The Sarandi del Yí Shear Zone (Uruguay): kinematics, deformation conditions and tectonic significance

ORIOLO, SEBASTIÁN; OYHANTCABAL, PEDRO; WEMMER, KLAUS; SIEGESMUND, SIEGFRIED

Congreso; XV Reunión de Tectónica; 2012

The Sarandí del Yí Shear Zone represents a major N to NNW structural feature in the basement of southern Uruguay. It was first defined by Preciozzi et al. (1979) as a structural lineament that separates different basement units, but Bossi and Campal (1992) suggested it as a boundary between tectonostratigraphic terranes. These authors also recognized it as a dextral transcurrent shear zone based mainly on the bending from ENE to WNW of the Florida dyke swarm. However, Oyhantçabal et al. (1993, 2010) have identified sinistral shear sense indicators and suggested a two?stage evolution: a first dextral shearing (Mesoproterozoic?) and a later sinistral reactivation during the Brasiliano Cycle (Neoproterozoic). Recently, Oyhantçabal et al. (2011) have also considered this shear zone as the eastern boundary of the Río de la Plata Craton. Despite the tectonic significance of this shear zone, related to the early evolution of Gondwana, structural and microstructural studies are almost absence. Detailed field mapping of mylonites and adjacent units as well as microstructural studies were carried out in two key areas (northern and southern) of the Sarandí del Yi Shear Zone in order to provide information that help to constrain its evolution. The northern area is located between the localities of Sarandí del Yí and Cerro Colorado, whereas the southern one locates to the south of the city of Minas. Field mapping confirms the presence of a mylonitic belt of about 2 km wide that made up the Sarandí del Yí Shear Zone. Granitic mylonites are the most frequent lithological type with variations from proto? to ultramylonites, though phyllonites are locally present. Mylonitic foliations show a mean attitude of 009º/88ºE whereas lineations are subhorizontal with a density maximum of 10º/002ºE. Tight isoclinal folds of the mylonitic foliation are also present, mostly towards the east, with centimetric to metric wavelengths. Fold axes are subparallel to lineations. The orientation of foliation regarding to the regional trend of the shear zone as well as kinematic indicators such as C? surfaces, asymmetric recrystallized tails on feldspar showing core and mantle structures and sheared late pegmatitic veins and veinlets support sinistral transcurrent kinematics, which is consistent with observations from Oyhantçabal et al. (1993, 2010). Microstructural analysis reveals the presence of polycrystalline quartz ribbons made up by subparallel elongated crystals that define an oblique foliation regarding to the long axis of the ribbons. Grain boundaries are cuspate and lobate. Therefore, subgrain rotation and grain boundary migration are interpreted to be the main recrystallization mechanisms for quartz. K?feldspar constitutes core and mantle structures, mostly σ?type, which supports recrystallization. They show frequent metamorphic myrmekites, bookshelf structures and flame perthites. Mica fish are also present as well as S-C? shear bands. These features suggest temperatures of ~500ºC during deformation. Evidences of cataclastic overprinting can be also recognized. Fractures filled with epidote and chlorite are locally present in the easternmost mylonitic belt. Results presented herein support sinistral transcurrent motions of the Sarandí del Yí Shear Zone probably related to the evolution of the Neoproterozoic Dom Feliciano Belt. This event has given rise to the development of a medium? to high?T mylonitic belt that was probably later reactivated under more brittle conditions. However, further structural, microstructural and geochronological studies will provide information to reveal a possible pre?Brasiliano evolution of the Sarandí del Yí Shear Zone