Geocronologia Pb-Pb em zircão (método de evaporação) das rochas do embasamento do setor meridional do Cinturão Araguaia - Região de Paraíso do Tocantins (TO)

Abstract

Recent geochronological studies of the basement orthogneisses of the northern region of the Araguaia belt showed the occurrence of both Archean (2.85 Ga) and Early Proterozoic (1.85 Ga) rocks. The oldest gneisses were grouped in the Colmeia Complex, the younger were named the Cantao Gneiss. In the southern portion of the Araguaia belt, the basement sequences include metavulcanic-sedimentary rocks, tonalitic, calc-silicate, and alkaline gneisses, and granitic rocks. All of these sequences have been considered as Archean in age, although no geochronological data are available to test this hypothesis. The metavolcanic-sedimentary rocks are included in the Rio do Coco Group, while the granitic rocks are represented by the Serrote and Malaga granites. The tonalitic and calc-silicate gneisses were grouped with Colmeia and Rio dos Mangues Complexes, and the alkaline gneisses in the Monte Santo Suite. These gneisses were investigated geochronologically by single zircon Pb-evaporation methods in order to define their stratigraphic interelationships, and to contribute to a greater understanding of the geological evolution of this crustal segment. The single zircon ages of the tonalitic and calc-silicate gneisses range between 1.8 and 2.1 Ga. An age of about 1.0 Ga. was obtained for the alkaline gneisses of Serra da Estrela (Monte Santo Suite). These data, along with the single zircon age of 1.85 Ga. published for the Serrote Granite, indicate the widespread occurrence of Proterozoic rocks in the basement of the southern part of Araguaia belt. However, the existence of Arquean sequences in this region can not be discarted since the volcanic-sedimentary rocks of the Rio do Coco Group have not as yet been dated. The correlation of tonalitic and calc-silicate gneisses with the Colmeia Complex was not confirmed, so these gneisses have been grouped with the Early Proterozoic Rio dos Mangues Complex. Thus, it is suggested that a significant part of the basement of the southern portion of the Araguaia belt may represent a younger crustal segment accreted to the Amazon Craton during the Early Proterozoic. The alkaline gneisses are interpreted as representative of the alkaline magmatism associated with the extensional phase that developed, in the Middle Proterozoic, the basin which received the sediments of Araguaia belt. This belt was deformed during the Brasiliano Cycle, and the published age of 510 Ma for the Matanca Granite may date the final phase of this deformation.