Metamorphic ages of low-grade units in the central region of Famatina: The signature of the Ocloyic Orogeny (Ordovician)

Abstract

Many of the metamorphic and deformational events associated to low-grade units in NW Argentina have been linked with ancient orogenies, like the Pampean (Cambrian) and the Ocloyic (Ordovician) cycles. The lack of specific ages in the low-grade metamorphic rocks of the Famatina belt, as well as the absence of a detailed stratigraphic analysis, have led to ambiguous interpretations respect to the Paleozoic evolution of the southern segment of the Central Andes. Recent work allows recognition of two units within the low-grade 'metamorphic basement'; the Negro Peinado and the Achavil formations. Provenance analysis relates deposition of both units to the final stages of the Pampean cycle during the Cambrian, whereas an Ordovician (Ocloyic) synorogenic nature is suggested for the La Aguadita Formation, also traditionally included within the low-grade 'metamorphic basement'. Thus, as a whole, these units areyounger than the traditionally correlated pre-Pampean Punco vise ana Formation. K-Ar ages obtained forFamatina indicate that low-grade metamorphism and deformation of these units are Ordovician, in contrast to a much older age (Proterozoic-Middle Cambrian) previously suggested. Ages of 457±9 Ma (whole rock) and 463±14 Ma (<2μm fraction) for the Negro Peinado Formation allow establishing a link with the climax of the Ocloyic Orogeny developed along western Gondwana during the early Paleozoic. Ages of 435±12 Ma (<2μm fraction) in the La Aguadita Formation and of 444±8 Ma (whole rock) in phyllonites of the Ángulos shear zone are consistent with an extent into the earliest Silurian for this cycle. Additionally, a 359±7 Ma (<2μm fraction) age for the Negro Peinado Formation (Las Trancas shear zone) indicates local resetting within the age range assigned to the Achalian Orogeny (Late Silurian-early Carboniferous).