Tectono-metamorphic evolution of a high- to medium-grade ductile deformed metagabbro/metadiorite from the Arenosa Creek Shear Zone, Western Sierras Pampeanas, Argentina

Abstract

A lower-granulite to middle-amphibolite facies ductile shear zone has been recognized affecting a quartzose metagabbro/metadiorite which is part of the Famatinian orogenic belt of Early Paleozoic age of the Sierra de La Huerta, Western Sierras Pampeanas, Argentina. Textural and compositional changes occurring during mylonitization allow reconstruction of its tectono-metamorphic evolution as well as the metamorphic conditions achieved. Kinematic analysis 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 microfabrics, three overprinted deformation events are differentiated: a) a relict ductile event developed entirely within the granulite facies, at temperatures exceeding 800 °C and in the range of pressures between ≤ 5 to 5.5 kbar; b) a two-stage mylonitic deformation event under conditions of high to medium-grade metamorphism, at temperatures 500-700 ºC and pressures in the interval 6-7 kbar and c) a brittle deformation event developed at low greenschist facies probably below 400 °C. Evidence of the former high-grade ductile event is preserved in the protolith notwithstanding being affected by the subsequent high- to medium-grade ductile deformation that sensibly obliterate it, generating typical mylonitic fabrics. Slight modal and chemical changes are appreciated between the protolith and its mylonitic products. Mineral compositions of main components such as plagioclase remain constant although amphibole composition shows a decreasing Mg/(Mg+Fe2+) ratio as deformation increases. Age dating of the mylonites gives deformation ages that vary from 441.9 ± 1.9 Ma to 438.7 ± 1.9 Ma. These ages are internally consistent and compatible 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 in time and space.