GEOLOGICAL EVOLUTION OF THE BOLIVIAN PRECAMBRIAN SHIELD, SW AMAZONIAN CRATON: U-PB ISOTOPIC CONSTRAINTS

The tectonic evolution of the Bolivian Precambrian shield extends from the Paleo- to the Mesoproterozoic; it can be related with the magmatic and metamorphic events in the Rondonian - San Ignacio (1.56-1.30 Ga) and Sunsás (1.25-1.00 Ga) provinces. Geochronological data and geological mapping allow to separate three temporally distinct granite suites before the San Ignacio orogeny (1.37-1.30 Ga):

1. a) The Correreca granite from 1.92 to 1.89 Ga (Pb-Pb evaporation) and Santo Corazon granite, this last one yielding 1850 Ma. It is suggested that the area of Correreca and Santo Corazon granites - which crop out in the southeastern of the Precambrian shield - could be part of the Apa Block and it could be collided to the Paraguá Terrane during Sunsas orogeny.
2. b) The Yarituses Granitoid Suite of 1.67 to 1.62 Ga - which crops out in the area of San Javier - is intruded into the San Ignacio Schist group and correspond to volcanic arc granite.
3. c) The San Ramon granodiorite – which yields an age of 1.43 Ga - could be considered as a pre-kinematic pluton associated to the Pensamiento Granitoid Complex of the San Ignacio orogeny. The magmatism, deformation and metamorphism of the San Ignacio orogeny with sin- to late-kinematic and late to post-kinematic plutons have crystallization ages of 1373 - 1340 and 1347-1275 Ma respectively. This orogeny represents a continental accretionary arc that built the architecture of the Rondonian-San Ignacio province (1.56-1.30 Ga) by the collision of the Paragua Terrane against the Rio Negro -Juruena province (1.78-1.60 Ga).
4. d) The evolution ends with the formation of the Sunsás collisional belt which produced sin- to late and late to-post-kinematic plutonism. The allochthonous and collisional nature of the Sunsás orogeny is marked by tectonic fronts converging towards the Paraguá Terrane.