THE FAMATINIAN MAGMATIC ARC IN THE CENTRAL SIERRAS PAMPEANAS: AN EARLY TO MID-ORDOVICIAN ARC ON THE GONDWANA MARGIN.

PANKHURST, R.J.; RAPELA, C.W.; SAAVEDRA, J.; BALDO, E.; DAHLQUIST, J.; PASCUA, I.; FANNING, C.M.

THE PROTO-ANDEAN MARGIN OF GONDWANA.

The Geological Society of London.

Lugar: 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 Córdoba to the east, are considered regionally equivalent to the Puncoviscana Formation; a mid-Cambrian Rb-Sr whole-rock ischron of 513 ( 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-rocks ages of 470-450 Ma, typical of granites in the Sierra de Famatina, but geochemical continuity with the main granodiorite suites raises the posibility that these are partially reset ages. A minor cordierite granite phase is ascribed to local anatexis caused by heat from the granodiorites. All the cal-alkaline rocks of the Los Llanos-Ulapes batholith have high initial 87Sr/86Sr (0.7075-0.7105) and low (Ndt (–4.6 to –6.3), inherited from lower crust. Sm-Nd models ages of 1600-1700 Ma 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 estableshed significantly before the arrival of the allochthonous Precordillera terrane in mid-Ordovician times.