Navegando por Assunto "Rochas metassedimentares"

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Proveniência das rochas metassedimentares do Cinturão Araguaia com base em datações Pb-Pb em zircão e idades-modelo Sm-Nd
(2011-06) PINHEIRO, Bruno Luís Silva; MOURA, Candido Augusto Veloso; GORAYEB, Paulo Sérgio de Sousa
This paper presents single zircon Pb-evaporation ages, obtained in detrital zircon grains, and Sm-Nd (TDM) for metassedimentary rocks of the Araguaia belt, to investigate the possible source areas of these rocks, and to contribute to understanding the geologic evolution of this belt in the context of the West Gondwana amalgamation. Quartzite of the Morro do Campo Formation, located in the north segment of the belt (Xambioá region) gave Archean 207Pb/206Pb zircon ages (2.65-3.0 Ga), while in the south segment (Paraíso do Tocantins region), the 207Pb/206Pb zircon ages were mainly Meso-Neoproterozoic (1.25-0.85 Ga), and, secondarily, Paleoproterozoic (1.85-1.70 Ga). These results suggest the existence of different sources areas for this quartzite. The Sm-Nd (TDM) model ages of metapelites (slate, phyllite and micaschist) present bimodal distribution with the main population occurring between 2.1 and 1.4 Ga (mode in 1.6-1.7 Ga), and a less abundant population between 2.7 and 2.4 Ga. These data have been interpreted as mixing of Paleoproterozoic (or even Archean) sources with younger, probably Meso-Neoproterozoic, sources. The main candidates to be the sources of the metassedimentary rocks of the Araguaia belt would be those crustal segments located in the southeastern (São Francisco Craton, Goiás Massif, Goiás Magmatic Arc). The sedimentary succession deposited in the Araguaia oceanic basin and the associated magmatic rocks were transported, later, towards the east margin of the Amazonian Craton, during the main tectonic that led to the structural development of the Araguaia belt as a result of the West Gondwana amalgamation.
Acesso aberto (Open Access)
Proveniência das rochas metassedimentares do cinturão Araguaia, com base em datações em idades modelo Sm-Nd em rocha total e datação Pb-Pb em zircão.
(Universidade Federal do Pará, 2005-06-30) PINHEIRO, Bruno Luís Silva; MOURA, Candido Augusto Veloso; http://lattes.cnpq.br/1035254156384979
The Araguaia belt is an important geotectonic unit of the Tocantins Structural Province, developed during the amalgamation of the West Gondwana Supercontinent during the Neoproterozoic. This belt is located on the border of the Amazon Craton and divides two different crustal segments: to the east there are terrains affected by thermo-tectonic events related to the Gondwana amalgamation. In the west side, terrains not affected by these PanAfrican/Brasiliano events occur. Generally, it has been suggested that the Araguaia belt, was installed in a continental rift basin, developed on Archean crust, without formation of expressive oceanic crust. Afterwards, tectonic inversion took place with mass transportation toward NW forming this belt. However, geochronological studies have shown that the Araguaia belt lies over Archean basement in the north and Paleoproterozoic rocks in the south. In addition, pillow basalts, located in the western region of the Araguaia belt, presented inherited (?) zircon crystals with ages around 2.05 Ga. These data suggest that the contribution of Paleoproterozoic crustal material may have been an important source for the metasedimentary rocks of the Araguaia belt, too. The answer of this question is fundamental for reconstruction of the paleogeographic evolution of the Araguaia belt in the context of the West Gondwana evolution. In this dissertation, single zircon Pb-evaporation ages in quartzites and Sm-Nd model ages in metasedimentary rocks of the Araguaia belt were determined, in order to identify the ages of the possible sources of these rocks and to contribute to understanding the paleogeographic evolution of this belt and West Gondwana. Detritic zircon crystals of two quartzite samples of the Morro do Campo Formation were dated by the single zircon Pb-evaporation technique. The sampling was carried out in the northern segment (Xambioá region), and in the southern segment (Paraíso do Tocantins region) of the Araguaia belt, both in the state of Tocantins. The zircon ages of the quartzite from Xambioá range from 3.0 to 2.65 Ga. On the other hand, a main contribution of Meso-Neoproterozoic terrains (1.25 – 0.85 Ga), with minor of Paleoproterozoic sources (1.75 – 1.85 Ga), is shown by detritic zircons of the southern segment of the belt. These data indicate the contribution of source areas with distinct ages for the metasedimentary rocks of the Araguaia belt. Sm-N model ages (TDM) of the metassedimentary rocks sampled along the Araguaia belt presented a bimodal distribution, with ages showing a major frequency between 1.4 and 2.1 Ga and a minor frequency between 2.4 and 2.7 Ga. These TDM ages suggest that the metasedimentary rocks of the Araguaia belt may result from Paleoproterozoic terrains mixed with younger sources, probably, of Meso- Neoproterozoic ages. Alternatively, this mixing may had a minor contribution from Archean crust. As a result of these isotopic data, it is suggested that the sources of the metassedimentary rocks of the Araguaia belt are located to the east of the belt. The sediments were deposited in oceanic basins along these segments, and were thrust over the eastern margin of the Amazon Craton, due to the amalgamation of the West Gondwana, resulting in the formation of the Araguaia belt. The presence of detritic zircon of Archean age in the quartzite from Xambioá does not necessarily implies in an Archean source located in the Amazon Craton, since Archean rocks are also present in the possible source areas situated to the east of the Araguaia belt. Moreover, these detritic zircon crystals may have been recycled during sedimentary processes and incorporated in rocks units of younger terrains. The main candidates for crustal sources of the metassedimentary rocks of the Araguaia belt are the rock units of the São Francisco Craton, the Goiás Massif and the Goiás Magmatic Arc. These source areas are located to the east of the Araguaia belt and have magmatic rocks with ages similar to those determined in the detritic zircon crystals. In addition, the mixing of rocks from these different terrains may result in the range of the observed TDM ages of the meatssedimentary rocks. The amalgamation of the Paraná Craton, presently under the Paraná sedimentary basin, with the São Francisco Craton, the Goiás Massif and the terrains of the Goiás Magmatic Arc, resulted in the formation of the Brasília belt and in a huge crustal block. The oblique collision of this block with the Amazon Craton, 50 to 100 Ma after the formation of this block, may have led to the SE to NW tectonic transport that thrust different rock units over the eastern margin of this craton, resulting in the formation of the Araguaia belt.