BOLETÍN DE LA SOCIEDAD GEOLÓGICA MEXICANA Vol 65, Núm.2, 2013, P. 373-395 http://dx.doi.org/10.18268/BSGM2013v65n2a18

Provenance and tectonic setting of Neoproterozoic to Early Cambrian metasedimentary rocks from the Cordillera Oriental and Eastern Sierras Pampeanas, NW Argentina

Aránzazu Piñán-Llamas^1,*, José C. Escamilla-Casas²

¹ Indiana University-Purdue University Fort Wayne, 2101 E. Coliseum Blvd. Fort Wayne, IN 46805. Phone: (260) 481-6253.
²Universidad Autónoma del Estado de Hidalgo. Área Académica de Ciencias de la Tierra y Materiales. Ciudad Universitaria Carretera Pachuca – Tulancingo Km. 4.5 C.P. 42184. Col. Carboneras, Mineral de la Reforma, Hidalgo. Phone: 0177171 72000.

* This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

Major, trace, and rare-earth element data for the Neoproterozoic to early Cambrian Puncoviscana Formation and related higher-grade metasedimentary rocks in Eastern Sierras Pampeanas, NW Argentina, were analyzed to evaluate their provenance, weathering, and tectonic setting during the upper Neoproterozoic-lower Paleozoic. The studied rocks are metapelites and metapsammites that were mainly affected by the lower Cambrian Pampean orogeny. Geochemical analyses indicate that the Puncoviscana Fm. and related rocks are characterized by moderate Chemical Index of Alteration (CIA) values. REE element distributions, Eu/Eu* values, and high Zr/Sc and Th/Sc ratios indicate that the samples were likely derived from predominantly upper-crust felsic sources. Tectonic discrimination diagrams based on immobile trace elements (e.g. Th-La-Sc, Sc-Th-Zr/10), multi-element patterns, REE characteristics, and diagnostic trace element ratios (i.e. Th/Sc, Zr/Hf, La/Th, La/Sc) imply that the tectonic setting of the source area was a continental island arc and/ or an active continental margin. Geochemical signatures of low-grade metapsammitic samples (high Hf and Zr content, and high Zr/ Sc ratios) indicate crustal recycling.

The geometry and sequence of structures and microstructures preserved in all samples are pervasive and suggest a common and uniform deformational history during the Pampean orogeny. Geochemical signatures and structures of the studied samples are consistent with protolithic sediments of the Puncoviscana Fm. and related rocks initially deposited in an arc-related basin that was subsequently deformed during the Pampean orogeny.

Keywords: Geochemistry, metasedimentary rocks, provenance, Pampean orogeny, Puncoviscana Formation.

Resumen

En este estudio, se caracteriza la proveniencia, grado de alteración, y el ambiente tectónico de rocas metasedimentarias de edad Neoproterozoica a Cámbrico Temprano del NW de Argentina, (Formación Puncoviscana y rocas equivalentes de mayor grado metamórfico aflorantes en las Sierras Pampeanas Orientales) durante el Neoproterozoico superior al Paleozoico inferior, en base al análisis de datos de elementos mayores, trazas, y tierras raras. Las rocas analizadas son metapelitas y metapsamitas que fueron afectadas principalmente por la orogenia Pampeana (Cámbrico Inferior). Los resultados geoquímicos indican que estas rocas se caracterizan por presentar valores moderados en el Índice Químico de Alteración (CIA). Las distribución de tierras raras, y los valores de Eu/Eu*, Zr/Sc, y Th/Sc indican que las muestras se derivaron muy probablemente y de forma predominante, de rocas fuente félsicas de la corteza superior. Diagramas de discriminación tectónica basados en elementos traza inmóviles (e.g. Th-La-Sc, Sc-Th-Zr/10), patrones multielementos, características de las tierras raras y relaciones de elementos traza útiles en la discriminación del ambiente tectónico (i.e. Th/Sc, Zr/Hf, La/Th, La/Sc) indican que el ambiente tectónico del área fuente pudo ser un arco de islas continental y/o un margen activo continental. La signatura geoquímica de las metapsamitas de bajo grado (que incluye altos contenidos de Hf y Zr y valores altos de Zr/Sc), indica la presencia de una componente sedimentaria de reciclado cortical en estas rocas.

La geometría y secuencia de estructuras y microestructuras preservadas en la totalidad de las muestras estudiadas, sugieren que estas rocas comparten una historia de deformación común y uniforme durante la orogenia Pampeana. Tanto las signaturas geoquímicas como las estructuras de las rocas estudiadas indican que los protolitos sedimentarios de la Formacion Puncoviscana y rocas asociadas se depositaron en una cuenca relacionada con un arco volcánico que, subsecuentemente, fue deformada durante la orogenia Pampeana.

Palabras Clave: Geoquímica, rocas metasedimentarias, proveniencia, orogenia Pampeana, Formación Puncoviscana.

1. Introduction

The Ediacaran-Early Cambrian Puncoviscana Formation (Turner, 1960; Figure 1a), a thick turbiditic sequence exposed in the Puna and Cordillera Oriental and a series of metaclastic successions in Eastern Sierras Pampeanas, considered as its higher-grade equivalents (Willner and Miller, 1985; Rapela et al., 1998; Schwartz and Gromet, 2004), were initially deposited on the western margin of Gondwana during the Neoproterozoic to lower Cambrian and deformed and metamorphosed during the lower Cambrian Pampean orogeny (Rapela et al., 2002; Siegesmund et al., 2010; Escayola et al., 2011). The study of the geological evolution of the Puncoviscana Fm. and related rocks is critical to understand the Pampean orogeny and the early geodynamic evolution of the western Gondwana margin. We investigate the geochemical signature of these rocks in order to constrain their provenance and depositional tectonic setting, and to discuss their evolution during the Neoproterozoic-early Paleozoic. Previous provenance studies have been carried out in the region resulting in different interpretations and tectonic models (Do Campo and Ribeiro-Guevara, 2002, 2005; Zimmermann, 2005; Rapela et al., 2007; Collo et al., 2009; Drobe et al., 2011; Hauser et al., 2011). Thus, the depositional setting and tectonic evolution of the Puncoviscana metasedimentary rocks and related rocks during the lower Paleozoic is still under debate.

Geochemistry has been traditionally used to determine the provenance of clastic and metaclastic rocks and to constrain the tectonic setting in which they were deposited (e.g. McLennan et al., 1983, 1990, 1993, 1995; Taylor and McLennan, 1985, 1995; Bhatia and Crook, 1986; Roser and Korsch, 1985, 1988; Gu, 1996a, 1996b). Petrographic analyses based on framework modes are usually less useful than geochemical analyses in the characterization of provenance and tectonic setting of metamorphic rocks if the samples contain considerable amounts of pseudo matrix. The transformation of labile fragments originally present in the rock (i.e. feldspar or rock fragments) into clay minerals (pseudo matrix) can bias framework modes toward the quartz apices of classification diagrams (Do Campo and Ribeiro-Guevara, 2005). Furthermore, the geochemistry of metasediments is particularly valuable in the study of fine-grained rocks that are difficult to characterize through petrographic studies.

Major and trace elements can constraint the effects of weathering and sedimentary sorting, while rare earth and certain trace elements (e.g. Th, Zr, Hf, Nb, and Sc) are considered particularly reliable in the discrimination of provenance and tectonic setting. This reliability is based on their low mobility during sedimentary processes, diagenesis, and metamorphism, thus reflecting the signature of the parent materials, and the composition and tectonic environment of the source (Taylor and McLennan, 1985; McLennan, 1989; McLennan et al., 1993; Roser et al., 1996; Condie, 1991, 1993). Provenance studies of medium and high metamorphic grade metasediments are relatively common in the literature (i.e. Li et al., 2005; Meinhold et al., 2007; Drobe et al., 2011; Verdecchia and Baldo, 2010).

In this paper, we present new major and trace element whole-rock geochemical analyses from the Puncoviscana Fm. and related metasedimentary rocks from Cumbres Calchaquies, Sierras de Quilmes, Ancasti, and Córdoba (Figure 1). We combine our results with previously published data (Willner et al., 1985; Do Campo and Ribeiro- Guevara, 2005; Zimmermann, 2005) to evaluate their source and tectonic depositional setting. The characterization of the provenance and tectonic setting of these units is important for understanding the geodynamic evolution of the western Gondwana margin during the early Paleozoic. We also present new structural and microstructural data to support the correlation of our samples along the orogen during the Pampean cycle.

2. Tectonic setting

The Eastern Sierras Pampeanas are isolated basement blocks exhumed as a result of Tertiary to recent high-angle reverse faulting (Jordan and Allmendinger, 1986) associated with the flat slab subduction of the Nazca Plate (Ramos et al., 2002). These blocks include large outcrops of metaclastic rocks that extend between 26° and 33° S (Figure 1), and were mainly affected by lower Cambrian deformation, metamorphism, and magmatism (the Pampean orogeny; e.g. Rapela et al., 1998, 2002; Siegesmund et al., 2010; Escayola et al., 2011). Based on isotope studies (e.g. Schwartz and Gromet, 2004; Steenken et al., 2004; Drobe et al., 2009 and references therein) these rocks have been correlated with the Ediacaran to Early Cambrian Puncoviscana Formation (Turner, 1960) that crops out along a NNE-SSW linear belt that extends from the Bolivian border (22°S) to the vicinity of Salta (~26°S; Figure 1a). Protoliths of the very low- to low-grade Puncoviscana Fm. and the medium- to high-grade metaclastic rocks of the Eastern Sierras Pampeanas have been interpreted by several authors as deposited in the same basin (Puncoviscana Basin) that extended from Bolivia to central Argentina (~33°S) roughly between 64°W and 68°W (Jezek, 1990; Rapela et al., 1990; Figure 1 of Zimmermann, 2005). However, the unexposed contact between low-grade and medium- to high-grade units, the discontinuity between outcrops, and the lack of a reliable stratigraphic column complicate the interpretation of the paleotectonism of the basin. Mon and Hongn (1991) subdivided the Pampean basement between 22°S to 28°S into several orogenic belts with a different tectonometamorphic evolution and suggest that higher-grade rocks are older than the Puncoviscana Fm.

Figure 1. (a) Simplified geological map of lower Paleozoic rocks in NW Argentina, modified from Willner and Miller (1985), and Simpson et al. (2003). Inset shows the location of the research area in Argentina (boxed). Dashed lines indicate approximate boundaries between peri-Gondwanan terranes. (b) Puncoviscana Fm. and higher-grade related rocks with location of samples and structural data plotted on lower hemisphere stereographic projections. Number of structural data (stereoplot number here between parentheses): Salta (1) S₀ n=52, S₁ n=13; F₁ axes n=8; Quilmes (2) S₀ n=12, S₁ n=17, F₁ axes n=29; Hualinchay (3) S₀ n=71, S₁ n=22, F₁ axes n=12; Sierra de San Javier (4) S₀ n=31; Guasayan (5) S₀ n=25, F₁ axes n=12; Ancasti (6) S₀ n=19, F₁ axes n=17; Tuclame (7) S₀ n=61, S₁ n=10, F₁ axes n=21.
Paleozoic mountain ranges and localities mentioned in the text: Tu. Tuclame; An. Ancasti; CC. Cumbres Calchaquies; C. Córdoba; SL. San Luis; N. Sierra del Norte; Ta. Sta. Rosa de Tástil; Ca. Cañaní; SQ Sierra de Quilmes.

The Puncoviscana Fm. includes successions of metamorphosed greywackes, siltstones, and mudstones, and locally interbedded metaconglomerates and discontinuous marble lenses (Jezek, 1990; Omarini et al., 1999; Porto et al., 1990); it also preserves sedimentary structures, and fossils of Ediacaran to Early Cambrian age (Aceñolaza and Durand, 1986; Durand, 1996). Sedimentological and facies analyses indicate a unidirectional pattern of sediment transport from east to west/northwest (present-day coordinates) from a common source area (Jezek et al., 1985; Jezek, 1990). Geochemical signatures of alkaline lava flows, sills, and basic dikes in the turbiditic section indicate oceanic-island to within-plate settings (Coira and Barber, 1987; Coira et al., 1990) consistent with a rift environment. Based on geochemical signatures obtained from basalts of uncertain stratigraphic position that are interbedded in metaclastic rocks of the Puncoviscana Fm., Omarini et al. (1999) suggest a transition from rift to a back-arc environment.

Idiomorphic detrital zircons from volcaniclastic beds in mid and upper stratigraphic levels of the Puncoviscana Fm. yielded an age range of 530-560 Ma (U-Pb, Lork et al., 1990), considered as a maximum sedimentation age. Adams et al., 2008a, 2008b reported maximum sedimentation ages of 636 ± 7-551 ± 5 Ma. More recently, Escayola et al. (2011) obtained ages of 536.4 ± 5.3 Ma and 537 ± 0.9 Ma from tuffaceous beds interlayered with metaclastic rocks of the Puncoviscana Fm. Detrital zircons from Puncoviscana Fm. rocks in the Cordillera Oriental yielded late Mesoproterozoic to early Neoproterozoic (850–1150 Ma), and late Neoproterozoic-Early Cambrian (650-520 Ma) U-Pb age maxima (Adams et al., 2011). A similar age distribution of detrital zircon population was obtained by Escayola et al. (2011) and Hauser et al. (2011), but these authors describe much stronger late Ediacaran peaks suggesting that deposition of the Puncoviscana Fm. likely started in the latest Ediacaran but is mainly Early Cambrian, and was coeval with a nearby magmatic arc (Escayola et al., 2011). The depositional age of the Puncoviscana Fm. is additionally constrained by the unconformably overlying shallow marine platform metasedimentary rocks of the Meson Group that contain poorly preserved middle to Late Cambrian trilobites (Sánchez and Salfity, 1999; Aceñolaza et al., 2002; Aceñolaza and Aceñolaza 2005). Based on U-Pb geochronology of detrital zircons, and the pre-depositional Santa Rosa de Tastil batholith, Augustsson et al. (2011) suggest a mainly middle Cambrian depositional age for the Meson Group. Adams et al. (2011) report a youngest zircon component at 500 Ma, suggesting an upper age limit of Late Cambrian for the Meson Group.

Low greenschist-facies metaclastic rocks in the vicinity of Tucumán (~26º to 27°S), upper greeschist to lower amphibolite-facies metaclastic rocks of the Ancasti Formation (~28° to 30°S Figure 1; Willner, 1983), and greenschist to upper amphibolite- and granulite-facies metaclastic rocks in Sierras de Cordoba (~31°S to 33°S, Figure 1), all included in the morphostructural province of Sierras Pampeanas (Ramos, 1999), have been considered as deep structural levels of the Puncoviscana Fm. (Willner and Miller, 1985; Toselli, 1990; Willner, 1990; Rapela et al., 1998; Steenken et al., 2004; Siegesmund et al., 2010; Steenken et al., 2011). Late Neoproterozoic (570–680 Ma) and Mesoproterozoic (960–1020 Ma) detrital zircon age maxima have been obtained from banded schists of the Ancasti Formation (U-Pb; Rapela et al., 2007). Similarly, Neoproterozoic (600–700 Ma) and Mesoproterozoic (900–1000 Ma) detrital zircon age maxima characterize metasedimentary rocks from the Sierras de Córdoba (Rapela et al., 1998; Schwartz and Gromet, 2004; Escayola et al., 2007; Drobe et al., 2009).

Sr and Nd-isotope data from Puncoviscana Fm. and higher-grade related rocks are relatively homogeneous, suggesting that most NW Argentinian basement rocks shared similar sources (Becchio et al., 1999; Lucassen et al., 2000). Nd-model (TDM) ages from the low-grade Puncoviscana Formation range from 1600 to 1800 Ma (Bock et al., 2000; Drobe et al., 2009), and from 1700 to 1800 Ma in Sierras de Córdoba metasedimentary rocks (Rapela et al., 1998; Steenken et al., 2011; Drobe et al., 2011). Similar TDM model ages (average TDM age ~ 1.76 ± 0.4 Ga) were reported from Sierra de Quilmes metasedimentary rocks (Figure 1; Lucassen et al., 2000).

In Sierras de Córdoba, a main migmatization event occurred at ~ 530 Ma (U/Pb SHRIMP zircon ages; Rapela et al., 1998, 2002). Although its significance is still not very well constrained and further geochronological work may be necessary, an earlier 543 ± 3.6 Ma (U/Pb zircon age) metamorphic event has been proposed by Siegesmund et al. (2010) for high-grade non-migmatitic gneisses of Sierras de Córdoba (Sierra Grande). Similarly, a first Pampean metamorphic event has been dated as 540–570 Ma (Rb/Sr whole rock data; Bachmann and Grauert, 1986; Bachmann et al., 1987) in banded schists of the Ancasti Formation.

The 523.7 ± 0.8 Ma calc-alkaline Cañaní pluton (U-Pb TIMS zircon age, Escayola et al., 2011), and the 536 ± 7 Ma or 534 ± 7 Ma Santa Rosa de Tástil calc-alkaline granite (Figure 1a; U-Pb zircon ages of Bachmann et al., 1987, and U-Pb Laser ICPMS zircon ages of Hauser et al., 2011, respectively) were emplaced in the chevron-folded Puncoviscana Fm. and are interpreted as part of a ‘Pampean’ magmatic arc (Omarini et al., 1999). In Sierras de Córdoba, the 530 ± 3 Ma calc-alkaline Sierra del Norte batolith (U-Pb zircon ages, Rapela et al., 1998) is considered to be the southward extension of the Pampean magmatic arc (Lira et al., 1997). Based on U-Pb zircon ages, Schwartz et al. (2008) suggested that the activity of this arc extended from ca. 555 to 525 Ma. The 522 ± 8 Ma (U-Pb monazite ages, Rapela et al., 1998) and 515–520 Ma (U-Pb monazite ages, Gromet and Simpson, 1999, 2000) peraluminous granites and migmatites in Sierras de Córdoba contain xenoliths of folded metasediments, suggesting that shortening of these rocks occurred prior to migmatization (Piñán-Llamas and Simpson, 2006). K/Ar and Ar/Ar muscovite ages starting at 502 Ma (Krol and Simpson, 1999; Steenken et al., 2010) and 505.7 ± 7.3 Ma ages (PbSL titanite age; Siegesmund et al., 2010) likely indicate the end of the Pampean metamorphism in the Córdoba region (Siegesmund et al., 2010; Steenken et al., 2011).

A number of collisional and non-collisional models have been proposed to explain the Pampean orogeny. In non-collisional models, the Puncoviscana turbiditic sediments were initially passive margin deposits (e.g. Durand, 1996) that likely evolved into an accretionary prism due to eastward subduction in the lower Cambrian (Rapela et al., 1998; Omarini et al., 1999; Piñán-Llamas and Simpson, 2006). During this process, the Puncoviscana Fm. and higher-grade metasediments of the Sierras de Córdoba were intruded by the calc-alkaline ‘Pampean’ magmatic arc (or ‘Tilcaric arc’, Omarini et al., 1999) and the Sierra del Norte complex (Figure 1a; Lira et al., 1997; Schwartz et al., 2008), respectively. The Pampean cycle ended with an episode of peraluminous magmatism, related to subduction of a mid-ocean ridge under the Puncoviscana accretionary prism as the arc magmatism was coming to a close (Fantini et al., 1998; Simpson et al., 2003; Schwartz et al., 2008). Schwartz et al. (2008) suggest that east-dipping subduction at ca. 555 Ma was likely followed by the ridge-trench collision at 525 Ma. In alternative models based on structural and geochronological data, the Puncoviscana sediments are foreland (Zimmermann, 2005; Ramos, 2008), fore-arc (e.g. Astini et al., 1996; Pankhurst et al., 1997), or back-arc basin deposits (Omarini et al., 1999; Escayola et al., 2007; Collo et al., 2009) tectonized in the mid-Cambrian Pampean orogeny.

In collisional models, the Puncoviscana Fm. and higher-grade related rocks were part of independent terranes (e.g. the 'Pampia terrane' of Ramos et al., 1986 and Ramos, 1988) that collided with the western Gondwana margin during the Pampean orogeny. In related models, the Pampean orogeny has been associated with the accretion of the allochtonous or para-autochtonous Pampia block (Rapela et al., 1998; Ramos, 2008; Collo et al., 2009) or the Western Sierras Pampeanas (Rapela et al., 2007; Siegesmund et al., 2009 and references therein) to the western margin of Gondwana. Consistent with collisional models is the presence of a discontinuous string of thin bodies of MORB-basalts in the Sierras de Córdoba that has been interpreted as a disrupted ophiolitic sequence (Escayola et al., 2007; Ramos et al., 2000; Rapela et al., 1998; Steenken et al., 2011).

Isotopic and geochronological affinities between the Puncoviscana Fm. and related metaclastic rocks to the south, and Gondwanan sources in South Africa, led some authors to propose an alternative model in which the Puncoviscana sediments were deposited in a trough marginal to the Kalahari craton and later emplaced against the western margin of the Rio de la Plata craton through a lateral strike-slip displacement (Rapela et al., 2007; Casquet et al., 2008; Drobe et al., 2009, 2011; Siegesmund et al., 2010). Based on Sm-Nd isotopic data and zircon provenance patterns, Collo et al. (2009) proposed that the Puncoviscana Fm. and equivalent protoliths within Sierras Pampeanas were deposited in a back-arc basin to the west of a Brasiliano magmatic arc that developed on a Mesoproterozoic basement extension of the Arequipa-Antofalla massif (Collo et al., 2009).

After the Pampean orogeny, the Pacific margin of Gondwana turned into a passive margin. Magmatism and tectonism were resumed in the Early Ordovician during the Famatinian Orogeny (Aceñolaza and Toselli, 1973). Ordovician magmatism, dated as ~470 Ma (Büttner et al., 2005; Dahlquist et al., 2012) locally affected the Puncoviscana Fm., and high-grade equivalents, resetting isotopic systems (e.g. Sierra de Ancasti, Sierra de Quilmes). According to several models, the Arequipa, Cuyania/ Precordillera, and Chilenia terranes were likely accreted to the Pacific margin of Gondwana (Ramos et al., 1986; Ramos, 1988; Bahlburg and Hervé, 1997; Thomas and Astini, 2003; Collo et al., 2009; Ramos, 2008, 2009; Steenken et al., 2011; Drobe et al., 2011) during the rest of the Paleozoic. Separation of the Pampean Orogen into isolated basement blocks with intervening pull-apart basins began in the late Carboniferous and continued through the Cretaceous. Exhumation of basement blocks on high-angle, Tertiary to Recent reverse faults has been related to the Andean orogeny (Jordan and Allmendinger, 1986).

3. Methodology

A total of 27 samples from the Puncoviscana Fm. (Cordillera Oriental) and higher-grade metaclastic sediments from Cumbres Calchaquies, Sierras de Ancasti, Quilmes, and Córdoba were collected for petrographic and geochemical analyses (approximate locations of the samples in Figure 1b; coordinates and description of the samples in Appendix I). The analyzed samples include metapelites, massive psammites, and banded metapsammites that are the main lithologies present in the sampled areas. All samples were crushed. Fifty-five major, trace, and rare earth elements were determined (Table 1) by Activation Laboratories Ltd. (Ontario, Canada) according to their Code 4Lithoresearch and Code 4B1 packages. These packages combine lithium metaborate/tetraborate fusion ICP analysis for major elements with ICP-MS analysis for trace elements, using international rock standards (Activation Laboratories Ltd. 2006). According to replicates and standard results, the analytical precision was typically better than ± 2.3% for most major elements and better than ± 10% for trace elements. In the discussion we also include previously published compositions of metaclastic rocks from Sierra de Ancasti, and the Puncoviscana Fm. (data from Do Campo and Ribeiro-Guevara, 2005; Willner et al., 1985; and Zimmermann, 2005). In the following sections we summarize lithological, structural, petrographic, and microstructural observations from the analyzed samples and their outcrops.

Table 1. Whole-rock major, minor, and rare-earth element abundances for Puncoviscana Fm. and higher-grade related rocks. Upper continental crust values (UCC) from McLennan, 2001.

Table 1. (Continuation).

4. Sample Description

4.1. Puncoviscana Formation-Anchizone grade

The Puncoviscana Fm. consists of anchizone-grade centimeter- to meter-thick massive metapsammitic beds alternating with centimeter-thick green and red metapelitic layers, and centimeter- to meter-thick layers of banded metapsammites. Massive metapsammitic layers containing graded- and cross-bedding and parallel lamination gradually transition into metapelitic layers. Three massive metapsammites (Punco 1, Punco 2, 56A), a banded metapsammite (67A), and four metapelitic samples (55, 54A, 67B, PV2) were collected for analysis (locations in Appendix I). In thin section, massive metapsammites and quartz-rich bands in banded metapsammitic samples are characterized by subangular, poorly sorted clasts of quartz, feldspar (mostly albite and minor K-feldspar), and rock fragments (quartzite, siltstone, shales, phyllites), clays, small detrital white micas, and few chlorite grains. Zircon, rutile, magnetite, and tourmaline are also present. In metapelitic rocks, elongated quartz grains with curved grain boundaries and pressure shadows are present. A compaction-related, bedding-parallel cleavage (Sc) is defined by aligned detrital micas, pressure-solution seams, and micabeards in banded and massive metapsammites, and metapelitic samples. Although Tertiary (Andean) deformation locally overprints older tectonic structures, predominant Pampean structures consist of N-S metric to decimetric chevron folds with mostly subhorizontal axes (Figure 1b) and subvertical axial planes that fold bedding and Sc. An S₁ fanning cleavage associated with chevron folds intersects bedding and Sc, and is observed mostly in metapelitic rocks (Piñán-Llamas and Simpson, 2006, 2009).

4.2. Metaclastic rocks from Cumbres Calchaquies (26°S - 27°S)- Low-Greenschist facies

Low greenschist-facies metaclastic sequences in northern Cumbres Calchaquies (Fm Medina, Gonzalez et al., 2000) include centimeter- to meter-thick levels of banded metapsammites, centimeter- to meter-thick massive metapsammitic beds, and centimeter-thick metapelitic beds. Cross- and graded-bedding, parallel lamination, slumps, and flame structures are preserved in metapsammites. Seven banded metapsammitic samples (63B, 63, 91, Anfama, 84B2, 94, 77) and one metapelitic sample (84B1) were analyzed for geochemistry (Figure 1b; Appendix I). Similarly to anchizone-grade samples, quartz-rich bands in banded metapsammitic samples are petrographically characterized by subangular quartz, feldspar (mostly albite), and rock fragments (quartzite, siltstone, shales). Local matrix recrystallization is observed. In both massive and banded metapsammitic samples, fine-grained detrital micas are preserved, but metamorphic white mica (illite; Toselli and Rossi de Toselli, 1983a, 1983b) and chlorite grains are more abundant than in lower-grade samples. Zircon, apatite, rutile, ilmenite, and tourmaline are common accessory minerals in metapsammitic samples. In thin section, a bedding-parallel cleavage (Sc) is defined in metapelitic and metapsammitic samples by preferentially oriented detrital and metamorphic white micas and chlorite, elongated quartz grains with partially dissolved borders, and pressure-solution seams. A tectonic cleavage (S₁) associated with NNE-SSW metric to decimetric chevron folds is present in metapelitic and metapsammitic rocks. These folds, are characterized by subhorizontal axes (Figure 1b) and subvertical axial planes. In thin section, S₁ is defined by asymmetric quartz grains with dissolved borders and pressure shadows, pressure-solution seams, preferentially aligned metamorphic white micas, and chlorite grains. At outcrop scale, S₁ is observed as a fanning cleavage overprinting bedding and Sc (Piñán- Llamas and Simpson, 2006, 2009).

4.3. NE Sierra de Quilmes

In northeastern Sierra de Quilmes, the Tolombon Complex (Toselli et al., 1978) includes centimeter- to meter-thick banded metapsammitic levels, and centimeter-scale metapelitic layers. Relict bedding surfaces and cross bedding are preserved. Banded metapsammites are defined by 2-10 mm-thick quartz-rich domains alternating with 0.25-2.5 mm-thick biotite-rich domains. This compositional banding is subparallel to relict bedding. A quartz-rich banded metapsammitic sample (63AC) was collected for geochemical analysis (Figure 1b). In thin section, banded metapsammites are characterized by quartz-rich domains of recrystallized quartz with few interspersed plagioclase and biotite grains; biotite-rich bands are predominantly formed by large corroded and kinked biotite grains with quartz and opaque inclusions. Zircon, apatite, ilmenite, and tourmaline are accessory minerals in these rocks. Relict bedding and compositional banding are folded by NNE-SSW trending metric to decametric-scale chevron folds (Figure 1b) and cm-scale parasitic folds (F₁) with subvertical axial planes and subhorizontal axis. Cleavage associated with these folds (S₁) crosscuts the relict bedding and compositional banding. S₁ is defined by preferentially aligned large idiomorphic metamorphic biotite grains and, locally, by small white micas.

4.4. Ancasti Formation

The Ancasti Formation (Figure 1) includes metric to cm-thick metapelitic levels with interlayered psammitic lenses, massive metapsammitic levels, and banded metapsammites (Willner and Miller, 1982; Willner, 1983). The compositional banding is subparallel to bedding relicts. Two samples of banded metapsammites (9306-2, 9307-2) and two samples of metapelites (9306-1, 9307-1) were geochemically analyzed (location and description of the samples in Appendix I). Banded metapsammites are characterized by alternating thin biotite-rich and thicker granoblastic quartz-rich layers that include small interspersed biotite grains and plagioclase (mostly albite). NNW to N-S trending upright cm-scale F₁ folds with an associated axial plane cleavage (S₁) affect bedding relicts and compositional banding. Preferentially oriented idiomorphic biotite grains define S₁.

4.5. Tuclame Formation

Centimeter- to meter-thick layers of metapelites and meter- to several meters-thick banded metapsammites occur along the westernmost edge of the Sierras de Córdoba (the Tuclame Formation, Figure 1). Banded metapsammites are characterized by a monotonous compositional banding, which is subparallel to bedding relicts, defined by cm-thick quartz-rich bands alternating with 2–5-mm thick biotite-rich layers. A very similar compositional banding occurs in lower-grade rocks of the Puncoviscana Fm., where primary structures are well preserved. Three samples of banded psammites (9846-2, 9677-2, 9847) and three metapelitic samples (9846-1, 9677-1, 9848-1) were geochemically analyzed (Figure 1b). In thin section, banded schists are characterized by granoblastic quartz-rich bands, that include sub-equant to elongate quartz grains, interspersed albite, and tabular biotite grains; occasionally, detrital quartz and feldspar grains are preserved. These quartz-rich bands alternate with biotite-rich bands defined by deformed biotite grains with quartz, feldspar, and muscovite inclusions. Centimeter- to decimeter-scale F₁ folds with N-S to NNW-SSE trending axes (Figure 1b) and subvertical axial planes affect the compositional banding and bedding relicts. Euhedral metamorphic biotites associated with F₁ folds define an axial planar disjunctive cleavage (S₁) that crosscuts the compositional banding.

5. Results

Geochemistry can be used to constrain the provenance and tectonic setting of sedimentary and metasedimentary rocks if their original whole-rock composition was not significantly altered during diagenesis, weathering, and/or metamorphism, (McLennan et al., 1993). In metamorphic rocks, immobile trace elements and REE are particularly reliable in the study of their provenance and depositional setting (McLennan, 1989; McLennan et al., 1990; 1993; Girty et al., 1994).

5.1. Weathering and sorting

The degree of weathering can be quantitatively measured with the Chemical Index of Alteration (Nesbitt and Young, 1982), defined as Al₂O₃/[Al₂O₃ + CaO* + Na₂O + K₂O] × 100 (molar contents, with CaO* being CaO content in the silicate fraction of the sample). The CIA measures the extent to which feldspars have been transformed into aluminous clays depleted in Ca, Na, and K relative to the unweathered source. On a scale of 40 - 100, unweathered igneous rocks have CIA values of 45 - 55, while values for moderately weathered shales range from 70 to 75 CIA. Intense weathering of feldspars in a rock results in high concentrations of residual clays (kaolinite and/or gibbsite), producing CIA values close to 100. CIA values for our samples (Table 1) vary between ~52 and 72, with an average value of 57 (n = 20) for metapsammites, and a mean of 67 for metapelitic samples (n = 11), thus showing a moderate degree of weathering. High-silica concentration in metapsammitic samples could mask the alteration, resulting in low CIA values (Nesbitt and Young, 1982).

Our data and previously published geochemical data for metapelitic and metapsammitic samples from the Puncoviscana and Ancasti formations (Zimmermann, 2005; Willner et al., 1985; Do Campo and Ribeiro-Guevara, 2005) are plotted on a ternary diagram using molecular proportions of Al₂O₃– (CaO* + Na₂O)–K₂O (A–CN–K diagram; Figure 2a and 2b) to evaluate deviation of the composition of the metasedimentary rocks from their original source (Nesbitt and Young, 1984; 1989). In this plot, an ideal trend almost parallel to the Al₂O₃–(CaO* + Na₂O) axis from a fresh unweathered homogeneous source indicates no significant change in the metasedimentary rocks (CaO* + Na₂O) and K₂O ratio with respect to the source. Our samples define a general trend toward illite-muscovite compositions (Figure 2a). K-feldspar is generally scarce in Puncoviscana Fm. metaclastic rocks (Do Campo and Ribeiro-Guevara, 2005; Zimmermann, 2005), and the composition of plagioclase is mostly albitic (Do Campo and Nieto, 2003). Therefore, the relatively high K content in the samples may be related to the transformation of aluminous clay minerals to illite (Fedo et al., 1995) or to secondary loss of Ca with respect to the parental material in the silicate fraction. Additionally, in an A–CN–K triangular plot, the intersection of the weathering trend with the feldspar join can be used to constrain initial compositions of source rocks (Fedo et al., 1995). The intersection of the weathering trend defined by our samples with the feldspar join suggests a source rock with a plagioclase/K-feldspar ratio of approximately 4:2 (Figure 2a). This indicates that, according to the present A–CN–K values, our sample set may have been derived from weathering of granodioritic and/or granitic sources. This interpretation has to be considered with caution, since some mobilization of alkalis may have occurred.

The index of compositional variability (ICV = [Fe₂O₃ + K₂O + Na₂O + CaO + MgO + TiO₂]/Al₂O₃) measures the maturity of a sample based on the abundance of alumina relative to other major cations (Cox et al., 1995). Non-clay minerals have higher ICV values than clay minerals and therefore, ICV values will decrease with increasing degree of weathering and/or maturity in the sample. Our samples display an average ICV = 1.03 (range = 0.87 - 1.24), suggesting moderate weathering and/ or maturity of the source.

In Table 3, sensitive discriminators of tectonic setting such as Th/Sc, Zr/Hf, La/Th, La/Sc, and Th/U ratios for the Puncoviscana Fm. and related rocks are compared to those for sandstones and greywackes from various tectonic settings (average values from Bhatia and Crook, 1986; McLennan et al., 1990; Floyd and Leveridge, 1987; Mazur et al., 2010). Average Th/Sc ratios in anchizone and greenschist-facies metapsammites are similar to those of trailing edge (passive margin), and dissected arc greywackes (McLennan et al., 1990; Bhatia and Crook, 1986). Zr/Hf values in our samples are similar to those of “continental island arc”, and “dissected arc” greywackes. La/Th, La/Sc, and Th/U ratios in our samples are comparable with those of “continental arc” and “dissected arc” sandstones (Table 3).

(La/Yb)n, (La/Sm)n, (Gd/Yb)n, and Eu/Eu* ratios, and ΣREE are sensitive discriminators of the tectonic setting (McLennan et al., 1990). In Table 2 our samples show values for (La/Yb)n, (La/Sm)n, and Eu/Eu* similar to those of sediments from “continental arc basins” and “passive margins” (Table 2; McLennan et al., 1990; McLennan, 2001). Eu anomalies in our samples are also typical for average UCC, recycled sedimentary rocks, and rocks from “differentiated continental volcanic arcs” dominated by felsic rocks that are exposed as the arc becomes dissected (McLennan et al., 1993).

Figure 7. (a) La-Th-Sc, (b) Th-Co-Zr/10, and (c) Th-Sc-Zr/10 tectonic discrimination diagrams for the Puncoviscana Fm. and higher-grade related metapsammitic rocks. Dotted lines represent fields for sandstones from various tectonic settings (after Bhatia and Crook, 1986). A, oceanic island arc; B, continental island arc; C, active continental margin; D, passive margin. Symbols as in Figure 3.

Figure 8. La/Sc vs. Ti/Zr tectonic discrimination diagram of the Puncoviscana Fm. and higher-grade related metapsammitic rocks. Dotted lines represent the fields for sandstones from various tectonic settings (after Bhatia and Crook, 1986). A, oceanic island arc; B, continental island arc; C, active continental margin; D, passive margin. Symbols as in Figure 3.

7. Conclusions

1- Major and trace element geochemistry (i.e. Th/Sc, Rb/Sr, Cr/V, Cr/Th, Zr/Sc, Th/Sc, Eu/Eu*) indicates that the Puncoviscana Fm. and higher-grade related metaclastic rocks are characterized by moderate degrees of weathering (low CIA and high ICV values) and that metaclastic materials were likely derived from predominantly upper-crust felsic sources.

2- The overall geochemical fingerprints for all analyzed samples, including concentrations of La, Th, Sc, Zr, La/Sc, Nb/Nb*, and Eu/Eu* ratios, or UCC-normalized patterns, suggest that the Puncoviscana and related samples were deposited on a “continental island-arc” and (or) an “active continental margin” setting.

3- A heavy mineral/recycled signature, characterized by high Zr/Sc ratios and high concentrations of HREE, Hf, Ti, and Zr, is observed in low-grade psammitic samples. This signature, which is characteristic of passive margins or dissected arcs, could reflect the dissection of a continental arc source, a change in the source area, or a stage of stronger reworking within the depositional basin.

4- Diagnostic trace element ratios (Th/Sc, Zr/Hf, La/Th, La/Sc, Th/U, LaN/YbN, LaN/SmN, GdN/ YbN), and large Eu anomalies, for the Puncoviscana Fm. and related rocks, are similar to those of sediments from modern continental arcs, continental island arc, and metasedimentary rocks related to dissected arcs.

5- Similarity in the style, orientation, and geometry of pre-Ordovician structures, and consistency in the sequence of deformation preserved in the Puncoviscana Fm. and higher-grade related rocks in the studied outcrops suggest a common deformation history during the Pampean orogeny.

6- Our results, in combination with previous geochronological, sedimentological, and isotopic data support a model in which the Puncoviscana and related sediments were deposited in a basin to the west of a Neoproterozoic Brasiliano or Pampean magmatic arc that formed along the margin of a Mesoproterozoic craton.

Acknowledgments

This research was partially financed through start-up funds to A.P.LL. by Indiana University-Purdue University Fort Wayne and by the Gobierno Federal de México (Secretaria de Educación Pública) through a PROMEP grant UAEHGO-PTC-325 to J. E. C. This study benefited from the logistic help and discussions with José Pablo Lopez. We appreciate constructive reviews and comments made by Dr. Verdecchia and an anonymous reviewer.

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Manuscript received: June 23, 2012.
Corrected manuscript received: October 23, 2012.
Manuscript accepted: October 26, 2012.395