Composición química de granitos batolíticos de las sierras pampeanas

Se describe en esta contribución la metodología químico-analítica y los resultados obtenidos mediante la misma a los efectos de la determinación de elementos mayoritarios y trazas en cuerpos graníticos mayores de las Sierras Pampeanas. Los óxidos mayoritarios SiO 2 , TiO 2 , Al 2 0 3 , Fe 2 0 3 (total), MnO, MgO ; CaO, Na 2 O, K 2 O yP 2 O 5 y los elementos traza Rb, Sr, Ba, La, Ce, Nd, V, Y, Nb y Zr fueron determinados por espectrometría de fluorescencia de rayos X en rocas granitoides de los batolitos de Achala, Capillitas y la Sierra de Velasco. Se efectuó un estudio de precisión y exactitud de las determinaciones mediante la comparación con patrones de roca internacionales

Estudios estadísticos de elementos traza en el basamento Igneo-Metamórfico de la Sierra de Quilmes

El conocimiento de los parámetros estadísticos de los elementos traza en rocas genéticamente asociadas es de fundamental importancia. En el campo de las rocas granitoídes, por ejemplo, el estudio de sus distribuciones es particularmente útil. En una secuencia de rocas graníticas la diferencia en composición de elementos mayoritarios entre las diferentes fases, es mínima; en cambio los tenores de elementos traza suelen diferir criticamente. En general, la variación relativa de elementos traza es mucho mayor que la de los elementos mayoritarios. Como consecuencia de ello, el grado de diferenciación y la acidez son mucho mejor indicados por la variación de aquéllos. A este respecto elementos como rubidio, estroncio, bario, cobalto y níquel, algunas de cuyas distribuciones se analizan en este trabajo, han resultado particularmente útiles como indicadores de diferenciación y del orden de intrusión de las diferentes fases que pueden componer un cuerpo complejo

U-Pb SHRIMP zircon dating of Grenvillian metamorphism in Western Sierras Pampeanas (Argentina) : correlation with the Arequipa-Antofalla craton and constraints on the extent of the Precordillera Terrane

The Sierras Pampeanas of Argentina, the largest outcrop of pre-Andean crystalline basement in southern South America, resulted from plate interactions along the proto-Andean margin of Gondwana, from as early as Mesoproterozoic to Late Paleozoic times (e.g., Ramos, 2004, and references therein). Two discrete Paleozoic orogenic belts have been recognized: the Early Cambrian Pampean belt in the eastern sierras, and the Ordovician Famatinian belt, which partially overprints it to the west (e.g., Rapela et al., 1998). In the Western Sierras Pampeanas, Mesoproterozoic igneous rocks (ca. 1.0–1.2 Ga) have been recognized in the Sierra de Pie de Palo (Fig. 1) (McDonough et al., 1993 M.R. McDonough, V.A. Ramos, C.E. Isachsen, S.A. Bowring and G.I. Vujovich, Edades preliminares de circones del basamento de la Sierra de Pie de Palo, Sierras Pampeanas occidentales de San Juán: sus implicancias para el supercontinente proterozoico de Rodinia, 12° Cong. Geol. Argentino, Actas vol. 3 (1993), pp. 340–342.McDonough et al., 1993, Pankhurst and Rapela, 1998 and Vujovich et al., 2004) that are time-coincident with the Grenvillian orogeny of eastern and northeastern North America (e.g., Rivers, 1997 and Corrievau and van Breemen, 2000). These Grenvillian-age rocks have been considered to be the easternmost exposure of basement to the Precordillera Terrane, a supposed Laurentian continental block accreted to Gondwana during the Famatinian orogeny (Thomas and Astini, 2003, and references therein). However, the boundaries of this Grenvillian belt are still poorly defined, and its alleged allochthoneity has been challenged (Galindo et al., 2004). Moreover, most of the Grenvillian ages so far determined relate to igneous protoliths, and there is no conclusive evidence for a Grenvillian orogenic belt, other than inferred from petrographic evidence alone (Casquet et al., 2001). We provide here the first evidence, based on U–Pb SHRIMP zircon dating at Sierra de Maz, for a Grenville-age granulite facies metamorphism, leading to the conclusion that a continuous mobile belt existed throughout the proto-Andean margin of Gondwana in Grenvillian times

Edad y origen de la fluorita del yacimiento de la Nueva (Cabalango, Córdoba, Argentina) en base a geoquímica de isótopos radiogénicos (Nd y Sr)

The fluorite mineralization from the La Nueva Mine, has a Sm-Nd age of 130 ± 19 Ma, and thus must be related to the Lower Cretaceous late-Gondwanic extensional and magmatic event that affected the Sierras Pampeanas Basement of Argentina. Hydrothermal fluids involved in the formation of fluorite were probably derived by mixing of two fluids, an ascending high 87Sr/86Sr one, probably equilibrated with basement metamorphics, as suggested by the very negative ENd(130) values, and a second, "descending", with a low 87Sr/86Sr value

K-bentonites in the Argentine Precordillera contemporaneous with rhyolite volcanism in the Famatinian Arc

New U–Pb radiometric dates for K-bentonite horizons within the Lower Cambrian to Middle Ordovician platform carbonates from the Precordillera terrane of NWArgentina provide further constraints on models for the allochthonous or parautochthonous accretion of this terrane. Two K-bentonite layers from the Talacasto section yield indistinguishable sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dates of 469.5 3.2 Ma and 470.1 3.3 Ma respectively. These are within uncertainty of the U–Pb SHRIMP zircon date of 468.3 3.4 Ma for a porphyritic rhyolite from the Famatinian magmatic arc, Sierra de las Planchadas, near Rio Chaschuil. Geochemical and isotope data also demonstrate the similarity of the Kbentonite and Chaschuil rhyolite parent magmas. Thus, it is highly probable that the Famatinian arc volcanoes provided the ash for the K-bentonite horizons, suggesting proximity to the Precordillera terrane during the deposition of the Lower Cambrian to Middle Ordovician platform carbonates. This implication supports a mid-Ordovician collision model, but could also be compatible with a parautochthonous model for docking of the Precordillera terrane, by movement along the Pacific margin of Gondwana, rather than across the Iapetus Ocean

Granate con alto contenido de tierras raras pesadas (HREE) y elevada relación Sm/Nd, en pegmatitas de la Sierra de Valle Fértil (Sierras Pampeanas, Argentina).

2 p.El trabajo se realizó en el marco de lso proyectos PB97-1246 (MEC) y BTE2001-1486 (MCYT) y PICT98-4189 (Argentina

Sr, C and O isotope composition of marbles from the Sierra de de Ancasti, Eastern Sierras Pampeanas, Argentina: age and constraints for the Neoproterozoic–Lower Paleozoic evolution of the proto-Gondwana margin

The Sierra Brava Complex on the eastern flank of the Sierra de Ancasti consists of marbles, metabasites, calc-silicate rocks, psammo-pelitic schists and gneisses. In the central part of this sierra a thick succession of banded schists (Ancasti Formation) crops out. Regional metamorphism of these rocks is attributed to the Famatinian orogeny (Ordovician), metamorphic grade increasing westwards and southwards and culminating in a migmatite complex on the western side of the Sierra. The meta-carbonate rocks are subdivided into a northeastern group (low-grade calcite marbles), and a southeastern group (high-grade calcite and calcite-dolomite marbles). Twenty-three marble samples were analysed for Sr isotope composition and Rb, Mn, Mg and Ca contents, and six for C and O isotope composition. An Ediacaran depositional age of 570 –590Ma is inferred by reference to the trend of 87Sr/86Sr in Neoproterozoic seawater. Thus the metacarbonates are probably slightly older than the Ancasti Formation (equivalent to the Puncoviscana Formation of northern Argentina), which has a maximum sedimentation age of ca. 570Ma. Ediacaran depositional ages have also been reported for metacarbonates elsewhere in Argentina, Uruguay and Brazil. We propose that the Sierra de Ancasti carbonates on one hand, and those in the Western Sierras Pampeanas (Difunta Correa Sequence) and –tentativelythe Corumbá Group of Brazil on the other, represent platforms on opposite margins of the extinct Clymene Ocean, whereas Neoproterozoic carbonate successions such as the Loma Negra Formation (Tandilia, southern Argentina) and the Arroyo del Soldado Group (Uruguay) were deposited on the eastern side (present coordinates) of the Rio de la Plata craton, which at the time occupied a position farther to the north.Peer reviewe

Involvement of the Argentine Precordillera terrane in the Famatinian mobile belt: U-Pb SHRIMP and metamorphic evidence from the Sierra de Pie de Palo

New data suggest that the eastern margin of the Argentine Precordillera terrane comprises Grenvillian basement and a sedimentary cover derived from it that were together affected by Middle Ordovician deformation and metamorphism during accretion to the Gondwana margin. The basement first underwent low pressure/temperature (P/T) type metamorphism, reaching high-grade migmatitic conditions in places (686 6 40 MPa, 790 6 17 8C), comparable to the Grenvillian M2 metamorphism of the supposed Laurentian counterpart of the terrane. The second metamorphism, recognized in the cover sequence, is of Famatinian age and took place under higher P/T conditions, following a clockwise P-T path (baric peak: 1300 6 100 Mpa, 600 6 50 8C). Low-U zircon overgrew detrital Grenvillian cores as pressure fell from its peak, and yields U-Pb SHRIMP ages of ca. 460 Ma. This is interpreted as the age of ductile thrusting coincident with early uplift; initial accretion to Gondwana must have occurred before this. The absence of late Neoproterozoic detrital zircons is consistent with a Laurentian origin of the Argentine Precordillera terrane

Sr, C and O isotope composition of marbles from the Sierra de Ancasti, Eastern Sierras Pampeanas, Argentina: age and constraints for the Neoproterozoic–Lower Paleozoic evolution of the proto-Gondwana margin

The Sierra Brava Complex on the eastern flank of the Sierra de Ancasti consists of marbles, metabasites, calc-silicate rocks, psammo-pelitic schists and gneisses. In the central part of this sierra a thick succession of banded schists (Ancasti Formation) crops out. Regional metamorphism of these rocks is attributed to the Famatinian orogeny (Ordovician), metamorphic grade increasing westwards and southwards and culminating in a migmatite complex on the western side of the Sierra. The meta-carbonate rocks are subdivided into a northeastern group (low-grade calcite marbles), and a southeastern group (high-grade calcite and calcite-dolomite marbles). Twenty-three marble samples were analysed for Sr isotope composition and Rb, Mn, Mg and Ca contents, and six for C and O isotope composition. An Ediacaran depositional age of 570 –590Ma is inferred by reference to the trend of 87Sr/86Sr in Neoproterozoic seawater. Thus the metacarbonates are probably slightly older than the Ancasti Formation (equivalent to the Puncoviscana Formation of northern Argentina), which has a maximum sedimentation age of ca. 570Ma. Ediacaran depositional ages have also been reported for metacarbonates elsewhere in Argentina, Uruguay and Brazil. We propose that the Sierra de Ancasti carbonates on one hand, and those in the Western Sierras Pampeanas (Difunta Correa Sequence) and -tentativelythe Corumbá Group of Brazil on the other, represent platforms on opposite margins of the extinct Clymene Ocean, whereas Neoproterozoic carbonate successions such as the Loma Negra Formation (Tandilia, southern Argentina) and the Arroyo del Soldado Group (Uruguay) were deposited on the eastern side (present coordinates) of the Río de la Plata craton, which at the time occupied a position farther to the north

The continental assembly of SW Gondwana (Ediacaran to Cambrian): a synthesis

SW Gondwana resulted from complex interplay between continental amalgamation and dispersal between ~ 650 and 490 Ma. The main cratons involved were Laurentia, Amazonia– MARA (Proterozoic Maz–Arequipa–Rio Apa, Casquet et al., 2012), Kalahari, Rio de la Plata (RPC), Congo and East Antarctica (Mawson block). Several collisional orogenic belts resulted, notably the East Africa–Antarctica, Brasiliano–Panafrican, Pampean–Saldania, and Ross– Delamerian orogens. East-Antarctica broke away from the western margin of Laurentia in Rodinia. After a long drift and counter-clockwise rotation (Dalziel, 2013) it collided with Congo and Kalahari to produce the southern part of the left-lateral transpressional East Africa–Antarctica orogen between 580 and 550 Ma, completing the amalgamation of East Gondwana. The Trans-Antarctic margin became an active one in the Ediacaran and subduction of the Pacific Ocean lithosphere occurred throughout the Paleozoic, forming a tract of the Terra Australis orogen. NW–SE directed compression in Late Cryogenian and Early Ediacaran times promoted closure of the Adamastor Ocean, resulting in the left-lateral transpressional Brasiliano–Pan African orogeny between 650 and 570 Ma. The Pampean orogenic belt to the west of the RPC resulted from right-lateral collision between Laurentia and its eastern extension MARA on the one hand and Kalahari–RPC on the other. Ocean opening started at ~ 630 Ma and subduction and further collision took place between 540 and 520 Ma, coeval with the northward drift of Laurentia (~ 540 Ma) away from MARA and the consequent formation of the proto-Andean margin of Gondwana. The margins of the intervening Puncoviscana ocean were covered by Laurentia-derived siliciclastic sediments and carbonates on the MARA side between 630 and ~ 540 Ma (Rapela et al, 2014; this symposium), and by the marine siliciclastic Puncoviscana Formation on the other. The latter formation, deposited between a 570 and ~530 Ma, received input from large alluvial fans descending from juvenile Mesoproterozoic and Neproterozoic sources (new Hf isotope evidence) largely located in the southern East Africa–Antarctica orogen. The Pampean orogen extended into the Saldania–Gariep orogen of southern South Africa (545–520 Ma) and was apparently discordant to the earlier Brasiliano–Pan African orogen. In late-Early to late Cambrian times the Pampean–Saldania realm evolved into a passive margin with siliciclastic platform sedimentation. The Pampean-Saldania realm was separated from the active Trans-Antarctic margin of East Antarctica by an inferred transform fault in Ediacaran to Cambrian times. Regional NW–SW shortening in the Ediacaran became N–S directed in the Cambrian, suggesting a major plate reorganization at this time.Peer reviewe