Provenance of Ordovician clastic sequences of the San Rafael Block (Central Argentina), with emphasis on the Ponón Trehué Formation

The Ordovician Ponón Trehué Formation is the only early Palaeozoic sedimentary sequence known to record a primary contact with the Grenvillian-age basement of the Argentinean Cuyania terrane, in its southwards extension named the San Rafael block. Petrographic and geochemical data indicate contributions from a dominantly upper continental crustal component and a subordinated depleted component. Nd isotopes indicate εNd of −4.6, ƒSm/Nd −0.36 and TDM 1.47 Ga in average. Pb-isotope ratios display average values for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb of 19.15, 15.69 and 38.94 respectively. U–Pb detrital zircon ages from the Ponón Trehué Formation cluster around values of 1.2 Ga, indicating a main derivation from a local basement source (Cerro La Ventana Formation). The Upper Ordovician Pavón Formation records a younger episode of clastic sedimentation within the San Rafael block, and it shows a more complex detrital zircon age population (peaks at 1.1 and 1.4 Ga as well as Palaeoproterozoic and Neoproterozoic detrital grains). Detailed comparison between the two Ordovician clastic units indicates a shift with time in provenance from localized basement to more regional sources. Middle to early Upper Ordovician age is inferred for accretion of the Cuyania terrane to the proto-Andean margin of Gondwana.Centro de Investigaciones Geológica

What is the real isotopic signature of dust emitted from Tierra del Fuego?

Fil: Gaiero, D.M. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones en Ciencias de la Tierra; Argentina.Fil: Gili, S. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones en Ciencias de la Tierra; Argentina.Fil: Strelin, J. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones en Ciencias de la Tierra; Argentina.Fil: Strelin, J. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto Antártico Argentino; Argentina.Fil: Koestner, E. IG-UFRGS. Laboratório de Geología Isotópica; Brasil.Fil: Chemale Jr, F. IG-UFRGS. Laboratório de Geología Isotópica; Brasil.Sugden et al. (2009) suggested an on/off switch mechanism that could explain the 20 times increase of dust deposition in Antarctica during the Last Glacial Maximum (LGM), as compared to present day. This suggested switch resides in Patagonian pro-glacial lakes; when glaciers retreat sediments are deposited in the lakes and dust emission ceased in opposition to cold periods, when glacial pour out sediments to the outwash plain, then increasing dust emission. The on/off switch mechanism proposed is supported on age analyses and strontium (Sr) and neodymium (Nd) isotope ratios of ancient lacustrine sediment samples from the Magellan Strait (MS) and from the North Patagonian icefield (NPI). The lacustrine sediments were used as representative of sediments that were disgorged to the outwash plain during the LGM. We argue on the type of samples used to characterize the possible Patagonian outwash sediments deflated during the LGM and accordingly, on the interpretation of the source of this dust. Modern sediments are deflated from the Patagonian surface mainly from widespread ephemeral lakes (Gaiero 2007), which are sporadically refilled with sediments supplied from the surrounding areas mainly through water runoff. The aim of this contribution is to discuss about this and contrast data from both set of samples and discuss about their significance for the interpretation of the isotopic signatures recorded on the sedimentary archives of the Southern Hemisphere (e.g., Antarctic ice cores). Fig. 1 shows that the mean isotopic composition of dry lake sediments (collected from the ancient outwash plain in the San Sebastián Bay area) and dust collected at Río Grande are significantly different compared to MS samples and very similar to sediments representing the Fuegian continental shelf (Basile et al. 2007). Similar to modern dust released from continental Patagonia (north of ~52° S), modern data from Tierra del Fuego could also be explained by a mixing between Jurassic rhyolites and Quaternary volcanic rocks (e.g., Gaiero et al. 2007; Fig. 1). On the contrary, the samples used by Sugden et al. (2009) seem to characterize discrete sources. The isotopic composition of most samples from the MS plots mostly within the compositional field corresponding to the Antarctic Jurassic rhyolites and similar rocks that outcrop out close along the Fuegian Cordillera (Gaiero et al. 2007). In the case of NPI samples, their compositions are similar to the local outcropping Paleozoic plutonic and metasedimentary rocks (Killian and Behrmann 2003). Nevertheless, directly to the W of the NPI, the isotopic composition of Pliocene/Pleistocene Chilean trench sediments (mean 87Sr/86Sr = 0.707 and δNd(0) = -2.1) indicates a variable contribution from other rocks cropping out in the area (e.g., Quaternary volcanic rocks). Why MS and NPI samples are different from modern Patagonian dust?Fil: Gaiero, D.M. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones en Ciencias de la Tierra; Argentina.Fil: Gili, S. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones en Ciencias de la Tierra; Argentina.Fil: Strelin, J. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro de Investigaciones en Ciencias de la Tierra; Argentina.Fil: Strelin, J. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto Antártico Argentino; Argentina.Fil: Koestner, E. IG-UFRGS. Laboratório de Geología Isotópica; Brasil.Fil: Chemale Jr, F. IG-UFRGS. Laboratório de Geología Isotópica; Brasil.Geoquímica y Geofísic

Rio grande cone tectono-stratigraphic model – brazil: seismic sequences

Análisis sísmicos integrados con técnicas de interpretación, procesamiento, información de velocidades y datos geofísicos permiten modelar diferentes rasgos de tipo tectónico, estructural o geomorfológico. Este trabajo se constituye en una primera aproximación a un modelo tridimensional de un rasgo geomorfológico ubicado en el fondo oceánico. El modelo inicial comprende una configuración bidimensional de una grilla interpretada a partir de secciones sísmicas llevadas a un volumen tridimensional, esto mediante la concepción de una aproximación dos y medio dimensional. Además de un cambio de configuración, se hizo un análisis de atributos sobre las imágenes de las secciones y se determinaron las características presentes desde un área de la plataforma, hasta el offshore del sudeste Brasilero. El rasgo geomorfológico a tratar es conocido como el Conode Rio Grande, el cual pudo ser caracterizando con sus principales estructuras tectónicas y estratigráficas, mediante la cartografía y modelamiento tridimensional del subsuelo, con la integración geológica, geofísica y geomorfológica. La geomorfología del cono, se ve influenciada por procesos gravitacionales, corrientes defondo, estructuras de tipo tectónico y composición litológica, como sus principales controladores que están presentes a partir del Neógeno. Así, a partir de la interpretación sísmica pueden ser extractados diferentes estructuras tectónicas, sedimentarias y geoformas, por ejemplo presencia de fallas, pliegues, canales, levee naturales, contornitos, etc

Whole-rock and isotope geochemistry of Ordovician to Silurian units of the Cuyania terrane, Argentina: insights for the evolution of SW Gondwana margin

The Cuyania terrane in central Argentina (Fig. 1) is characterized by a Mesoproterozoic (Grenvillianage) basement with depleted Pb isotopic signatures and Mesoproterozoic Nd model ages resembling basement rocks of the same age from Laurentia (Ramos, 2004; Sato et al., 2004 and references therein). Several authors have proposed para-autochthonous (Aceñolaza et al., 2002; Finney et al., 2005) versus allochthonous (e. g. Ramos et al., 1986; Dalziel et al., 1994; Astini et al., 1995; Thomas and Astini, 1996) geotectonic models for the early Palaeozoic evolution of the Cuyania terrane. The tectonic evolution of the Cuyania terrane is a substantial part of the understanding of the evolution of the western border of southwest Gondwana. Several morphostructural units form the Cuyania composite terrane (Fig. 1; Ramos et al., 1996): The Precordillera s.s., the Western Pampeanas Ranges and the San Rafael and Las Matras blocks. However, the boundaries of the terrane are still not well-constrained (Astini and Dávila, 2004; Porcher et al., 2004; Casquet et al., 2006). A combination of several methodologies including geochemistry, Sm-Nd, Pb-Pb and U-Pb detrital zircon dating was applied to several clastic Ordovician (Los Sombreros, Gualcamayo, Los Azules, La Cantera, Yerba Loca, Empozada, Trapiche, Sierra de la Invernada, Portezuelo del Tontal, Las Vacas, Las Plantas and Alcaparrosa Formations) and Ordovician to Silurian (Don Braulio and La Chilca Formations) units of the Cuyania terrane (Fig. 2). The combination of these different approaches can give accurate information in order to constrain the probable sources that provided detritus to the Cuyania terrane and ultimately to constrain the existing models about its origin.Centro de Investigaciones Geológica

Nd isotopes from Yerba Loca Formation (Upper Ordovician), Cuyania terrane, Argentina

Several arguments as well as geochemical data from the basement rocks support the allochthonous models of the Precordillera (Cuyania) Terrane as derived from Laurentia (e.g. Ramos et al., 1986; Dalla Salda et al., 1992; Astini et al., 1995). Less certain are the time and type of collision with Gondwana. However, other authors have proposed a parautochthonous evolution based on biostratigraphical and structural data, displaced during Ordovician–Devonian times (e.g., Aceñolaza et al., 2002; Finney et al., 2005). To contribute to the discussion about tectonic models for the Precordillera, preliminary Nd isotope data from an ongoing provenance study on the Caradocian rocks from the Yerba Loca Formation are here presented.Centro de Investigaciones Geológica

Provenance of Ordovician clastic sequences of the San Rafael Block (Central Argentina), with emphasis on the Ponón Trehué Formation

The Ordovician Ponón Trehué Formation is the only early Palaeozoic sedimentary sequence known to record a primary contact with the Grenvillian-age basement of the Argentinean Cuyania terrane, in its southwards extension named the San Rafael block. Petrographic and geochemical data indicate contributions from a dominantly upper continental crustal component and a subordinated depleted component. Nd isotopes indicate εNd of −4.6, ƒSm/Nd −0.36 and TDM 1.47 Ga in average. Pb-isotope ratios display average values for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb of 19.15, 15.69 and 38.94 respectively. U–Pb detrital zircon ages from the Ponón Trehué Formation cluster around values of 1.2 Ga, indicating a main derivation from a local basement source (Cerro La Ventana Formation). The Upper Ordovician Pavón Formation records a younger episode of clastic sedimentation within the San Rafael block, and it shows a more complex detrital zircon age population (peaks at 1.1 and 1.4 Ga as well as Palaeoproterozoic and Neoproterozoic detrital grains). Detailed comparison between the two Ordovician clastic units indicates a shift with time in provenance from localized basement to more regional sources. Middle to early Upper Ordovician age is inferred for accretion of the Cuyania terrane to the proto-Andean margin of Gondwana.Centro de Investigaciones Geológica

U-Pb detrital zircon data from the paleozoic Sierra Grande Formation, North Patagonian Massif, Argentina

The Sierra Grande Formation is a siliciclastic sedimentary unit with oolithic iron levels of economical importance which crops out in the Argentinean North Patagonian Massif. It was deposited within an open marine environment, with littoral to sub-littoral conditions. A Silurian-Lower Devonian age is assigned based on fossil content. It is mainly composed of medium to fine quartz arenites, wackes, silts, shales and conglomerate beds. In order to establish the source rocks for the Silurian-Devonian basin and also to constrain the sedimentation age, different U-Pb geochronological studies have been carried out on detrital zircons. The zircons have been studied with comparative morphology analysis using SEM images to asses the different population types. For dating using U-Pb isotopes in zircon detrital grains, the high resolution SHRIMP and LA-ICP-MS methodologies were used. The predominant obtained age groups are: Neoproterozoic (547–991 Ma), Cambrian (497-546 Ma) and Ordovician (443-495 Ma). All the analyzed samples also comprise Mesoproterozoic ages of about 1009 to 1382 Ma and smaller proportion Paleoproterozic (1641 to 2248 Ma) and Neo-Archean (2649-2657 Ma) ages. It is important to note that Silurian ages (428 and 440 Ma) were obtained from a group of zircon crystals, constraining the sedimentation age of the Sierra Grande basin to be not older than Middle Silurian. With these isotopic data it is possible to discuss the provenance of the Silurian-Devonian siliciclastic sedimentary cover of the North Patagonian Massif.Centro de Investigaciones Geológica