Untangling the Neoproterozoic-Early Paleozoic Tectonic Evolution of the Eastern Sierras Pampeanas Hidden in the Isotopical Record

Abstract

The Sierras Pampeanas of Central Argentina are an example of a continuous and fast overlap of episodes of high- to medium- and low-grade metamorphism, deformation, anatexis, magmatism and mineralization along the belts that bounded the margins of the South American cratons during the Neoproterozoic and early Paleozoic. A compilation and critical revision of the massive isotopic and geochemical data for the basement rocks of the Eastern Sierras Pampeanas is presented. The Eastern Sierras Pampeanas are defined by three main events: the Ediacaran to early Cambrian (580–530 Ma) Pampean, the late Cambrian–Ordovician (500–460 Ma) Famatinian and the Devonian-Carboniferous (400–350 Ma) Achalian orogenies. The mean average crustal residence age (Sm–Nd TDM) varies between 1.8 and 1.7 Ga with εNd(540) (−6 to −8). Pampean and Famatinian granitoids exhibit a similar TDM interval except for the Ordovician TTG suites of the Sierras de Córdoba (TDM 1.3–1.0 Ga). Achalian magmatism exhibits more radiogenic εNd(540) values (0.5 to −4) and TDM ages younger than 1.3 Ga. Two types of Pampean-related mafic rocks are recognized: one with a depleted mantle signature and another younger group with an enriched mantle signature, which is associated with the peak of metamorphism. Ordovician mafic-ultramafic rocks result from mixing/assimilation of depleted mantle and crustal magmas. Detrital zircon data for the metaclastic sequences indicates mainly Grenvillian and Brasiliano sources. The difference between the measured crystallization age for detrital zircon grains and the depositional age of the succession indicates that most of the Pampean basins are collisional, i.e. foreland basins except for Sierra Norte metaclastic host rocks that correspond to a convergent setting signature. The results for the post-Pampean Famatinian basins indicate mostly collisional convergent settings for the Ambato and La Cébila (type locality) metamorphic complexes and the Olta (northern sector) and Achavil formations. The Negro Peinado Formation is the only post-Pampean basin that corresponds to a collisional setting. Data from Green Quarry, Nogolí Metamorphic Complex, and Olta Formation in the central part of the Sierra de Chepes, Suri Formation and La Cébila Metamorphic Complex at Quebrada La Rioja yielded convergent margin settings. These exhibit the highest correlation among the post-Pampean basins showing a relatively large number of Middle Cambrian age detrital zircons apart from having Ordovician magmatic zircons in the detrital record. Sources were apparently more restricted than in the rest of the Famatinian post-Pampean basins. The two samples of post-Pampean basins that exhibit Río de la Plata age peaks (2.2–2.0 Ga; Paleoproterozoic) belong to the collisional convergent group. Therefore the exhumed Pampean rocks probably formed a drainage divide that blocked westward transport of the Río de la Plata-derived sediments. Intense erosion owing to an unstable tectonic scenario would have led to the progressive appearance of the Río de la Plata signature.