Petrografia, análise deformacional e geocronologia (U-Pb) dos Gnaisses do terreno Paraguá: provável arco vulcânico Orosiriano – SW do Cráton Amazônico.

Abstract

The southwestern portion of the Amazonian Cráton consists of amalgamated autochthonous terrenes (Rio Alegre, Jauru, Nova Brasilândia, Paraguá, Alto Guaporé) juxtaposed against over a long period of time before until the formation of the supercontinent Rodinia. The Paraguá Terrene, according to previous definitions, represents a Paleoproterozoic continental fragment agglutinated to the margin of the proto-Amazonian Cráton during the San Ignácio Orogeny (1,38 to 1,30 Ga). It was initially called Paraguá Craton, a term applied to the Precambrian of eastern Bolivia not affected by the Grenvilian (Sunsás-Aguapeí) mobile belts. There were few changes in the lithostratigraphic stacking of the Paraguá Terrene, first proposed in Bolivia, which established three lithostratigraphic units for the Paleoproterozoic basement: the Lomas Manechis Granulitic Complex, the Chiquitanía Gneiss Complex and the San Ignacio Schist Supergroup. Into the basement, it was intruded the Pensamiento Granitoid Suite/Complex, with ages ranging from 1,3 to 1,38 Ga, during the San Ignacio Orogeny. In Brazil, the orthogneiss to the Terrene Paraguá were named Serra do Baú Complex, and the paraderivade granulites inserted into Ricardo Franco Metamorphic Complex, as corresponding to Lomas Manechis Granulitic Complex from semi-detailed systematic geological mapping as well as petrographic, geochemical and geochronology (U-Pb) studies, and deformation analysis tend to corroborate the understanding of origin and evolution of the gneissic terrene rocks encompassed into the Serra do Baú Complex in the boundary region between Brazil and Bolivia. Field and petrographic data showed the existence of a small body of paraderived gneiss Sillimanite-Corierite. Under the microscope, it displays intense alteration of cordierite, and the presence of sillimanite and garnet; seven other ortho-derived gneiss bodies are identified mostly as biotite gneisses. These rocks are an felsic rock association derived from subalkaline magmas, medium-K to high-K calc-alkaline type, metaluminous to peraluminous; crystallization ages range from 1,6 to 1,8 Ga. The tectonic context of the Paraguá Terrene, correlated to the ortho-derived gneisses of the Chiquitanía Gneissic Complex, shows evidences of three orogenic events. The first deformation phase (F1), associated with the Lomas Manechis Orogeny, developed gneissic banding (S1) at deep crustal levels; F1 is intensely affected by the second deformation phase (F2) which is related to the San Ignácio Orogeny and caused transposition banding (S1) resulting in the axial-surface foliation (S2); the third deformation phase (F3) is attributed to the Sunsás Orogeny and marked by the development of foliation S3 in which the S1 foliation is locally transposed into S2 foliation. F3 is a non-penetrative foliation and occurs associated with open and gentle folds (D3). The Paraguá Terrene possibly was bonded to the proto-craton Amazon during Orogeny San Ignacio and his last tectonic event, the Sunsás Orogeny, weakly reworks the rocks of gneiss Rio Fortuna and other geological units of the basement paleo the Mesoproterozoic. The Paraguá Terrene possibly was bonded to the proto-Amazonian craton during Orogeny San Ignacio and his last tectonic event, the Sunsás Orogeny, weakly reworks the rocks of gneiss Rio Fortuna and other geological units of the paleo the mesoproterozoic basement.