Metamorphic evolution of ductile shear zones related to the Famatinian orogeny, Sierra de La Huerta, Western Sierras Pampeanas, NW Argentina

CASTRO DE MACHUCA, B.; PREVILEY, L.; MORATA, D.; DELPINO, S.

Brisbane

Congreso; 34th International Geological Congress; 2012

Ductile shear zones with NW to NNE trending and NE to E dipping mylonitic foliation have been recognized elsewhere in the Sierra de La Huerta, Western Sierras Pampeanas, Argentina. They affect both metamorphic host rocks and intrusive rocks which are part of the Early Paleozoic (Famatinian) orogenic belt. Textural and compositional changes experienced during mylonitization allow reconstruction of the tectono-metamorphic evolution of the area as well as the metamorphic conditions achieved. Kinematic analysis of these shear zones indicates reverse-sense of shear with a minor component of dextral displacement. On the basis of paragenetic associations and microfabrics, three overprinted deformation events are differentiated: a) a relict ductile event developed within the granulite facies, at temperatures exceeding 800 °C and pressures between 5 to 5.5 kbar; b) a two-stage mylonitic deformation event under conditions of high to medium-grade metamorphism, at temperatures between 500-700 ºC and pressures between 6-7 kbar and c) a brittle deformation event developed at low greenschist facies probably below 400 °C. Evidences of the former event are preserved in the protoliths notwithstanding being affected by the subsequent deformation that strongly obliterates them and generates typical medium-grade mylonites. Slight modal and chemical changes are appreciated between the protoliths and their mylonitic products. Deformation ages of the mylonites 40Ar/39Ar) that vary from 441.9±1.9 Ma to 432±4 Ma suggest that the ductile deformation events would have been related to the later stages of the Famatinian orogenesis, associated with the accretion of the Precordillera terrane to the southwestern Gondwana margin.