasted from Middle Permian to Permotriassic times.Fil: Martínez Dopico, Carmen Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Lopez, Monica Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; ArgentinaFil: Wemmer, Klaus. Universität Göttingen; AlemaniaFil: Rapalini, Augusto Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires; Argentin

Geochemistry of the (meta-)mafic rocks from the Gonzalito mining district, northern Patagonia

In spite of hosting one of the most important Pb–Ag–Zn mineralizations in Patagonia, the metamorphic history of the rocks of the Mina Gonzalito Complex (MGC; east of the North Patagonian Massif) is still unclear. The complex consists of schists, para- and ortho-derived gneisses, ranging from greenschist to amphibolite facies, and metamafic rocks. Leucogranites and pegmatites were intruded synkinematically. Field, petrological and thermochronological evidence indicates that the MGC experienced an early prograde path and metamorphic peak during the Early Ordovician (ca. 472 Ma), magmatism and localized post-peak deformation and re-equilibrium at lower pressure, followed by uplift during the Late Permian. The MGC is intruded by the calc-alkaline Santa Rosa Diorite (SiO2 = 58.7–60.4 wt%; LaN/YbN = 7.2–10.5) and trachyte dike swarms in the Late Permian- Early Triassic. The mafic intrusives of the MGC form small schistose, massive and banded bodies interlayered within the gneisses and granites and recorded recrystallization of hornblende + plagioclase + quartz + titanite ± clinopyroxene ± biotite ± ilmenite. The metamafic rocks are mostly tholeiitic gabbros having SiO2 (45.4–52.1 wt%), TiO2 (0.62–2.88 wt%), flat REE patterns (LaN/YbN = 0.48–2.76), although some pyroxene-banded varieties show higher ratios. Initial P–T modelling in the NCKFMASHTO system for the metamafic rocks defined P-T conditions between 550 and 730 °C and 1–4 kbar. Our data suggest that the protolith of the metamafic rocks was emplaced in a shallow environment, associated with underplating of mantle-derived magmas slightly modified by crustal contamination. The intrusion of mantle-derived magmas may have been related either to a magmatic arc or to a continental rift environment. The model involving an Ordovician intracontinental back-arc basin is favored herein because it can reasonably explain many other geological features of Early Paleozoic basement rocks from northern Patagonia.Fil: Martínez Dopico, Carmen Irene. 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: Cutts, Kathryn Ann. Universidade Federal de Ouro Preto; BrasilFil: Lopez, Monica Graciela. 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: Pugliese, Franco Eduardo. 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; Argentin

Geochemistry of the (meta-)mafic rocks from the Gonzalito mining district, northern Patagonia

In spite of hosting one of the most important Pb–Ag–Zn mineralizations in Patagonia, the metamorphic history of the rocks of the Mina Gonzalito Complex (MGC; east of the North Patagonian Massif) is still unclear. The complex consists of schists, para- and ortho-derived gneisses, ranging from greenschist to amphibolite facies, and metamafic rocks. Leucogranites and pegmatites were intruded synkinematically. Field, petrological and thermochronological evidence indicates that the MGC experienced an early prograde path and metamorphic peak during the Early Ordovician (ca. 472 Ma), magmatism and localized post-peak deformation and re-equilibrium at lower pressure, followed by uplift during the Late Permian. The MGC is intruded by the calc-alkaline Santa Rosa Diorite (SiO2 = 58.7–60.4 wt%; LaN/YbN = 7.2–10.5) and trachyte dike swarms in the Late Permian- Early Triassic. The mafic intrusives of the MGC form small schistose, massive and banded bodies interlayered within the gneisses and granites and recorded recrystallization of hornblende + plagioclase + quartz + titanite ± clinopyroxene ± biotite ± ilmenite. The metamafic rocks are mostly tholeiitic gabbros having SiO2 (45.4–52.1 wt%), TiO2 (0.62–2.88 wt%), flat REE patterns (LaN/YbN = 0.48–2.76), although some pyroxene-banded varieties show higher ratios. Initial P–T modelling in the NCKFMASHTO system for the metamafic rocks defined P-T conditions between 550 and 730 °C and 1–4 kbar. Our data suggest that the protolith of the metamafic rocks was emplaced in a shallow environment, associated with underplating of mantle-derived magmas slightly modified by crustal contamination. The intrusion of mantle-derived magmas may have been related either to a magmatic arc or to a continental rift environment. The model involving an Ordovician intracontinental back-arc basin is favored herein because it can reasonably explain many other geological features of Early Paleozoic basement rocks from northern Patagonia.Fil: Martínez Dopico, Carmen Irene. 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: Cutts, Kathryn Ann. Universidade Federal de Ouro Preto; BrasilFil: Lopez, Monica Graciela. 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: Pugliese, Franco Eduardo. 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; Argentin

The Neoproterozoic-early Paleozoic metamorphic and magmatic evolution of the Eastern Sierras Pampeanas: an overview

The Eastern Sierras Pampeanas were structured by three main events: the Ediacaran to early Cambrian (580–510 Ma) Pampean, the late Cambrian–Ordovician (500–440 Ma) Famatinian and the Devonian-Carboniferous (400–350 Ma) Achalian orogenies. Geochronological and Sm–Nd isotopic evidence combined with petrological and structural features allow to speculate for a major rift event (Ediacaran) dividing into two Mesoproterozoic major crustal blocks (source of the Grenvillian age peaks in the metaclastic rocks).This event would be coeval with the development of arc magmatism along the eastern margin of the eastern block. Closure of this eastern margin led to a Cambrian active margin (Sierra Norte arc) along the western margin of the eastern block in which magmatism reworked the same crustal block. Consumption of a ridge segment (input of OIB signature mafic magmas) which controlled granulite-facies metamorphism led to a final collision (Pampean orogeny) with the western Mesoprotrozoic block. Sm–Nd results for the metamorphic basement suggest that the TDM age interval of 1.8–1.7 Ga, which is associated with the less radiogenic values of εNd(540) (−6 to −8), can be considered as the mean average crustal composition for the Eastern Sierras Pampeanas. Increasing metamorphic grade in rocks with similar detrital sources and metamorphic ages like in the Sierras de Córdoba is associated with a younger TDM age and a more positive εNd(540) value. Pampean pre-540 Ma granitoids form two clusters, one with TDM ages between 2.0 and 1.75 Ga and another between 1.6 and 1.5 Ga. Pampean post-540 Ma granitoids exhibit more homogenous TDM ages ranging from 2.0 to 1.75 Ga. Ordovician re-activation of active margin along the western part of the block that collided in the Cambrian led to arc magmatism (Famatinian orogeny) and related ensialic back-arc basin in which high-grade metamorphism is related to mid-crustal felsic plutonism and mafic magmatism with significant contamination of continental crust. TDM values for the Ordovician Famatinian granitoids define a main interval of 1.8–1.6, except for the Ordovician TTG suites of the Sierras de Córdoba, which show younger TDM ages ranging from 1.3 to 1.0 Ga. In Devonian times (Achalian orogeny), a new subduction regime installed west of the Eastern Sierras Pampeanas. Devonian magmatism in the Sierras exhibit process of mixing/assimilation of depleted mantle signature melts and continental crust. Achalian magmatism exhibits more radiogenic εNd(540) values that range between 0.5 and −4 and TDM ages younger than 1.3 Ga. In pre-Devonian times, crustal reworking is dominant, whereas processes during Devonian times involved different geochemical and isotopic signatures that reflect a major input of juvenile magmatism.Fil: Steenken, André. Universität Greifswald; AlemaniaFil: Lopez, Monica Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; ArgentinaFil: Martínez Dopico, Carmen Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotópica; ArgentinaFil: Drobe, Malte. Universität Göttingen; AlemaniaFil: Wemmer, Klaus. Universität Göttingen; AlemaniaFil: Siegesmund, Siegfried. Universität Göttingen; Alemani

Fábrica magnética de los diques permo-triásicos del área de La Esperanza, macizo norpatagónico, Argentina

Se presentan los resultados preliminares de un estudio de anisotropía de susceptibilidad magnética (ASM) en diques de composiciones ácidas y básicas que intrusionan a rocas del Complejo Plutónico La Esperanza (Provincia de Río Negro, Macizo Norpatagónico, Argentina), con el fin de entender su mecanismo de emplazamiento y posibles edades relativas. El mismo fue complementado con estudios de mineralogía magnética y petrografía. Se distinguen diversas facies composicionales y texturales en los diques. El portador magnético más destacado sería magnetita multi-dominio. Se observa una buena correlación entre la fábrica magnética y el rumbo de las estructuras, pudiéndose asignar esto a direcciones de flujo y ascenso magmático.Preliminary results from anisotropy of magnetic susceptibility (AMS) studies of acid and basic dikes intruding the La Esperanza Plutonic Complex (Rio Negro province, North Patagonian Massif, Argentina) are presented, with the purpose of understanding their emplacement mechanism and possible relative ages. These studies were complemented with magnetic mineralogy and petrographic analysis. Several types are distinguished, both compositional and textural. The most important magnetic carrier is interpreted to be multi-domain magnetite. A very good correlation is observed between the magnetic fabric of the dikes, and their attitudes, which is interpreted to reflect magmatic flow and emplacement kinematics.Fil: Miguez, Maximilano R.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Geología; ArgentinaFil: Martínez Dopico, Carmen Irene. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotopica; ArgentinaFil: Rapalini, Augusto Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Lopez, Monica Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geocronología y Geología Isotopica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geocronología y Geología Isotopica; ArgentinaFil: Luppo, Tomas. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Basicas, Aplicadas y Ambientales de Buenos Aires; Argentin

Upper Triassic U-Pb Age from The Agua de La Zorra Formation in Paramillos area, Mendoza Precordillera

The Agua de la Zorra Formation (Uspallata Group) constitutes the rift infill of the Paramillos depocenter from the Cuyo basin located in the Argentine Precordillera. This unit hosts the Paramillos de Uspallata polymetallic vein-deposit. In this contribution we report a new LA-ICP- MS zircon U-Pb age of 230.2 ± 2.2 Ma (Upper Triassic) for a tuff level that belongs to the Agua de la Zorra Formation. The obtained age is consistent with the age of the volcanism in other areas of the Cuyo basin.Fil: Orellano Ricchetti, Ana Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Gómez, Anabel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Rubinstein, Nora Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Martínez Dopico, Carmen Irene. 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: Lana, Cristiano. Universidade Federal de Ouro Preto; Brasi

U–Pb ages and Hf-isotope data of detrital zircons from the late Neoarchean-Paleoproterozoic Minas Basin, SE Brazil

Because of its world-class iron ore deposits and promising Au and U mineralizations, the late Neoarchean to Paleoproterozoic Minas Basin (Minas Supergroup, SE of Brazil) is one of the best-studied basins in South America. However, the lack of datable interlayered volcanic rocks prevented discourse over ages of the strata, the sources and the nature of its ore deposits. In this paper, we present detrital zircon U–Pb age patterns coupled with Lu–Hf data for 18 samples, representing different stages of the Minas Basin evolution (∼2000 analyzed zircons). Age spectra for the main basal unit (Moeda Formation) show a classic rift-related detrital zircon pattern, characterized by multiple autochthonous sources, which in turn are much older than the age of deposition. Maximum age for the rifting event is constrained at ca. 2600 Ma. Detritus accumulated at the base of the Minas Supergroup were derived from Archean source rocks and their sedimentation was marked by differential uplift of the Archean crust, shortly after the 2730–2600 Ma high-K calc-alkaline magmatism (Mamona Event). The age of the BIF deposits is younger than 2600 Ma, most likely coinciding with the great oxygenation event between 2400 and 2200 Ma and the precipitation of banded iron deposits worldwide. Detrital zircons from the topmost units of the Minas strata suggest that tectonic inversion and closure of the basin took place at ca. 2120 Ma with the deposition of the synorogenic Sabará Group. Rhyacian zircon supply showing juvenile Hf signatures gives evidence of a late Rhyacian amalgamation between the Mineiro Belt and the craton. The εHf signatures support the hypothesis that the Archean crystalline crust of the craton was mostly built by crust–mantle mixing processes, with a successive decrease of εHf values in zircons crystallized after 3250 Ma and minor mantle-like additions after Paleoarchean times. Regionally, our dataset supports previous interpretations of a long-lived evolution of the southern São Francisco Craton comprising a succession of convergent island arcs, small microplate collisions, and development of Archean convergent and divergent basins that evolved between Archean and Paleoproterozoic times.Fil: Martínez Dopico, Carmen Irene. Universidade Federal de Ouro Preto; Brasil. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Lana, Cristiano. Universidade Federal de Ouro Preto; BrasilFil: Moreira, Hugo S.. University of Portsmouth; Reino UnidoFil: Cassino, Lucas F.. Universidade Federal de Ouro Preto; BrasilFil: Alkmim, Fernando F.. Universidade Federal de Ouro Preto; Brasi

Upper Triassic U-Pb Age from The Agua de La Zorra Formation in Paramillos area, Mendoza Precordillera

The Agua de la Zorra Formation (Uspallata Group) constitutes the rift infill of the Paramillos depocenter from the Cuyo basin located in the Argentine Precordillera. This unit hosts the Paramillos de Uspallata polymetallic vein-deposit. In this contribution we report a new LA-ICP- MS zircon U-Pb age of 230.2 ± 2.2 Ma (Upper Triassic) for a tuff level that belongs to the Agua de la Zorra Formation. The obtained age is consistent with the age of the volcanism in other areas of the Cuyo basin.Fil: Orellano Ricchetti, Ana Paula. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Gómez, Anabel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Rubinstein, Nora Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Martínez Dopico, Carmen Irene. 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: Lana, Cristiano. Universidade Federal de Ouro Preto; Brasi

Magnetic fabrics and microstructures of Paleoproterozoic intrusive bodies of Piedra Alta Terrane, Río de la Plata Craton, Uruguay: A reconnaissance study

Small to medium-sized intrusive bodies with ages between 2.1 and 2.0 Ga characterize Piedra Alta terrane and represent the final stabilization of Río de la Plata Craton. A reconnaissance study of petro- and magnetic fabrics based on microscopic observations, anisotropy of magnetic susceptibility and anhysteretic remanent magnetization, was carried out on 41 sites from 14 intrusive bodies, in order to obtain preliminary information on kinematics of their emplacement, degree of deformation, and regional stress regime during this magmatic event. The magnetic fabrics coupled with the microstructural analysis are interpreted as generally reflecting magma kinematics near solidus temperature, although few cases of tectonically modified fabrics at local levels have been interpreted. Field evidence and magnetic fabrics in most plutons show subvertical to high angle E-W to NE-SW trending foliations and subvertical lineations. These data suggest that plutons were probably intruded under a regional extensional to transtensional regime, which controlled the magma ascent and distribution of the intrusions. Lack of metamorphism and low to null deformation shown by the study plutons suggest a late-orogenic character for this magmatism, which has been associated with crustal stabilization after accretionary processes that led to the formation of Río de la Plata Craton in the Orosirian.Fil: Franceschinis, Pablo Reinaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Rapalini, Augusto Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Sánchez Bettuchi, Leda. Universidad de la República. Instituto de Ciencias Geológicas; UruguayFil: Martínez Dopico, Carmen Irene. 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: Milanese, Florencia Nidia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; Argentin

Geochemistry and geochronology of the shallow-level La Esperanza magmatic system (Permian-Triassic), Northern Patagonia

The La Esperanza plutonic-volcanic complex is the largest Late Paleozoic-Early Triassic composite magmatic system of northern Patagonia. This paper reports new SHRIMP U-Pb zircon ages and K-Ar muscovite dating as well as whole-rock geochemical data for selected units. In addition, we present some new and reprocessed whole-rock Sr isotopic compositions. On the basis of the new and published data, three compositionally and isotopically distinct high-K magnesian calc-alkaline series were distinguished. Two of these are characterized by high Ba-Sr: (i) biotite and muscovite bearing rhyolites and granites (265 ± 2 Ma; 260 ± 2 Ma) and (ii) metaluminous amphibole-biotite bearing granodiorites (273 ± 2 Ma), monzogranites (255 ± 2 Ma), dacites (253 ± 2 Ma), and slightly peraluminous granites (dated herein as 251 ± 2 Ma). There is also a low Ba-Sr series of high-silica metaluminous rocks (granites and acid dike swarms; 250 ± 2 Ma and ≈244 ± 2 Ma). Geochemistry coupled with geochronology revealed a pulsatory multi-sourced open magmatic system with mafic magma replenishment and reactivation of crystal mushes that occurred before upward migration to upper crustal levels. Mafic magmas alternated with crust-derived magmas incrementally assembled in subvolcanic levels over 30 Ma. Zircon crystallization and mica cooling ages in the granite units allowed detection of two magmatic lulls, between 270 and 265 Ma and between 260 and 255 Ma. Both episodes coincide with a period of exhumation in upper crustal levels. The new temporal and geochemical constraints allow correlation of the La Esperanza plutonic-volcanic complex with the Los Menucos Group (258-248 Ma), encompassing a volume of magmatism comparable to a moderately sized large igneous silicic province. These mid-to-late Permian to Middle Triassic rocks record the transition between subduction-related magmatism (>273 Ma) and post-orogenic extensional magmatism (<250 Ma) in the Gondwana margin. Even though this magmatism would be coeval with the proposed collision of the Patagonia terrane, no expected syn-collisional magmatism or associated deformation were found in upper crustal levels. However, the different nature and melting conditions of the inferred sources of the magmas that crystallized before 270 Ma, between 265 and 260 Ma, and from 255 to 245 Ma, suggest that the La Esperanza plutonic-volcanic complex was assembled during a 30 Ma period of major plate reorganization.Fil: Martínez Dopico, Carmen Irene. 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: Lopez, Monica Graciela. 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: Rapalini, Augusto Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Fanning, Christopher Mark. The Australian National University; AustraliaFil: Antonio, Paul Yves Jean. Universidade de Sao Paulo; Brasi