THE ROBORÉ MICROCONTINENT, SW AMAZONIAN CRATON: NEW INSIGHTS ON THE OROSIRIAN-ECTASIAN CRUSTAL EVOLUTION FROM U-PB GEOCHRONOLOGY AND ISOTOPIC CONSTRAINTS

We present new and compiled U-Pb zircon ages and Nd and Hf zircon constraints for rocks of the Roboré microcontinent, the Eastern Precambrian Shield of Bolivia, located in the SW of the Amazonian Craton. The database together with updated geologic information provides new insights into the Proterozoic evolution. This microcontinent is subdivided into the San Diablo and Paraguá terranes. The oldest San Diablo terrane comprises amphibolite facies gneissic rocks (1941 ± 40 Ma), which were intruded by plutonic bodies of 1874 and 1849 Ma. The country rocks yield Sm-Nd T_DM ages and ε_Ndt values from 1.96 to 2.29 Ga, and +1.76 to -2.73 respectively, suggestive of derivation from short-lived protoliths. The bulk isotopic signatures for the San Diablo crust are consistent with magma genesis in a juvenile-like accretionary arc, have a calc-alkaline character, and show a subduction-related tectonic. The northern Paraguá terrane contains a granulitic crust dated to 1820 Ma. The available Sm-Nd T_DM model ages spread from ca. 1.7 to ca. 2.2 Ga and ε_Ndt values range from +3.0 to -2.9, with Paleoproterozoic protoliths. The basement rocks are crosscut by the Yarituses Suite (1683-1610 Ma) located to the west. The latter includes the following granites: La Cruz, Refugio, San Pablo, San Miguel, and Rosario. These granites exhibit Sm-Nd T_DM model ages of 1.8 to 2.5 Ga and ε_Ndt of +4.06 to -3.8. From a tectonic point of view, these units characterize the Suruquiso accretionary orogeny characterized here that coalesced the San Diablo and Paraguá terranes. Eventually, the Paraguá crust is crosscut by the San Ignacio granitoids, collectively known as the Pensamiento Granitoid Complex (1440-1270 Ma). These rocks show Sm-Nd T_DM model ages between 1.6 and 2.4 Ga, and predominantly crustal-like isotopic signatures of +5.2 to -4.0 akin to a convergent arc setting. On a broader scale, the granitoids are products of the Alto Guaporé orogeny in the Brazilian counterpart, distinguished by an accretionary phase (ca. 1440 Ma) and a collisional one (ca. 1330 Ma).. The polycyclic evolution of the Microcontinent ends with the Sunsás/Grenville collision leading to the tectonic stability of the Amazonian Craton.