Front San Diablo na região de Miraflores, Faixa Sunsás, Bolívia: implicações tectônicas e estratigráficas

Abstract

The Front San Diablo is a tectonic structure of the rocks in the Paraguá Terrane and the metasedimentary cover of the Sunsás Group. The geologic and geochronologic data obtained in this work defined three lithostratigraphic units: the Chiquitania Complex composed of biotite gneiss with monzogranitic to granodiorite composition; the Suite Pensamiento represented by the Colmena Granite, a mylonitic monzogranitic intrusive body mylonatized with monzogranitic composition, exposed in the major deformation zone; and the Sunsás Group, consisting of micaschist and quartzite. Three phases of deformation were identified: F1 generated the gneissic banding and S1 with a N-S trend in the Chiquitania Complex rocks; F2 resulted in the transposition of the S1 banding, and gave rise to folds, and S2 foliation parallel to the regional E-W trend; phase F3 is marked by open folds and spaced foliation of crenulation type (S3) orthogonal to the regional structures. The SHRIMP U-Pb geochronological data indicated the crystallization age of 1,685 ± 4.9 Ma for the Chiquitania complex gneiss; on the other hand, the LA-ICPMS U-Pb data obtained for detrital zircon grains suggest maximum age of sedimentation of ca.1.2 Ga for the deposition of the Sunsás Group. Paleo- and mesoproterozoic sources are also recognized. The structural data characterize the San Diablo Front as a dextral NE shear zone generated during the Sunsás Orogeny around 1.0 Ga, as indicated in the K-Ar previous data. Based on the data reported here, it becomes clear that the Front San Diablo does not represent a boundary between the Paraguá and San Pablo allochthonous terrane, but rather, a Neoproterozoic structure separating crustal blocks within the Paraguá Terrane.