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

Textural and compositional changes affecting a quartzose metagabbro/metadiorite in a ductile shear zone which is part of the Early Palaeozoic (Famatinian) orogenic belt of the Western Sierras Pampeanas, Argentina, allow reconstruction of its tectono-metamorphic evolution as well as the metamorphic conditions achieved. 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 ≤5–5.5 kbar; b) a two-stage mylonitic deformation event under high-to medium-grade metamorphic conditions, at temperatures 500–700 °C and pressures between 6 and 7 kbar and c) a brittle deformation event developed at low greenschist facies below 400 °C. Evidence of the first event is preserved in the protolith although the subsequent deformation obliterated it partially. Slight modal and chemical changes were detected between the protolith and its mylonitic products. Deformation ages of the mylonites that vary from 441.9 ± 1.9 Ma to 438.7 ± 1.9 Ma 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.