Petrografia das rochas metamórficas da região de São Valério da Natividade (TO) e litoquímica e geocronologia dos gnaisses

Abstract

The geological context of the São Valério da Natividade region in the Tocantins state consist of the Goiás Massif and the Cavalcante-Natividade Crustal Block, both related to the Brasília Belt basement. The studied area is composed mainly by Paleoproterozoic rocks such as orthogneisses of granodioritic and tonalitic composition and migmatites. Xenoliths of metapelitic schists with garnet and staurolite porphyroblasts, psamitic gneisses and quartzites occur locally. Orthogneisses show a continuous banding with NE direction and subvertical deeps and the migmatites consist of metatexites formed under compressional regime and shear zones. The metapelite schistosity is concordant to the gneisses and its mineralogical association record a metamorphic peak of the upper amphibolite facies with a green schist facies retrometamorphism. Based on the internal foliation of the staurolite, two ductile deformational phases were identified and the third phase is marked by the schistosity and the gneissic banding. A fourth deformational phase, of brittle character, can also be observed The chemical composition of the orthogneisses suggests a granodioritic protholith with affinity with TTG calc-alkaline suites, crustal reworking and contribution of MORB sources, the crustal block Cavalcante-Natividade, and crustal reworking in the Goiás Massif gneisses. Geochronological data (U-Pb LA-ICP-MS) 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, interpreted as the metamorphism younger age. Similar crystallization ages were obtained in the Almas-Conceição Domain, also related to calc-alkaline TTG suites (Almas-Dianópolis Suite). The metamorphism age is consistent with the Brazilian-Pan African Cycle in the Brasília Belt.