Age constraints on the Cambrian Mesón Group (NW Argentina) based on detrital zircons U–Pb geochronology and magnetic polarity bias

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Highlights

  • U–Pb LA-ICP-MS studies on detrital zircons from the Cambrian Mesón Group.

  • Maximum depositional ages: 525 (Lizoite), 520 (Campanario) and 509 Ma (Chalhualmayoc).

  • Magnetic polarity bias for Campanario Formation further constrain depositional ages.

Abstract

The Cambrian Mesón Group represents the first infill stages of the NW Argentina Paleozoic basin, given after the major orogenic event represented by the Tilcaric orogenic phase. This unit extends for several hundred kilometers along the Eastern Cordillera in the provinces of Salta and Jujuy, but its age is poorly constrained due to the paucity of fossil evidence and scarce geochronological data. U–Pb LA-ICP-MS geochronological studies were carried out on detrital zircons from the Lizoite, Campanario and Chalhualmayoc formations, that make up the Mesón Group, in outcrops in the Santa Victoria sub-basin in northernmost Argentina. Maximum depositional ages of 524.8 ± 4.1, 519.7 ± 2.4 and 508.6 ± 1.7 Ma were obtained for these formations, respectively. These new data, together with previous detrital zircon ages from southernmost outcrops of this Group, points to a possible diachronism in deposition, with younger ages towards the south. In our studied units, a dominant peak of Cambrian to late Ediacaran zircons is characteristic for the three formations, although a somewhat larger contribution from older crystals is observed in the Campanario and Chalhualmayoc formations. This pattern is similar to most other reported detrital zircon age spectra for the Mesón Group along the basin. Published paleomagnetic data for the Campanario Formation indicates a very strong reverse magnetic polarity bias, which allows us to infer three more likely intervals for the deposition of this formation (roughly 512–510, 509–505 and 497-493 Ma). These three alternatives are analyzed with respect to either major diachronous or roughly synchronous deposition of the Mesón Group sediments across the whole basin. Published magnetic fabric data for the Campanario Formation at three distant localities suggest possible paleocurrent directions from W to E.

Introduction

One fundamental tool for understanding the tectono-magmatic and paleogeographic evolution of the proto-margin of southwestern Gondwana is the study of provenance of detrital zircons based on U–Pb geochronology (e.g. Rapela et al., 2007, 2011, 2015; Augustsson et al., 2011, Adams et al., 2011, and many others). The Ediacaran to Early Cambrian Puncoviscana Formation, and units broadly equivalent exposed in northwestern Argentina (also named as “Puncoviscana Complex”, see discussion in Zimmermann, 2005; Aparicio González et al., 2014) show a major presence of Neoproterozoic zircons with subordinate input of Mesoproterozoic crystals (Adams et al., 2011; Escayola et al., 2011; Augustsson et al., 2011; Hauser et al., 2011; Aparicio González et al., 2014). U–Pb dating on plutons intruding this unit has provided useful constraints on the ages of the Cambrian magmatism as well as minimum ages for the proper Puncoviscana Formation. The ages of the Cañani pluton (523.7 ± 0.8 Ma, Escayola et al., 2011), the Cerro Negro orthogneiss (523.0 ± 0.7 Ma, Escayola et al., 2011), the Mojotoro granite (533 ± 2 Ma, Aparicio González et al., 2011) and the Tastil batholith (534.0 ± 7.0 Ma, Hauser et al., 2011) intruding the Puncoviscana Formation overlap with ages of granites found in the Eastern Sierras Pampeanas, such as the Juan Garcia and Villa Albertina granites (537.0 ± 4.0 Ma and 530.0 ± 4.0 Ma, respectively, Iannizzotto et al., 2013). All of them are considered to be part of the Pampean magmatic arc (e.g., Casquet et al., 2018; Ramos et al., 2010; Schwartz et al., 2008 and references therein) recording eastward (present-day coordinates) subduction, which could have begun about 570 Ma (Escayola et al., 2007). Different sources for the zircons have been proposed for the Puncoviscana Formation and equivalents. Adams et al. (2011) proposed the Brazilian shield as the most likely source, whereas Rapela et al. (2007) proposed the orogenic belts generated from the collision of the Kalahari craton with the Antarctic and Rio de la Plata cratons.

A major unconformity separates the deformed Puncoviscana Formation from the overlying, scarcely deformed and unmetamorphosed Middle to Late Cambrian sedimentary sequence. This unconformity has been assigned to the Tilcaric orogenic phase (Turner and Méndez, 1975). The collision of the Arequipa-Antofalla block against the Córdoba terrane (Escayola et al., 2011) or the Pampia terrane (Ramos et al., 2010) has been proposed as the origin of the Tilcaric deformation that affected the Puncoviscana Formation and their southern equivalents, generating tight folding and low-grade metamorphism (Willner and Miller, 1986; Willner et al., 1987; Mon and Hongn, 1991; Do Campo and Nieto, 2003, among others). The subsequent collapse of the orogen occurred in the middle to late Cambrian (Hongn et al., 2010), resulting in the deposition of the clastic succession known as the Mesón Group (Turner, 1960). Sánchez and Salfity (1999) characterized this group by stratigraphic analysis as the infill of intracratonic sub-basins along the suture zone between the Arequipa-Antofalla blocks and the Pampia terrane. According to Augustsson et al. (2011) sources of zircons for the Mesón Group are probably derived from unexposed rocks of the Rio Apa block or in the Eastern Sierras Pampeanas. Adams et al. (2011) indicated that there is a greater amount of Palaeoproterozoic zircons and a lower Mesoproterozoic input in the Mesón Group when compared with the underlying Puncoviscana Formation. These authors proposed that this may be because the Río de la Plata craton was not an available source of sediments until after the Middle Cambrian. Rapela et al. (2007) proposed a right-lateral strike-slip displacement between the Río de la Plata craton and the Eastern Sierras Pampeanas (Pampia) terrane during the Early Cambrian to explain the apparent lack of zircons provided by the Río de la Plata craton. Spagnuolo et al. (2012) and Franceschinis et al. (2016) provided some paleomagnetic data in favor of such a tectonic model. However, more recent paleomagnetic data (Franceschinis et al., 2020) cast serious doubts on its feasibility.

Beyond different tectonic models for the evolution of this region of southwestern Gondwana in the Cambrian, major uncertainties remain regarding basic knowledge of the timing of basin evolution along this margin. In particular, the age of deposition of the Mesón Group has been a matter of controversy for a long time (see Adams et al., 2011 and references therein), mainly due to the scarcity of biostratigraphic evidence. Radiometric ages from detrital zircons of this succession obtained in the last decade (Adams et al., 2011; Augustsson et al., 2011; Aparicio González et al., 2014) have placed some constraints but, by default, these data indicate only maximum depositional ages, and to infer from them actual chronology of deposition is not straightforward (Rossignol et al., 2019). In order to provide further geochronological constraints on the deposition of the Mesón Group succession, we present new U–Pb ages for detrital zircons from the three formations that compose the Mesón Group at their type locality, Santa Victoria Oeste, in the extreme north of Argentina. We attempt an analysis of these new and previous radiometric data together with a conspicuous magnetic polarity bias shown by the intermediate Campanario Formation. We further analyze whether the sedimentary infill of the basin was fast and virtually synchronous or diachronous.

Section snippets

Geologic framework: The Mesón Group

The Mesón Group is exposed in northwestern Argentina (Fig. 1) and consists mainly of a siliciclastic succession of sandstones and conglomerates with interbedded shales. Their depositional environment is inferred as a shallow platform setting along the western proto-margin of Gondwana. This succession was developed on top of a suture zone between the Pampia and the Arequipa-Antofalla terranes (Sánchez and Salfity, 1999). As already mentioned, the Mesón Group was deposited on top of the Tilcaric

Analytical methodology for U–Pb zircon geochronology

The isotopic analyses were carried out at the Geochronology and Isotope Geochemistry Laboratory of the University of Brasilia, Brazil. The analyzed samples were: 13-ME-206 (Lizoite Formation), 13-ME-207 (Campanario Formation) and 13-ME-208 (Chalhualmayoc Formation) (Fig. 3, Table 1).

Zircon concentrates were extracted from 10 kg of rock samples by panning at different sizes (100–400 μm), and by magnetic separation using a Frantz isodynamic separator; no chemical treatment was applied. After

Sample 13-ME-206 (Lizoite Formation)

Forty-two zircon grains were analyzed and large grain sizes are predominant. The crystals vary from colourless to brownish yellow. The colourless crystals analyzed are elongated with euhedral faces and show concentric zonation, characteristic of provenance from felsic igneous rocks. This suggests coeval igneous felsic magmatism with the near outcrop of the Cañani Pluton, which shows similar crystallization ages (Escayola et al., 2011). Few zircon grains are subrounded, crystals show irregular

Mesón Group and Puncoviscana Formation zircon age patterns

Previous geochronological data from zircons of the Mesón Group (Adams et al., 2011; Augustsson et al., 2011; Aparicio González et al., 2014) plus those published in this work cover an area of around 300 km in N–S direction, involving three of the sub-basins defined by Sánchez (1994) and Sánchez and Salfity (1999).

The age pattern distribution from detrital zircons along the basin (or sub-basins) may provide important clues to identify the main sources of sediments that were deposited in each

Conclusions

We report U–Pb LA-ICP-MS studies on detrital zircons from the Lizoite, Campanario, and Chalhualmayoc formations of the Cambrian Mesón Group in the Santa Victoria sub-basin, in northernmost Argentina. Maximum depositional ages of 524.8 ± 4.1, 519.7 ± 2.4, and 508.6 ± 1.7 Ma were obtained for these formations, respectively. When analyzed together with previous zircon age determinations on the Mesón Group from more southerly outcrops, an apparent diachronism, with a pattern of younger depositional

CRediT authorship contribution statement

Pablo R. Franceschinis: Conceptualization, Methodology, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Mónica P. Escayola: Conceptualization, Methodology, Validation, Formal analysis, Resources, Data curation, Writing - review & editing, Supervision, Funding acquisition. Augusto E. Rapalini: Conceptualization, Methodology, Validation, Formal analysis, Resources, Writing - review & editing, Supervision, Funding acquisition. Constanza Rodríguez Piceda:

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was supported by the Universidad de Buenos Aires (Grant 20020130100465BA) and CONICET (PIP 112-201101-00294). Remasoft program (AGICO SA) was used to analyze the data. The authors specially thank to Wolf Uwe Reimold for his detailed and thorough revision of the English grammar. Thorough and constructive comments by reviewers F. Hongn and V. Ramos and the editors are gratefully acknowledged. This contribution is dedicated to the memory of Márcio Pimentel.

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