The Famatinian magmatic arc in the central Sierras Pampeanas: an Early-to-Middle Ordovician continental arc on the Gondwana margin

PANKHURST, R.J; RAPELA, C.W.; SAAVEDRA, J.; BALDO, E.G.; SAAVEDRA, J.; GALINDO, C.; FANNING, C.M

The Proto-Andean Margin of Gondwana

Geological Society of London

Año: 1998; p. 343 - 367

A new multi-disciplinary study of the central Sierras Pampeanas encompasses fieldwork, petrography, metamorphic and micro-structural analysis, geochemistry and geochronology. Remnants of a low-to-medium grade metasedimentary sequence, which also occurs in the Sierras de Cdrdoba to the east, are considered regionally equivalent to the Puncoviscana Formation; a ?mid-Cambrian Rb-Sr whole-rock isochron of 513 +- 31 Ma probably dates their main metamorphism. The predominant granitoids of the Los Llanos- Ulapes batholith constitute a calc-alkaline suite representative of the Famatinian subduction-related magmatic arc. The main granodiorite phase of the batholith is associated with an S2 fabric and shear zone formation, and was emplaced late during the deformational history of the metasediments. Conventional and SHRIMP U-Pb zircon dating yielded a combined age of 490 +- 5 Ma. Younger monzogranites gave Rb-Sr whole rock ages of 470-450 Ma, typical of granites in the Sierra de Famatina, but geochemical continuity with the main granodiorite suite raises the possibility that these are partially reset ages. A minor cordierite granite phase is ascribed to local anatexis caused by heat from the granodiorites. All the calc-alkaline rocks of the Los Llanos-Ulapes batholith have high initial 87Sr/86Sr (0.7075-0.7105) and low ENdt (-4.6 to -6.3), inherited from lower crust. Sm-Nd model ages of 1600-1700Ma indicate that the underlying crust is identical to that beneath the foreland to the east. This part of the Famatinian arc was thus a continental magmatic arc and was established significantly before the arrival of the allochthonous Precordillera terrane in mid-Ordovician times.