Estudo petrográfico e geoquímico do embasamento e dos granitóides San Ignácio e Sunsás da região San Ramon, Concepción, SW do cráton amazônico da Bolívia.

Abstract

The main objectives of this work were the petrographic characterization and geochemistry studies of the Sunsas granitic intrusions and their country rocks in the Bolivian sector of the SW Amazonian craton. The studied units comprise the Sunsas Orogeny granitoids (1,30 Ga-950 Ma), the basement (>1400 Ma) including La Chiquitania gneiss, enderbitic and granitic gneiss of the Lomas Maneches unit and San Ignacio granitoids (1400-1300 Ma). The studied area is located in the west sector of Bolivia and involves the Santa Cruz de La Sierra and Beni states. The justificative for this study is the absence of investigation focusing the area since the 1980 decade, when the Bolivian Geological Survey with the Geological British Survey mapped the Bolivian pre-Cambrian. The Lomas Maneches unit comprises enderbitic and granitic gneiss from metaluminous to peraluminous composition. The tectonic setting indicated by the tectonic diagrams suggest late-orogenic to post-tectonic origin and the REE patterns suggest fractional crystallization processes for the rocks formation. The La Chiquitania unit presents two types of rocks (granitic gneiss and the garnet gneiss) here interpreted as similar to the Lomas Maneches rocks. The San Ignácio granitoids include San Andrés, El Refugio and San Ramon granites. According to the geochemistry results the rocks are characterized as metaluminous and peraluminous and the tectonic setting where the rocks were formed vary from pre-collision to post-orogenic and the REE patterns suggest the existence of two groups of rocks originated in different sources or as result of different processes of magma fractionation.The Sunsas granitoids here studied included the Talcoso, Cachuela, Naranjito, Taperas and Primavera intrusions. The petrography study allowed to classificate the first three granitoids as granites and the other two as granodiorite. The geochemical study of all the granites indicates metaluminous trend and according to the REE patterns, the Naranjito, Primavera and Talcoso granites are product of the fractional crystallization processes; the Cachuela granite represents the more primitive, and the Taperas granite with intermediate position in the magmatic fractional processes.The present study and previously works suggest that the La Chiquitania paragneiss were formed as result of erosion and sedimentation from sources dating at 1,76 Ga. At about 1680-1660 occurred an important accretionary event in this region (here defined as Lomas Maneches Orogeny). Following the geological time, the San Ignácio event was recorded by granitogenesis ca. 1,34 Ga and 1,33 Ga, whose tectonic environment probably is related to a continental magmatic arc. The Sunsas event granitoids may be classificated as I-type granites, resulted of the continental magmatic arc setting at 1,07 Ga. The REE patterns allow to suggest these granites were generate from a magmatic fractionating processes, with mantelic source during which subduction finished with the Grenvillian collision, according to the literature, responsible for the Rodinia supercontinent assembly.