Geology of the region of Rio Bonete: Northern edge of the Precordillera terrane

Abstract

The Jagüé shear zone is an important structure with NW strike within the Andean foreland in the La Rioja Province (28º31'S - 68º30'W), partly exposed along the Río Bonete. It has been suggested that this structure represents the northern boundary of the Precordillera Terrane. Consists of intensely deformed foliated ortho-amphibolites locally grading into gabbros, Grenville mylonitized and ultramylonitized granites and pods of banded, fine grained, calcitic and dolomitic marbles. These rocks were traditionally included within the Río Bonete Formation, whereas the marbles were separated as Member Caliza Las Damas. In addition, the Río Bonete Formation included a rhythmic succession of greywackes and shales of Late Ordovician age, affected by very low-grade metamorphism. According to new mapping these would represent two different units separated by an unconformity. For this reason, we suggest the use of Río Bonete Metamorphic Complex for the highly-sheared metamorphic unit and Chuscho Formation for the metasedimentary rocks, which are in turn, overlain by basic pillow lavas included in the Famatinan ophiolite belt. Also in order to avoid confusions, we propose replacing the term Caliza Las Damas for Mármol Las Damas that better represents their composition. Considering that the Chuscho Formation, despite its subgreenschist facies metamorphism, has not been affected by the ductile deformation and the fact that given the banded composition the marbles would correlate with the Cambrian units of the Precordillera terrane platform, the age of the shear zone is restricted to the Lower Paleozoic, and included within the Ocloyic Orogeny. Structural analysis allows interpreting a left-lateral to normal kinematics for the regional ductile shear linked to the emplacement of the Precordillera terrane with the western margin of Gondwana. The striking geological contrast observed to the north and south of the Jagüé Shear Zone is consistent with the hypothesis that indicates this region as the northern boundary of the Precordillera terrane.