Provenance of the late Proterozoic to early Cambrian metaclastic sediments of the Sierra de San Luis (Eastern Sierras Pampeanas) and Cordillera Oriental, Argentina

Abstract

Provenance studies have been performed utilising major and trace elements, Nd systematics, whole rock Pb–Pb isotopes and zircon U/Pb SHRIMP data on metasedimentary rocks of the Sierra de San Luis (Nogolí Metamorphic Complex, Pringles Metamorphic Complex, Conlara Metamorphic Complex and San Luis Formation) and the Puncoviscana Formation of the Cordillera Oriental. The goal was the characterisation of the different domains in the study area and to give insights to the location of the source rocks. An active continental margin setting with typical composition of the upper continental crust is depicted for all the complexes using major and trace elements. The Pringles Metamorphic Complex shows indications for crustal recycling, pointing to a bimodal provenance. Major volcanic input has to be rejected due to Th/Sc, Y/Ni and Cr/V ratios for all units. The εNd_{(540 Ma)} data is lower for the San Luis Formation and higher for the Conlara Metamorphic Complex, as compared to the other units, in which a good consistency is given. This is similar to the T_DM ages, where the metapsammitic samples of the San Luis Formation are slightly older. The spread of data is largest for the Pringles Metamorphic Complex, again implying two different sources. The whole rock ²⁰⁷Pb/²⁰⁶Pb isotopic data lies in between the South American and African sources, excluding Laurentian provenances. The whole rock Pb–Pb data is almost indistinguishable in the different investigated domains. Only the PMC shows slightly elevated ²⁰⁸Pb/²⁰⁴Pb values. Possible source rocks for the different domains could be the Quebrada Choja in the Central Arequipa–Antofalla domain, the Southern domain of the Arequipa–Antofalla basement, the Brazilian shield or southern Africa. Zircon SHRIMP data point to a connection between the Puncoviscana Formation and the Conlara Metamorphic Complex. Two maxima around 600 Ma and around 1000 Ma have been determined. The Nogolí Metamorphic Complex and the Pringles Metamorphic Complex show one peak of detrital zircons around 550 Ma, and only a few grains are older than 700 Ma. The detrital zircon ages for the San Luis Formation show age ranges between 590 and 550 Ma. A common basin can be assumed for the Conlara Metamorphic Complex and the Puncoviscana Formation, but the available data support different sources for the rest of the Complexes of the Sierra de San Luis. These share the diminished importance or the lack of the Grenvillian detrital peak, a common feature for the late Cambrian–early Ordovician basins of the Eastern Sierras Pampeanas, in contrast to the Sierras de Córdoba, the PVF and the Conlara Metamorphic Complex.

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).