Early Ordovician magmatism in the Sierra de Ancaján, Sierras Pampeanas (Argentina): implications for the early evolution of the proto-Andean margin of Gondwana

The Ancaján pluton is a small-scale (ca. 5.34 km2) elongated igneous body of granodiorite to monzogranite composition that crops out in the Sierra the Ancaján (Eastern Sierras Pampeanas), intruding marbles and metasedimentary rocks of the Ediacaran Ancaján series. New SHRIMP and LA-MC-ICP-MS U–Pb zircon analyses from one granodiorite sample yielded a likely Ordovician crystallization age of ca. 473 Ma. Inherited Cambrian, Neoproterozoic, Mesoproterozoic and Paleoproterozoic ages have also been recorded. The Ancaján granitoids are calc-alkalic, magnesian and slightly peraluminous with medium to high K2O (2.44–3.74 wt%) and high Na2O (4.05–4.51 wt%) contents. These geochemical characteristics are comparable to those of the Ordovician Na-rich (TTG-like) magmatism of the Foreland Famatinian Domain (FFD) in the Sierras de Córdoba. Therefore, the Ancaján pluton could represent the northernmost outcrop of such magmatism. Isotopes data (Sr/Sri = 0.7052–0.7055; εNdi = − 0.7 to − 0.4; TDM = 1.24–1.27 Ga) and inherited zircon ages suggest that the parental magma probably resulted from partial melting of a combined source, mainly formed by older subcontinental mantle with mafic–ultramafic oceanic lithosphere composition, along with recycling/assimilation of continental crust involving Pampean granitoids and/or Ediacaran to Cambrian sedimentary protoliths. This interpretation is coincident with that postulated for the Ordovician Na-rich granitoids from the Sierras de Córdoba. The evidence shown here further implies that metasedimentary rocks of the Sierra de Ancaján were part of the continental upper plate during the Famatinian subduction, and corroborate the previous imbrication of the Ancaján and Puncoviscana series during the early Cambrian Pampean Orogeny.