La Rioja, Argentina
La Rioja, Argentina

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Dahlquist J.A.,CONICET | Pankhurst R.J.,British Geological Survey | Gaschnig R.M.,University of Maryland University College | Rapela C.W.,CONICET | And 5 more authors.
Gondwana Research | Year: 2013

We report the first study integrating in situ U-Pb and Hf isotope data from magmatic zircon and whole-rock Sm-Nd isotope data for granitic rocks of the Sierras Pampeanas, Argentina, in order to evaluate the Palaeozoic growth of the proto-Andean margin of Gondwana. Early-Middle Ordovician granitic magmatism is by far the most voluminous of the Sierras Pampeanas and represents the most significant magmatic event. These calc-alkaline granitoids were intruded at an active continental margin. εHft values range from -3.3 to -14.7 and εNdt from -3.3 to -6.3 (t=473Ma), with average TDM Hf and TDM Nd ranging from 1.5 to 2.2Ga and 1.4 to 1.7Ga, respectively. Middle-Late Devonian magmatism occurred in the foreland, away from the orogenic front in the west, and included F-U-REE rich A-type granites. The Achala granite, the largest batholith in the Sierras Pampeanas, has εHft and εNdt values ranging from -3.6 to -5.8 and -4.0 to -6.5, respectively (t=369Ma). Small scattered Early Carboniferous A-type granite plutons were intruded in a dominantly extensional setting and have εHft and εNdt values ranging from -6.7 to +2.2 and -0.5 to -3.6, respectively (t=341Ma). The generation of Ordovician and Devonian magmas dominantly involved crustal reworking and stabilization rather than the formation of new continental crust by juvenile material accretion, whereas Carboniferous magmatism resulted in part from reworking of supracrustal material, but with variable addition of juvenile magmas. © 2012 International Association for Gondwana Research.

Alasino P.H.,CONICET | Dahlquist J.A.,INGeReN CENIIT UNLaR | Dahlquist J.A.,CONICET | Pankhurst R.J.,British Geological Survey | And 5 more authors.
Gondwana Research | Year: 2012

A recently discovered granitic intrusion at Cerro La Gloria in western Sierra de Famatina (NW Argentina) is representative of sub- to mid-alkaline Carboniferous magmatism in the region. The main rock type consists of microcline, quartz and plagioclase, with amphibole, magnetite, ilmenite, biotite, epidote, zircon, allanite and sphene as accessory minerals. We report a U-Pb zircon SHRIMP age for the pluton of 349±3Ma (MSWD=1.1), i.e., Tournaisian. Whole-rock chemical composition and Nd isotope analyses are compatible with an origin by melting of older mafic material in the lower crust (εNdt between -0.58 and +0.46 and T DM values of about 1.1Ga). The pluton is intruded by penecontemporaneous to late alkaline mafic dykes that are classified as back-arc basalts. Coeval, Early Carboniferous A-type granites occur farther east in the Sierras Pampeanas, probably generated during lithospheric stretching. Overall, the Early Carboniferous granitic rocks show a west-to-east mineralogical and isotopic zonation indicating that magma genesis involved a greater contribution of juvenile material of mantle character to the west. Based on the observed patterns of geochronology, geochemistry and field relationships we suggest that A-type magma genesis in the Eastern Sierras Pampeanas was linked to an Andean-type margin where the lithospheric mantle played a role in its generation. © 2012 International Association for Gondwana Research.

Dahlquist J.A.,CONICET | Rapela C.W.,CONICET | Pankhurst R.J.,British Geological Survey | Fanning C.M.,Australian National University | And 7 more authors.
Journal of South American Earth Sciences | Year: 2012

The granitic rocks of the Sierra de Ancasti represent one of the easternmost outcrops of the Famatinian arc of the Sierras Pampeanas. We report here new U-Pb SHRIMP and LA-ICP-MS Ordovician ages and a petrological and geochemical study of the Las Cañadas I-type granitic complex in the central sector of Sierra de Ancasti. Numerical modelling indicates that calc-alkaline and metaluminous monzogranites (SiO 2=69.8-73.0wt.%) were produced by fractional crystallization of plagioclase-rich tonalite (∼60% plagioclase and SiO 2=60.9-62.5wt.%) from an inferred parental magma of granodioritic composition (SiO 2=67.4wt.%). Sm-Nd isotopic data on the dated rocks indicate a dominant continental crustal input, as reported for other Ordovician granitic rocks (Famatinian magmatism) farther west in the Sierras Pampeanas. Combined ages and isotopic data are consistent with previous studies indicating that Famatinian magmatism was short-lived (∼20Ma) and lacked a significant asthenospheric contribution. The occurrence of inherited zircon and the petrological and geochemical data from the Las Cañadas complex indicate that the parental magma was derived from mixed igneous and sedimentary material in the source or by partial melting of igneous rocks of different ages. Systematic dating of inherited zircon cores is required to clarify this issue. Zircon saturation geothermometry combined with geochemical data indicates that the onset of zircon crystallization, together with variable amounts of other minerals such as plagioclase, quartz, biotite, oxides and apatite, occurred when the parental magma reached 67wt.% SiO 2 with ∼440ppm of Zr at ∼878°C. © 2011 Elsevier Ltd.

Dahlquist J.A.,Av. Velez Sarsfield 1611 | Pankhurst R.J.,British Geological Survey | Rapela C.W.,Calle 1 No. 644 | Basei M.A.S.,University of Sao Paulo | And 5 more authors.
International Journal of Earth Sciences | Year: 2015

New geochronological, geochemical, and isotopic data are reported for the Capilla del Monte two-mica granite pluton in the northeastern Sierras de Córdoba. An Early Carboniferous age is established by a U–Pb zircon concordia (336 ± 3 Ma) and a Rb–Sr whole-rock isochron (337 ± 2 Ma). Zircon saturation geothermometry indicates relatively high temperatures (735–800 °C). The granites have high average SiO2 (74.2 %), Na2O + K2O (7.8 %), and high field-strength elements, high K2O/Na2O (1.7) and FeO/MgO ratios (5.1), with low CaO content (0.71 %). REE patterns with marked negative Eu anomalies (Eu/Eu* 0.14–0.56) indicate crystal fractionation, dominantly of plagioclase and K-feldspar, from a peraluminous magma enriched in F. Isotope data (87Sr/86Srinitial = 0.7086, εNd336 = −5.5 to −4.4 with TDM = 1.5 Ga, zircon εHf336 +0.8 to −6.1; mean TDM = 1.5 Ga) suggest a Mesoproterozoic continental source, albeit with some younger or more juvenile material indicated by the Hf data. The pluton is the easternmost member of a Carboniferous A-type magmatic suite which shows an increase in juvenile input toward the west in this part of the pre-Andean margin. The petrological and geochemical data strongly suggest a similar intraplate geodynamic setting to that of the nearby but much larger, Late Devonian, Achala batholith, although Hf isotope signatures of zircon suggest a more uniformly crustal origin for the latter. Further studies are required to understand whether these bodies represent two independent magmatic episodes or more continuous activity. © 2015 Springer-Verlag Berlin Heidelberg

Dahlquist J.A.,CONICET | Alasino P.H.,INGeReN CENIIT UNLaR | Alasino P.H.,CONICET | Bello C.,Comision Nacional de la Energia Atomica
Mineralogy and Petrology | Year: 2014

A new LA-ICP-MS crystallization age of 370 ± 8 Ma is presented for monzogranite from the Achala batholith, the largest Devonian igneous body in the Sierras Pampeanas, confirming previous U-Pb zircon ages and indicating emplacement within a relatively short episode. Granitic rocks from the central area of the batholith display restricted high SiO2 contents (69.8-74.5 wt.%). Major element plots show ferroan and alkaline-calcic to calc-alkaline compositions with an A-type signature. High concentrations of the high field-strength elements such as Y, Nb, Ga, Ta, U, Th, and flat REE patterns with significant negative Eu anomalies, are also typical of A-type granites. The aluminium saturation index (1.10-1.37) indicates aluminous parent magmas which are further characterised by high FeO/MgO ratios (2.6-3.3) and F contents of igneous biotites (0.9-1.5 wt%), as well as relatively high AlIV (2.39-2.58 a.p.f.u.) in biotites and the occurrence of primary muscovite. Petrogenetic modelling supports a source enriched in plagioclase and progressive fractional crystallization of feldspar. The central area of the batholith displays small-scale bodies composed predominantly of biotite (80 %), muscovite (10 %) and apatite (10 %), yielding rock compositions with 2.3-5.4 wt. % P2O5, and 6-7 wt.% F, together with anomalous contents of U (88-1,866 ppm), Zr (1081-2,581 ppm), Nb (257-1,395 ppm) and ΣREE (1,443-4,492 ppm). Previous studies rule out an origin of these bodies as metasedimentary xenoliths and they have been interpreted as cumulates from the granitic magma. An alternative flow segregation process is discussed here. © 2013 Springer-Verlag Wien.

The crystallization age of subvolcanic dykes emplaced in Carboniferous granitic plutons of the Sierras Pampeanas Orientales remains uncertain. The first age precise U-Pb zircon LA-ICP-MS obtained from a dyke emplaced in the Los Árboles pluton (Sierra de Fiambalá) yield a crystallization age of 311 ± 3 Ma.

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