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

The Ancajan pluton is a small-scale (ca. 5.34 km(2)) elongated igneous body of granodiorite to monzogranite composition that crops out in the Sierra the Ancajan (Eastern Sierras Pampeanas), intruding marbles and metasedimentary rocks of the Ediacaran Ancajan 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 Ancajan 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 Cordoba. Therefore, the Ancajan pluton could represent the northernmost outcrop of such magmatism. Isotopes data (Sr/Sr-i = 0.7052-0.7055; epsilon Nd-i = - 0.7 to - 0.4; T-DM = 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 Cordoba. The evidence shown here further implies that metasedimentary rocks of the Sierra de Ancajan were part of the continental upper plate during the Famatinian subduction, and corroborate the previous imbrication of the Ancajan and Puncoviscana series during the early Cambrian Pampean Orogeny.