Provenance of the late Proterozoic to early Cambrian metaclastic sediments of the Sierra de San Luis (Eastern Sierras Pampeanas) and Cordillera Oriental, Argentina
Introduction
The geodynamic evolution of the southwestern margin of Gondwana, based on the geochemical and isotopic provenance studies of the (meta-) sediments, has been the focus of investigation for the last 10 years in the Eastern Sierras Pampeanas and the Cordillera Oriental (Sims et al., 1998, Rapela et al., 1998, Rapela et al., 2007, López de Luchi et al., 1999, López de Luchi et al., 2003, Bock et al., 2000, Brogioni, 2001, Aceñolaza et al., 2002, Cerredo and López de Luchi, 2002, Thomas et al., 2002, Thomas and Astini, 2003, Finney et al., 2003, Schwartz and Gromet, 2004, Steenken et al., 2004, Steenken et al., 2006, Zimmermann, 2005, Escayola et al., 2007, Adams et al., 2008). Several scenarios like passive or active margin settings as well as autochthony or allochthoneity were suggested for the Neoproterozoic to early Palaeozoic evolution of the metaclastic units of the Eastern Sierras Pampeanas and the Cordillera Oriental, two of the main morphotectonic units along the southwestern margin of Gondwana in Argentina (e.g. Ramos, 1988, Bahlburg, 1990, Astini et al., 1995, Pankhurst and Rapela, 1998, Rapela et al., 1998, Rapela et al., 2007, Bock et al., 2000, Lucassen et al., 2000, Zimmermann and Bahlburg, 2003, López de Luchi et al., 2003, Schwartz and Gromet, 2004, Steenken et al., 2004, Steenken et al., 2006, Prozzi and Zimmermann, 2005, Zimmermann, 2005, Schwartz et al., 2008, Adams et al., 2008, Casquet et al., 2008).
Geochemical and isotope studies (e.g. Schwartz and Gromet, 2004, Steenken et al., 2004, Zimmermann, 2005) support the hypothesis that large parts of the low to high-grade metaclastic successions of the Eastern Sierras Pampeanas (Fig. 1) constitute an extension of the Puncoviscana Formation, the very low to low-grade metaclastics of northwestern Argentina and southernmost Bolivia. Metamorphism, deformation and magmatism affecting these metaclastic units result from the early Palaeozoic Pampean orogenic cycle (Toselli and Aceñolaza, 1978, Aceñolaza and Toselli, 1981, Omarini, 1983, Aceñolaza et al., 1988, Aceñolaza et al., 1990, Rapela et al., 1998, Rapela et al., 2007, Bock et al., 2000, Steenken et al., 2004, Steenken et al., 2006, Steenken et al., 2007, Zimmermann, 2005).
The investigation of the geochemical characteristics of (meta-) clastic deposits is suitable to constrain not only the tectonic setting, but also the geological provinces from which the clastic material was eroded. Studies to solve provenance related problems of (meta-) clastic sediments can be carried out in different ways: (i) lithoclasts may be analysed statistically in thin sections (Dickinson and Suczek, 1979, Zimmermann, 1999, von Eynatten et al., 2003), (ii) the major and trace element composition of the whole rock and mineral samples may be considered (e.g. Floyd and Leveridge, 1987, McLennan et al., 1990, McLennan et al., 1993, Zimmermann, 1999), and (iii) isotopic whole rock systems like Pb–Pb or Sm–Nd and single grain studies such as U–Pb dating of zircons can also be applied.
The advantage of trace elements in contrast to major elements is the coherent behaviour of specific element groups, like the HFSE (including the REE) during weathering, diagenesis and metamorphism. The fact that those elements are insoluble at surface conditions as well as during metamorphic processes makes them far more reliable in ascertaining the provenance of the clastic material and the different tectonic settings where they are deposited (e.g. Bhatia, 1983, Taylor and McLennan, 1985, McLennan et al., 1993, Roser et al., 1996). REE, Ni, Y, Sc, Cr, Th, V, Ti, Hf and Zr are most commonly used in the discrimination between different tectonic settings and/or sources. Contributions of acidic or basic material to the sediments, as well as the degree of recycling may be identified by specific elements. Ratios between certain trace elements will reflect the different contributions to the clastic material characterising the geotectonic setting from where they were derived or in which they were deposited. However, discrimination between source rock characteristics and the plate tectonic setting is not that easy to establish. The combination of Nd and Pb isotopes with major and trace elements provides the possibility for examining the geological setting of the provenance area and to determine the different terranes, their boundaries and origins. SHRIMP dating of zircon yields characteristic inheritance patterns reflecting the detritus of different orogenic events in the provenance area of the sediments. Therefore, the SHRIMP data provide an additional constraint to discriminate between different tectonic provinces that might show similar whole rock Nd and Pb isotope ratios.
The aim of this study is to provide new insights on the Ediacaran/early Palaeozoic geodynamic evolution along a part of the southwestern margin of Gondwana. Major and trace elements, Nd and Pb isotope systematics as well as SHRIMP U/Pb zircon data are used to both characterise and to show differences and/or similarities in the provenance and the tectonic setting of the Puncoviscana Formation (Cordillera Oriental) and higher-grade metaclastic sediments from the Sierra de San Luis [southern tip of the Eastern Sierras Pampeanas (Fig. 1)]. Previously suggested tectonic models for the early Palaeozoic evolution of the Pacific margin of Gondwana will be discussed by the combination of data presented here and results already published (e.g. Rapela et al., 1998, Sims et al., 1998, López de Luchi et al., 1999, López de Luchi et al., 2003, Bock et al., 2000, Brogioni, 2001, Steenken et al., 2004, Steenken et al., 2006, Prozzi and Zimmermann, 2005, Zimmermann, 2005, Rapela et al., 2007, Adams et al., 2008).
Section snippets
Geological setting
The Eastern Sierras Pampeanas (Fig. 1) consist of uplifted basement blocks, produced by Miocene to recent flat-slab subduction in the 27–33°30′S Andean segment (Ramos et al., 2002). The eastern belt of this morphostructural unit is known as the Pampean orogen, which is characterised by late Neoproterozoic sedimentation and Cambrian deformation, magmatism and metamorphism. The western sector, the Famatinian orogen, is characterised by Upper Cambrian to Middle Ordovician marine sedimentation and
Sample material
A total of 36 samples of metasedimentary rocks from the PVF and the metamorphic units of the Sierra de San Luis were analysed for major and trace elements with ICP-MS and XRF (Table 1). Sm–Nd model ages were calculated for 22 of these samples. The Pb isotopic composition was calculated for 23 whole rock samples. The SHRIMP technique was applied on four samples (A73-05, A93-05, A7-06 and A8-06). The locations are indicated in Fig. 2, Fig. 3.
Chemical index of alteration
Major elements are useful in provenance analyses if the rocks were not strongly affected by metamorphism, diagenesis or weathering (McLennan et al., 1993). Major elements may provide some indication of the sources for the clastic erosion material (e.g. Boles and Franks, 1979, Nesbit et al., 1996, Zimmermann and Bahlburg, 2003). The deviation from the original composition of the source material by alteration processes can be inferred from the chemical index of alteration (CIA) that is calculated
Influence of weathering, sorting and alteration
The effects of variable degrees of weathering in the source area would result in a relative depletion of alkali and alkaline earth elements and in an enrichment of the Al2O3 and TiO2 concentration in the resulting terrigenous sedimentary rocks (Nesbit et al., 1980, Grandstaff et al., 1986, Harnois, 1988). The chemical index of alteration (CIA) for the sample collection varies between 53 and 85, with an average for the metapsammitic rocks of 59 (n = 25), indicating moderate feldspar alteration and
Major and trace elements implications
Most of the samples are pelites, greywackes, litharenites or arkoses (Fig. 5). The samples plot in areas comparable to the samples from Sims et al., 1998, Brogioni, 2001, López de Luchi et al., 2003 and Zimmermann (2005), except for one metapsammitic rock from the SLF (A12-06) and two samples from the PMC (A13-06 and A19-06) that are shifted due to higher SiO2/Al2O3 ratios. One metapelitic sample from the SLF yielded a high Al2O3 concentration (21%) and a low log (Fe2O3/K2O).
The PMC and the SLF
Conclusion
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A tectonic discrimination of the provenance areas based on geochemical results, between the different domains of the Sierra de San Luis and the PVF points to an active margin setting without being able to discriminate between fore-arc and back-arc setting. The PMC metapsammites are enriched in Th, pointing to a more felsic provenance. This unit shows the broadest dispersion of data with a mixed provenance of recycled, possibly passive margin material and relatively new felsic material. This can
Acknowledgements
We are grateful for the German Science Foundation (DFG) Grant Si 438/28-1;2 that funded the research project. M.D. is grateful for the financial support from the DAAD for a 6-month short fellowship for Ph.D. students in Argentina. The manuscript was significantly improved by two anonymous reviewers.
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