Turmalinitos do Grupo Brusque na região entre São João Batista e Tijucas, Santa Catarina, Brasil

The Dom Feliciano Belt, in the State of Santa Catarina, is represented by the Itajaí and Brusque groups and the Florianópolis Batholith. The basal Rio do Oliveira Formation of the Brusque Group consists of metabasic/calc-silicate, metavolcanic-exhalative, metapelitic and metapsammitic units. A discontinuous tourmalinite sequence integrating the metavolcanic-exhalative unit stretches out from São João Batista to Tijucas. Two types of tourmalinites were distinguished and named Rio do Oliveira and Morro do Carneiro. The first is very fine-grained, banded, and composed of greenish-yellow or brown tourmaline. The second is coarser-grained, massive to slightly foliated, and composed of color-zoned tourmaline. Microprobe and laser ablation-ICPMS analyses showed that the Rio do Oliveira tourmaline is richer in Al, alkalis, and poorer in Ti, Mg, Fe and Ca when compared to the Morro do Carneiro tourmaline. Rio do Oliveira tourmalinites were formed by selective substitution of pelitic and psammitic sediments by percolation of B-rich fluids through the volcanic-sedimentary unit during diagenesis and metamorphism. Morro do Carneiro tourmaline has detrital cores and rims enriched in Mg, Fe, Ti, Mn, Sr, Co and Ca, when compared to the Rio do Oliveira tourmaline. An intermediate dark zone is Ti- and Fe-rich. The abrupt increase in Ca from core to rim may have resulted from the equilibrium with the surrounding Ca-rich environment (underlying calc-silicate rocks). Even if the REE patterns obtained for Rio do Oliveira and Morro do Carneiro tourmalines are practically identical, it is proposed that the fluids that generated the Morro do Carneiro tourmalinites have also been metasomatic/igneous.O Cinturão Dom Feliciano no Estado de Santa Catarina é representado pelos grupos Itajaí e Brusque e o Batólito Florianópolis. A Formação Rio do Oliveira, basal do Grupo Brusque, constitui-se de unidades metabásicas/cálcio-silicáticas, metavulcânicas-exalativas, metapelíticas e metapsamíticas. Uma sequência descontínua de turmalinitos integrante da unidade metavulcânica-exalativa estende-se desde São João Batista até Tijucas. Dois tipos de turmalinitos foram distintos e denominados Rio do Oliveira e Morro do Carneiro. O primeiro é muito fino, bandado e composto por turmalina amarela-esverdeada ou castanha. O segundo é mais grosso, de textura maciça a levemente foliada, e formado por turmalina zonada. Análises por microssonda eletrônica e laser ablation-ICPMS mostraram que a turmalina do Rio do Oliveira é mais rica em Al, álcalis e mais pobre em Ti, Mg, Fe and Ca quando comparada com a do Morro do Carneiro. Os turmalinitos do Rio do Oliveira formaram-se por substituição seletiva de sedimentos pelíticos e psammíticos resultante da percolação de fluidos ricos em boro pela unidade vulcano-sedimentar durante a diagênese e metamorfismo. A turmalina do Morro do Carneiro possui núcleos detríticos e bordas mais ricas em Mg, Fe, Ti, Mn, Sr, Co e Ca que a turmalina do Rio do Oliveira. Uma zona intermediária escura é mais rica em Ti e Fe. O aumento brusco em Ca do núcleo para a borda pode ter resultado do equilíbrio com o ambiente circunvizinho rico em Ca (rochas cálcio-silicáticas subjacentes). Mesmo que os padrões definidos pelos ETR para as turmalinas do Rio do Oliveira e do Morro do Carneiro sejam praticamente idênticos, propõe-se que os fluidos formadores do turmalinito do Morro do Carneiro possam também ter tido origem metassomática/ígnea

The espinal formation in the outcrops near the northern limit within the pampean ranges and the precordillera of Jagüé, La Rioja (Argentina)

Se analizan los afloramientos de la Formación Espinal en el margen occidental de la Sierra de Toro Negro, al norte del Bolsón de Jagüé, La Rioja, cerca del límite tectónico con los afloramientos de la denominada Precordillera de Jagüé. Esta unidad corresponde al basamento de medio a alto grado de Sierras Pampeanas Occidentales. Se presentan datos de petrografía, geoquímica y una edad U-Pb TIMS en circones detríticos de 905 +14/-13 Ma, (MSWD = 0,74; discordia) ligeramente más joven que otras edades obtenidas para esta unidad pero aun comparables con las del ciclo Grenvilliano. Las litologías predominantes son gneises y esquistos anfibólicos, granatíferos y biotíticos con evidencias de retrogradación. La composición y textura de las muestras son coherentes con un origen sedimentario e ígneo para estos afloramientos. Estas rocas se distinguen de aquellas del basamento de la Precordillera-Cuyania principalmente por el mayor grado metamórfico.The outcrops of the Espinal Formation in the western margin of the Toro Negro Range, north of the Bolsón de Jagüé, La Rioja, near the tectonic limit with the outcrop of the Precordillera of Jagüé are analyzed. This unit corresponds to the medium to high grade metamorphic basement of the Western Pampean Ranges. Petrography, geochemistry and a U-Pb TIMS age on detrital circóns of 905 +14/-13 Ma, (MSWD = 0,74; discordia)are presented. This age is slightly younger than others obtained for this unit but still comparable with the Grenvillian cicle. The dominant lithologies are amphibolic, granatiferous and biotitic gneises and schists with evidences of retrogradation. The composition and texture of the samples points to a sedimentary and igneous origin for these outcrops. These rocks distinguishes from the ones that forms the basement of the Precordillera-Cuyania, mainly because the higher metamorphic grade.Facultad de Ciencias Naturales y MuseoCentro de Investigaciones Geológica

The espinal formation in the outcrops near the northern limit within the pampean ranges and the precordillera of Jagüé, La Rioja (Argentina)

Se analizan los afloramientos de la Formación Espinal en el margen occidental de la Sierra de Toro Negro, al norte del Bolsón de Jagüé, La Rioja, cerca del límite tectónico con los afloramientos de la denominada Precordillera de Jagüé. Esta unidad corresponde al basamento de medio a alto grado de Sierras Pampeanas Occidentales. Se presentan datos de petrografía, geoquímica y una edad U-Pb TIMS en circones detríticos de 905 +14/-13 Ma, (MSWD = 0,74; discordia) ligeramente más joven que otras edades obtenidas para esta unidad pero aun comparables con las del ciclo Grenvilliano. Las litologías predominantes son gneises y esquistos anfibólicos, granatíferos y biotíticos con evidencias de retrogradación. La composición y textura de las muestras son coherentes con un origen sedimentario e ígneo para estos afloramientos. Estas rocas se distinguen de aquellas del basamento de la Precordillera-Cuyania principalmente por el mayor grado metamórfico.The outcrops of the Espinal Formation in the western margin of the Toro Negro Range, north of the Bolsón de Jagüé, La Rioja, near the tectonic limit with the outcrop of the Precordillera of Jagüé are analyzed. This unit corresponds to the medium to high grade metamorphic basement of the Western Pampean Ranges. Petrography, geochemistry and a U-Pb TIMS age on detrital circóns of 905 +14/-13 Ma, (MSWD = 0,74; discordia)are presented. This age is slightly younger than others obtained for this unit but still comparable with the Grenvillian cicle. The dominant lithologies are amphibolic, granatiferous and biotitic gneises and schists with evidences of retrogradation. The composition and texture of the samples points to a sedimentary and igneous origin for these outcrops. These rocks distinguishes from the ones that forms the basement of the Precordillera-Cuyania, mainly because the higher metamorphic grade.Facultad de Ciencias Naturales y MuseoCentro de Investigaciones Geológica

U-Pb (ID-TIMS) zircon ages on pyroclastic events from Balcarce Formation, Tandilia System, Argentina: unreworked or reworked origin?

In eastern Argentina the southernmost outcrops of the Río de la Plata cratonic region are exposed in the Tandilia System in the central part of the Buenos Aires province. The evolution of Tandilia comprises mainly a juvenile igneous-metamorphic Paleoproterozoic (2.2 to 2.1 Ga) basement named Buenos Aires Complex. After a long paleoweathering process the Sierras Bayas Group (c. 185 m thick) is a record of the first Neoproterozoic sedimentary unit (siliciclastic, dolostones, shales, limestones), covered by the Cerro Negro Formation (c.150- 400 m thick, siliciclastics) and assigned to the Upper Neoproterozoic (both were firstly called “La Tinta Group”). The final sedimentary transgression at the Early Paleozoic is the Balcarce Formation (c. 90-450 m thick) (Iñiguez et al., 1989) reflecting an independent marine basin evolution in a passive margin setting. It is important to mention that pyroclastic levels were recognized in Tandilia System by Cuomo et al. (1983) from boreholes (Olavarría region) and Dristas and Frisicale (1987, 2003) in outcrops for the Cerro del Corral, Sierras de la Tigra and Los Barrientos areas. Also there are several evidence (kaolinite clay deposits, quartz crystals, alunite and iron-rich levels) originated by subsequent hydrothermal activity (Dristas and Frisicale, 2003; Dristas and Martínez, 2003). These authors described an alteration of original fall out pyroclastic material related to mafic rocks at Cerro del Corral area in the sedimentary cover, showing a kaolinite and rutile-anatase assemblage from low-temperature process. Other deposits involving mainly reworked pyroclastic material were also recognized along the Tandilia System. The main purpose of this work is to present U-Pb zircon ages by ID-TIMS methodology, on samples taken from pyroclastic levels outcropped in Cerro del Corral area and discuss the implications on two lines: the study zircons helps to constrain the age of the Balcarce Fm or represent reworking material?Centro de Investigaciones Geológica

Neoproterozoic rhyolite ignimbrites in the Sierra Norte de Córdoba: evidence of an early magmatic arc in the Pampean cycle?

La Sierra Norte forma parte de las Sierras Pampeanas de Córdoba por lo cual las principales intrusiones graníticas y el metamorfismo están relacionadas al ciclo orogénico Pampeano. La mayor diferencia con las Sierras de Córdoba es que en Sierra Norte el grado metamórfico es bajo y las intrusiones son superficiales. La Sierra Norte está compuesta por pequeños afloramientos de metasedimentitas, plutones de granodiorita y granito y cuerpos subvolcánicos relacionados a los últimos estadios del batolito. En este trabajo describimos mantos de ignimbritas intercalados en los metaconglomerados de la Formación La Lidia. Las ignimbritas están moderadamente soldadas y está compuestas por aproximadamente 15 % de porfiroclastos de cuarzo, feldespato potásico y plagioclasa totalmente alterada. La pasta consiste de trizas vítreas devitrificadas. La edad U/Pb convencional en circones dio 584+22/-14 Ma y es interpretada como la edad de cristalización. Tanto las ignimbritas como la roca de caja han sido afectadas por un metamorfismo en facies prehnita-pumpellita que alteró la plagioclasa en zoicita, sericita y pumpellita. Las ignimbritas silícicas neoproterozoicas, conjuntamente con el batolito calcoalcalino que le suceden sugiere la existencia de un arco magmático asociado al margen continental activo de Gondwana. Este arco habría estado activo desde el Neoproterozoico hasta el Cámbrico, con una duración de más de 60 Ma. La correlación de las metasedimentitas y volcanitas neoproterozoicas con la Formación Puncoviscana es posible.The Sierra Norte belongs to the Sierras Pampeanas of Córdoba and like in the Sierras de Córdoba the main granitic intrusions, metamorphism and deformation are related to the Pampean orogenic cycle. The most important difference with Sierras de Córdoba is that Sierra Norte shows a weaker deformation and a lower grade metamorphism. Sierra Norte consists of small outcrops of metasedimentary rocks intruded by granodioritic and monzogranitic plutons of the Sierra Norte-Ambargasta batholtih, of Cambrian age. Dacitic to rhyolitic domes and dikes were intruded during the last stages of the batholith. In this paper we describe rhyolitic ignimbrite sheets within the metaconglomerates of the La Lidia Formation. The ignimbrites are moderately welded and consist of about 15 % of quartz, K-feldspar and entirely altered plagioclase porhyroclasts. Groundmass consists of devitrified glass shards. The conventional U/Pb zircon age is 584+22/-14 Ma and it is considered as the crystallization age. Very low grade metamorphism of the prehnite-pumpellyte facies altered the plagioclase to zoicite, sericite and pumpellyte. The Neoproterozoic silicic ignimbrites, together with the Cambrian calc-alkaline batholith and the related volcanics indicate the existence of a magmatic arc associated with the active margin of the Gondwana continent. This arc was active since Neoproterozoic times, with a duration of more than 60 Ma. Correlation of the Neoproterozoic metasedimentary and volcanics of Sierra Norte with the Puncoviscana Formation of Northwestern Argentina is considered.Fil: Llambias, Eduardo Jorge. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Gregori, Daniel Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Geológico del Sur. Universidad Nacional del Sur. Departamento de Geología. Instituto Geológico del Sur; ArgentinaFil: Stipp Basei, Miguel Angelo. Universidade de Sao Paulo. Instituto de Geociencias. Centro de Pesquisas Geocronológicas.; BrasilFil: Varela, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Prozzi, César. Universidad Nacional del Sur. Departamento de Geología; Argentin

New insights on the interpretation of the provenance and evolution of the Silurian units in the central Precordillera, Argentina

In the central region of the Precordillera, San Juan Province, Silurian silicoclastic sedimentites of the Los Espejos Formation crop out in the Jáchal River area. To the south of this region, an equivalent unit is recognized in the San Juan River area (Tambolar Formation). Both units present similar lithological characteristics, however, it has not yet been defined if they share source areas of detrital contributions. On the other hand, for the Jáchal River sector, it is proposed to establish if there were changes in the regions from where the sub-basin received sediment contributions during the Devonian, which can be seen reflected in the detrital zircon contribution patterns of the overlying unit (Talacasto Formation). The present work is part of a series of studies tending to determine the nature and provenance of the Silurian-Devonian sequences of the Central Precordillera. On this occasion, detrital zircon patterns of the detrital sources of the Los Espejos Formation are analyzed and compared with the information obtained for the Silurian Tambolar Formation (San Juan River area) and the overlying Devonian Talacasto and Punta Negra formations (Jáchal and San Juan rivers areas). To characterize and compare the studied units, different methodologies were applied, namely sedimentary petrography, heavy minerals studies, and morphological and isotopic analyses of detrital zircons. The analysis of thin sections allowed determining textural and compositional parameters. Through the predominance of detrital minerals, it was possible to establish that the studied units are composed of quartzite-type rocks coming from mature areas, with low percentages of lithic components and abundant opaque heavy minerals of the hematite group. The study of heavy minerals, especially morphological and typological parameters of detrital zircons, allowed to establish recycled and plutonic sources as main modes, as well as the changes that occurred during the basin filling dynamics for Silurian and Devonian times. On the other hand, U–Pb isotopic analysis in detrital zircons indicate that the Pampean-Brasiliano orogenic cycle composes the main source of detritus with ages between 511 and 816 Ma. In second place are the Mesoproterozoic ages, represented by the interval from 1000 to 1350 Ma. The youngest detrital ages show a maximum sedimentation age of 478.5 ± 4.4 Ma (Tremadocian), indicating that younger sources of contribution correspond to the Famatinian Orogen. The Kolmogorov-Smirnoff test revealed that the studied Silurian-Devonian units studied have similar patterns of sedimentary contributions, which suggests that the sources of provenance were common in both regions and remained active throughout the entire time interval, without significant changes.Fil: Arnol, Jonatan Ariel. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División geología; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; ArgentinaFil: Cretacotta, Agustina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo; ArgentinaFil: Uriz, Norberto Javier. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. División geología; ArgentinaFil: Cingolani, Carlos Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo; ArgentinaFil: Angelo Stipp Basei, Miguel. Universidade de Sao Paulo; Brasi

The Cuesta de Rahue Basement Inlier (Southern Neuquén Precordillera, Argentina): A Devonian to Triassic polyphase orogenic record in northern Patagonia

New geological, structural, microstructural, and K-Ar biotite and illite geochronological data of igneous-metamorphic rocks exposed in the Cuesta de Rahue Basement Inlier are presented to reconstruct the Late Palaeozoic to Mesozoic tectonometamorphic and magmatic history of northwestern Patagonia. This block comprises a medium-grade metasedimentary sequence (Cuesta de Rahue Metamorphic Complex), Late Carboniferous granitoids and a low-grade metavolcano-sedimentary unit (Arroyo Coloco Metamorphic Complex). The Cuesta de Rahue Metamorphic Complex was deposited during the middle Palaeozoic and underwent Devonian low-pressure regional metamorphism, succeeded by the intrusion of granitoids at ca. 300 Ma. On the other hand, the Arroyo Coloco Metamorphic Complex record deformation and metamorphism at epizonal conditions (> 300 °C), constrained at ca. 232-199 Ma by K-Ar and XRD illite data. The Cuesta de Rahue Basement Inlier thus records a protracted orogenic evolution, recording Devonian metamorphism, Late Carboniferous-Permian Gondwanide tectonomagmatic processes, and Late Triassic deformation and metamorphism. Afterwards, this block was also affected by Mesozoic normal faulting and, finally, by Miocene-Pliocene Andean deformation. The latter was intimately related to reactivation of inherited basement fabrics, favouring a transpressional deformation regime.Fil: Oriolo, Sebastián. 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: González, Pablo D.. Servicio Geologico Minero Argentino. Delegacion General Roca.; ArgentinaFil: Alegre, Pablo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias Geológicas; ArgentinaFil: Wemmer, Klaus. Universität Göttingen; AlemaniaFil: Varela, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Centro de Investigaciones Geológicas. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Stipp Basei, Miguel Angelo. Universidade de Sao Paulo; Brasi

Geochemical characterization of the Devonian and Carboniferous-Permian arc magmatism in northwest Patagonia, Argentina

Sobre la base de análisis químicos incluyendo elementos mayores y trazas de 23 rocas ígneas del basamento cristalino del noroeste patagónico, provincias de Neuquén, Río Negro y Chubut, se caracteriza el magmatismo y ambiente tectónico para la región entre 40º y 42º S, al oeste de los 70º O. Las rocas analizadas son mayormente granitoides (mas una migmatita y una meta-basita), con deformación dúctil. El conjunto ha sido datado anteriormente por los métodos U-Pb, K-Ar y Sm-Nd, por lo que se tiene información sobre edades de cristalización, de metamorfismo y deformación y de tiempo de extracción de los protolitos desde el manto. Por sus edades geológicas han sido separadas en dos grupos, uno Chánico (Devónico) y otro Gondwánico (Carbonífero-Pérmico). En ambos casos quedan caracterizados como granitoides subalcalinos, de tendencias calcoalcalinas normales, preferentemente peraluminosos y de tipo I, relacionados con sendos arcos magmáticos desarrollados sobre margen continental. Los resultados obtenidos con las rocas gondwánicas analizadas fueron comparados con datos de otros autores en unidades de basamento del Macizo Norpatagónico, Cordillera Norpatagónica y Cordillera Frontal de Mendoza y Chile.Based on 23 whole rock chemical analyses of the igneous basement rocks from the Northwest Patagonia, Neuquén, Río Negro and Chubut provinces, we characterize the magmatism and tectonic setting for the region between 40º and 42º S, further west of 70º W. The analysed rocks are mainly granitoids (with one migmatite and another meta-basite), with ductile deformation. Previously, all these rocks have been isotopically dated by U-Pb, K-Ar and Sm-Nd methods, and then the crystallization and metamorphism-deformation ages, and extraction time from the mantle are known. According to their geological ages, they have been separated into Chanic (Devonian) and Gondwanic Groups (Carboniferous-Permian). In both cases, the granitoids are subalkaline, normal calc-alkaline, I- type, peraluminous, and related to their respective magmatic arcs, developed on continental margin. The Gondwanic granitoids were also compared with the granitoids of the same age ranging from Northpatagonian Massif, Cordillera Norpatagónica, and Cordillera Frontal from Mendoza (Argentina) and Chile.Facultad de Ciencias Naturales y MuseoCentro de Investigaciones Geológica

Middle Proterozoic and Lower Paleozoic of the Sierra de Umango, Andean Foreland (29°S), Argentina: U-Pb ages and isotopic characteristics

En el basamento cristalino de la sierra de Umango se han distinguido varias unidades ígneo-metamórficas, formadas principalmente entre el Proterozoico medio y el Paleozoico inferior. Incluye ortogneises graníticos, una sucesión de esquistos y mármoles, anfibolitas, y tres plutones graníticos de reducido tamaño. Se han realizado análisis químicos de granitoides y anfibolitas, y determinaciones de edad por los métodos U-Pb y K-Ar. La edad U-Pb en circón del Ortogneis Juchi (1108±13 Ma) ratifica la asignación al Proterozoico medio por anterior datación Rb-Sr (1030±30 Ma). La edad U-Pb en circón del Granito El Peñón (473±17 Ma) es ligeramente mayor que la obtenida por el método Rb-Sr (469±9 Ma), aunque concordante si se tiene en cuenta el error de las determinaciones. Las edades K-Ar sobre muscovitas del Granito El Peñón (409±14 y 381±11 Ma) son relacionadas a deformación y metamorfismo ocurridos en el Devónico. Los isótopos de Nd y Sr permitieron establecer diferencias entre un conjunto de unidades relacionadas con el Ciclo Grenvilliano y otro con los Ciclos Pampeano-Famatiniano. Los datos obtenidos en la sierra de Umango indican que en las Sierras Pampeanas Occidentales hay núcleos de un basamento del Proterozoico medio (Ciclo Grenvilliano), más una sucesión de esquistos y mármoles con anfibolitas, y plutones graníticos de reducido tamaño, formados en el lapso Proterozoico superior-Devónico (Ciclos Pampeano-Famatiniano). La estructura interna de mayor relevancia, formada en el Ordovícico-Devónico muestra clara vergencia tectónica occidental y estaría relacionada con láminas de corrimiento (nappes) de una tectónica colisional. Las mismas se proyectarían desde el interior orogénico (Sierras Pampeanas Orientales) hacia el contiguo antepaís de una de las placas interactuantes. Las sedimentitas clásticas del Ordovícico superior-Devónico en Precordillera (cuenca de antepaís) completan el escenario geodinámico.Igneous and metamorphic units formed mainly during the Middle Proterozoic and Early Paleozoic times have been distinguished in the Sierra de Umango crystalline basement. These include granitic orthogneisses, a sequence of schists and marbles, amphibolites, and three small granitic plutons. Geochemical analyses of granitoids and amphibolites, as well as age determinations using U-Pb and K-Ar methods were carried out. The U-Pb age in zircón of the Juchi Orthogneiss (1108 ± 13 Ma) confirms the previous Rb-Sr Mesoproterozoic dating (1030 ± 30 Ma). The U-Pb in zircón age of El Peñón Granite (473 ± 17 Ma) is slighty older than the Rb-Sr age (469 ± 9 Ma). The K-Ar mouscovite dates from El Peñón Granite (409 ± 14 and 381 ± 11 Ma) are related to a Devonian deformation and metamorphism. On the basis of Nd and Sr isotopic characteristics, two different sets of units can be identified, one related to the Grenvillian cycle and the second to the Pampean-Famatinian cycles. According to the data obtained for the Sierra de Umango, we interpret that in this part of the Western Sierras Pampeanas there are relics of a Mesoproterozoic basement (Grenville cycle), together with more important exposures of a schists and marble succession, with amphibolites and small granitic plutons, that were formed during Late Proterozoic to Devonian times (Pampean and Famatinian cycles). The major basement structure, formed during Ordovician to Devonian, shows a clear tectonic vergence toward the west, and consists of thrust sheets (nappes) typical of collisional tectonics. These sheets are transported from the inner parts of the orogen (Sierras Pampeanas Orientales) toward the contiguous foreland of the interacting terrane. The Upper Ordovician to Devonian sedimentary rocks of the Precordillera (foreland basin) could complete the geodynamic environment.Centro de Investigaciones Geológica

Geochemical characterization of the Devonian and Carboniferous-Permian arc magmatism in northwest Patagonia, Argentina

Sobre la base de análisis químicos incluyendo elementos mayores y trazas de 23 rocas ígneas del basamento cristalino del noroeste patagónico, provincias de Neuquén, Río Negro y Chubut, se caracteriza el magmatismo y ambiente tectónico para la región entre 40º y 42º S, al oeste de los 70º O. Las rocas analizadas son mayormente granitoides (mas una migmatita y una meta-basita), con deformación dúctil. El conjunto ha sido datado anteriormente por los métodos U-Pb, K-Ar y Sm-Nd, por lo que se tiene información sobre edades de cristalización, de metamorfismo y deformación y de tiempo de extracción de los protolitos desde el manto. Por sus edades geológicas han sido separadas en dos grupos, uno Chánico (Devónico) y otro Gondwánico (Carbonífero-Pérmico). En ambos casos quedan caracterizados como granitoides subalcalinos, de tendencias calcoalcalinas normales, preferentemente peraluminosos y de tipo I, relacionados con sendos arcos magmáticos desarrollados sobre margen continental. Los resultados obtenidos con las rocas gondwánicas analizadas fueron comparados con datos de otros autores en unidades de basamento del Macizo Norpatagónico, Cordillera Norpatagónica y Cordillera Frontal de Mendoza y Chile.Based on 23 whole rock chemical analyses of the igneous basement rocks from the Northwest Patagonia, Neuquén, Río Negro and Chubut provinces, we characterize the magmatism and tectonic setting for the region between 40º and 42º S, further west of 70º W. The analysed rocks are mainly granitoids (with one migmatite and another meta-basite), with ductile deformation. Previously, all these rocks have been isotopically dated by U-Pb, K-Ar and Sm-Nd methods, and then the crystallization and metamorphism-deformation ages, and extraction time from the mantle are known. According to their geological ages, they have been separated into Chanic (Devonian) and Gondwanic Groups (Carboniferous-Permian). In both cases, the granitoids are subalkaline, normal calc-alkaline, I- type, peraluminous, and related to their respective magmatic arcs, developed on continental margin. The Gondwanic granitoids were also compared with the granitoids of the same age ranging from Northpatagonian Massif, Cordillera Norpatagónica, and Cordillera Frontal from Mendoza (Argentina) and Chile.Facultad de Ciencias Naturales y MuseoCentro de Investigaciones Geológica