Resumo

ZIMMERMANN, Udo et al. Provenance studies of the Falda Ciénaga Formation (Middle Ordovivian, Argentinean Puna) using sedimentary petrography, trace elements and Nd isotopes. Rev. Asoc. Argent. Sedimentol. [online]. 2002, vol.9, n.2, pp.165-188. ISSN 1853-6360.

The objective of this paper is to provide a provenance study of the Falda Ciénaga Formation and a paleotectonic scenario for the Middle Ordovician combining new data with published information from Ordovician sedimentary successions of the Puna, a highland in the northwestern part of Argentina. The Falda Ciénaga Formation oucrops between the Salar de Pocitos (24° 24' 59.5''S 66° 55' 18.0''W) and the Sierra Calalaste (26° 11' 52.5''S 67° 41' 02''W). The abundance of black shales containing pyrite, the absence of shallow water sedimentary structures and the lack of debris flows suggest that the Falda Ciénaga Formation was deposited in a relatively deep basin. The material was deposited by turbidity currents in a depositional lobe, lobe fringe and basin plain environment (Göttert, 1997; Zimmermann et al., 1997). Deposits in the northern region (Salar de Pocitos; Fig. 1b) show different lithofacies to those to the south of Salar de Hombre Muerto (Fig. 1c). Voluminous sandstones and greywackes outcrop in the northern area, while shale dominate in the southern outcrop region. Furthermore, the succession in the north contains Phyllograptus sp. (Zimmermann et al., 1998) indicating a time lapse between Upper Arenigian and Llanvirnian, whereas Glossograptus sp. and Glossograptus hieckinsis found in the southern outcrops (Aceñolaza y Toselli, 1971; this paper) suggests a Llanvirnian age. Further to the south, between Salar de Hombre Muerto and Antofagasta de la Sierra (Fig. 1b), the Falda Ciénaga Formation is composed of massive turbidites and shales, rare siltstones and subordinate coarse-grained greywackes. The latter are interpreted as submarine channel deposits, as observed to the east of Los Nacimientos locality (Fig. 1c). Feldspathic to quartzitic arenites, medium to very-coarse grained greywackes and fine layers of pelites occur as lithotypes of the Falda Ciénaga Formation. The composition of the sandstones changes between 65-78 % Q_t , 3-19 % F and 0-22 % L. The sedimentary rocks do not contain volcanic lithoclasts or synsedimentary volcanics. The detrital material was transported from southern sources (SW to SE) to northern depositional areas. This coincides with the pronounced axis of the Ordovician Puna basin for the Lower Ordovician (Bahlburg, 1990; Göttert, 1997; Zimmermann y Bahlburg, 1998, Zimermann, 1999). Whole rock geochemistry was focussed on trace elements, because a significant major element mobility occurs in abundance in Ordovician sedimentary rocks of the Puna, illustrated by element relations (K-Rb), spider diagrams of LILE as well as provenance discrimination diagrams after Bhatia (1983) and Roser y Korsch (1988) (Bahlburg, 1998; Zimmermann y Bahlburg, 1998; subm.; Zimmermann, 1999, 2000). Furthermore, isotope analysis of K-Ar and Rb- Sr systems indicate similar major element mobility for the entire Ordovician sedimentary record of the southern Puna (Zimmermann, 1999). INAA trace element and rare earth element (REE) analysis show typical concentrations of an evolved upper continental crustal composition (UCC) as Th/Sc 0,75-2,81 (average 1,8); Th/U 3-5,9 (average 4,15) and La_N/Yb_N 4,69-8,57 (average 6,75). A negative Ta anomaly in some samples could reflect an influence of volcanic arc material of unknown age in the detritus. This source is not visible in the petrographic data (see Table 1). The concentrations and relations of Hf, Co, Th, Sc and La show compositional ranges typical for passive or rifted margin environments and support the petrographic data. Interestingly, slightly enriched concentrations of Cr (2-6x composition of UCC, Upper Continental Crust) do not coincide with negative anomalies of other compatible elements such as Ni and Sc, that show depleted UCC concentrations. Cr enrichment may be due to the introduction of detrital material from a less fractionated source that delivered heavy minerals like chromite, for example and reflect the loss, during mechanical sorting, of Ni and Sc bearing minerals. Nd-isotope data for the Formation Falda Ciénaga indicates e_Ndt=450Ma between -6 y -4,5 and T_DM of 1,4 -1,6 Ga. These data are similar to those of the Tollilar Formation (Fig. 2; Zimmermann et al., 1999a), the Puncoviscana Formation (Fig. 2; Bock et al., 2000), the Sierras Pampeanas (Rapela et al., 1998) and the magmatites of the Ordovician Sierra Famatina (Fig. 1; Pankhurst et al., 1998). Mixing models after DePaolo et al. (1991) show that the input of volcaniclastic material available in the region (Diablo Formation or Volcanic Successions; Fig. 2, Table 4) or mafic to ultra-mafic sources (Salar de Pocitos, Sierra Calalaste, Sierra Quebrada Honda, Fig. 1b) is not realistic. Furthermore, an input of mafic material could be excluded by petrographic data (Table 1, Figs. 6 and 9). Ordovician volcanics of the Puna have isotopic compositions that could not be mixed with any other exposed source to model a provenance according to the petrographic data for the Falda Ciénaga Formation. Mixing model using higher fractionated end members (e.g. Tolar Chico Formation (Fig. 2), Puncoviscana Formation) and as less fractionated end member magmatites of the Sierra Famatina give results according to the petrographic and geochemical data. Detrital material of the Puncoviscana Formation and metasedimentary as well as plutonic rocks of the Sierras Pampeanas are the most probable source areas for the Falda Ciénaga Formation. These sources could have been delivered the high amount of quartz grains as well as the sedimentary and metamorphic lithoclasts. Feldspar, especially plagioclase as well as less stable lithoclasts were lost by recycling and sorting processes that affected the petrographical composition. Recycling and sorting have changed the geochemical composition as shown by Hf-La- Co ratios (Fig. 10b). Petrographical, geochemical and isotope geochemical comparisons with the Upper Turbidite System (UTS) of the Northern Puna, which holds the same stratigraphic position (Fig. 2), show similarities regarding the sedimentary facies and Nd-isotope systematics (this paper; Bock et al., 2000; Zimmermann y Bahlburg, subm.) but slight differences in the petrographic composition. The UTS is marked by an observable input of volcanic lithoclasts in contrast to the Falda Ciénaga Formation. The plutonic and volcanic rocks of the so-called "Faja Eruptiva de la Puna Occidental" (after Mendez et al., 1973) as well as the probable extinct Puna- Famatinian volcanic arc are regarded as a possible source for volcaniclastic debris in the UTS. Nevertheless, the petrographic data of the Upper Turbidite System points to a collisional orogen or a interior cratonic provenance (Bahlburg, 1990) in accordance to the Falda Ciénaga Formation. The data presented for the Falda Ciénaga Formation coincides with the proposed evolution for the Ordovician basin in the Puna (Bahlburg y Zimmermann, 1999, Zimmermann, 2000; Zimmermann y Bahlburg, subm.). The differences in the petrographic record may be explained by a heterogeneous basin morphology (Zimmermann, 1999), for example the so-called "Cobres High" (Moya, 1997), rather than different tectonic entities. Additionally, in a retro-arc setting such as the Ordovician Puna basin (Bahlburg, 1990, 1998; Zimmermann, 1999, Zimmermann y Bahlburg, 1998; Bock et al., 2000) variations in petrographical, geochemical and isotope geochemical signatures are common. These are due to lithologically heterogeneous source areas, combined with variable transportation and sedimentation processes (e. g. McLennan y Taylor, 1991; Einsele, 1992; Marsaglia, 1995; Pouclet et al., 1995; Pre-Piper et al., 1995). However, this case study shows that the geochemical and isotope geochemical data obtained can be explained by petrographic results. It further suggests that the Falda Ciénaga Formation marks the extinction of the volcanic arc in the Puna and that the plate tectonic setting could be interpreted as a retro-arc or foreland basin deposit.

Palavras-chave : Middle Ordovician; Geochemistry; Nd-isotopes; Provenance analysis.

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