Evolution of a high-grade ductile shear zone through the analysis of textural and compositional changes. An example from the Famatinian Orogen, Sierra de La Huerta, San Juan, Argentina

CASTRO DE MACHUCA, BRÍGIDA; DELPINO, SERGIO; MOGESSIE, ABERRA; PONTORIERO, SANDRA; PREVILEY, LORENA; BJERG, ERNESTO

Río Cuarto (Córdoba)

Otro; XIV Reunión de Tectónica y III Taller de campo de Tectónica; 2009

ComTec (Asociación Geológica Argentina)-Univ. Nacional de Rio Cuarto

A lower granulite to upper amphibolite facies ductile shear zone with NNE trending and E dipping mylonitic foliation has been recognized in the Arenosa Creek, Sierra de La Huerta, province of San Juan, affecting an outcrop of quartzose metagabbro-metadiorite (Pl-Hbl-Bt-Qtz±Cpx±Opx-opaques) which is part of the Famatinian magmatic arc of the Western Sierras Pampeanas. Kinematic analysis of the Arenosa Creek Shear Zone (ACSZ) indicates reverse-sense of shear with a minor component of dextral displacement. This is compatible with the kinematics of other ductile deformation zones in the area, collectively forming a network. On the basis of paragenetic associations and deformation textures, at least three overprinted deformation events can be differentiated in the ACSZ: a) a relict ductile event developed entirely within the granulite facies field, at temperatures exceeding 700 °C and in the probable range of pressures between 5 to 6 kbar; b) a two-stage mylonitic deformation event occurring under conditions of high to medium-grade metamorphism, at temperatures that ranges between >550-600 ºC  and pressures in the interval 6-7 kbar; c) a brittle deformation event developed at low greenschist facies, probably at temperatures below 400 °C. Evidences of relict high-grade ductile event are well preserved in the quartzose metagabbro-metadiorite (protolith) notwithstanding it is affected by a stronger mylonitic high- to medium-grade ductile deformation that sensibly obliterate them, generating typical mylonitic fabrics. Significant modal and chemical changes are appreciated between the protolith and its mylonitic products. Nevertheless, mineral compositions of main components such as plagioclase remain essentially constant. Amphibole composition shows a slight decreasing in the Mg/(Mg+Fe2+) ratio as deformation increases. The textural and compositional changes experienced by the quartzose metagabbro-metadiorite during mylonitization allow reconstructing its tectono-metamorphic evolution as well as the metamorphic conditions achieved. Age dating (40Ar/39Ar hornblende) of the mylonite formed at upper amphibolite-facies conditions gives deformation ages that vary from 441.9±1.9 Ma to 438.7±1.9 Ma. These ages are internally consistent and compatible broadly with existing geological and geochronological data for the region, suggesting that during the Famatinian orogeny the western margin of Gondwana was characterized by several episodes of ductile deformation that varied greatly in time and space.