The proterozoic history of eastern Bolivia

About 100 000 km² of the previously unmapped Bolivian sector of the Central Brazil shield has been studied by “Proyecto Precámbrico”, an Anglo-Bolivian technical cooperation programme. The Lower Proterozoic is represented by the Lomas Maneches Granulite Group and the bulk of the Chiquitania Paragneiss Complex, which were formed during the Trans-Amazonic orogenic cycle (± 2000 Ma). The Middle Proterozoic spans the orogenic cycles of San Ignacio (± 2000-1300 Ma) and Sunsas (<1300-950 Ma). The San Ignacio cycle included the deposition of the San Ignacio Schist Group, now belts of pelitic schists with basic/ultrabasic sills, and the subsequent mobilisation of these and older rocks within a north-trending orogenic belt, accompanied by granitoid development. The Sunsas cycle began with the deposition of the molassic Sunsas Group and closed with the growth of a westnorthwest-trending orogenic belt, bordered to the north by a marginal zone and a stable craton, which was accompanied by granitoid phases and major basic/ultrabasic igneous activity. The close of the Sunsas orogeny marked the cratonization of the shield at about 950 Ma.Unmetamorphosed Upper Proterozoic and possibly Cambrian sediments on the southern and eastern flanks of the shield represent marine transgressions related to the intra-continental Braziliano orogenic cycle. East-trending dolerite dykes were probably intruded during this period within the shield.