77 research outputs found
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
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
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
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 country rocks of Devonian magmatism in the North Patagonian Massif and Chaitenia
Previous work has shown that Devonian magmatism in the southern Andes occurred in two contemporaneous
belts: one emplaced in the continental crust of the North Patagonian Massif and the other in an oceanic island arc
terrane to the west, Chaitenia, which was later accreted to Patagonia. The country rocks of the plutonic rocks consist
of metasedimentary complexes which crop out sporadically in the Andes on both sides of the Argentina-Chile border,
and additionally of pillow metabasalts for Chaitenia. Detrital zircon SHRIMP U-Pb age determinations in 13 samples
of these rocks indicate maximum possible depositional ages from ca. 370 to 900 Ma, and the case is argued for mostly
Devonian sedimentation as for the fossiliferous Buill slates. Ordovician, Cambrian-late Neoproterozoic and “Grenvilleage”
provenance is seen throughout, except for the most westerly outcrops where Devonian detrital zircons predominate.
Besides a difference in the Precambrian zircon grains, 76% versus 25% respectively, there is no systematic variation
in provenance from the Patagonian foreland to Chaitenia, so that the island arc terrane must have been proximal to the
continent: its deeper crust is not exposed but several outcrops of ultramafic rocks are known. Zircons with devonian
metamorphic rims in rocks from the North Patagonian Massif have no counterpart in the low metamorphic grade Chilean
rocks. These Paleozoic metasedimentary rocks were also intruded by Pennsylvanian and Jurassic granitoids
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