Geologia e petrologia dos gnaisses da regi?o de S?o Valério da Natividade, Tocantins

摘要

The S?o Valério da Natividade region in Tocantins state consists of the Goiás Massif and the Cavalcante-Natividade Block, both related to the Brasília Belt basement and composed mainly by Paleoproterozoic rocks, such as migmatite-orthogneiss complex of granodioritic and tonalitic composition. Xenoliths of metapelitic schists with garnet and staurolite porphyroblasts and quartzites occur locally. Orthogneisses show a continuous banding with NE direction and subvertical deeps. Migmatites consist of metatexites formed under compressional regime and shear zones. Metapelite schistosity is concordant to the gneisses and its mineralogical association records a metamorphic peak of the upper amphibolite facies with a green schist facies retrometamorphism. Based on the staurolite’s internal foliation, two ductile deformational phases are recognized. The schistosity and the gneiss banding mark the third phase. A fourth deformational phase of brittle character can also be observed. The orthogneisses’ chemical composition suggests a granodioritic protholith with affinity with TTG calc-alkaline suites, evolved by crustal reworking and contribution of MORB sources in the Cavalcante-Natividade crustal block, and by crustal reworking in the Goiás Massif gneisses. U-Pb LA-ICP-MS in zircon data of the orthogneiss show in the concordia diagram an upper intercept of 2229 ± 13 Ma, interpreted as the protholith age of crystallization, and a lower intercept of 614 ± 85 Ma interpreted as the metamorphism younger age. Similar crystallization ages were obtained in the Almas-Concei??o Domain, also related to calc-alkaline TTG suites (Manoel Alves Suite). The metamorphism age is consistent with the Brazilian-Pan African Cycle in the Brasília Belt.