GEOCRONOLOGIA DA GRANITOGÊNESE DA PROVÍNCIA AMAZÔNIA CENTRAL BRASILEIRA: UMA REVISÃO

Abstract

A review of the Rb-Sr data of granitic rocks from the brazilian Central Amazonian Province is presented. Three major periods of magmatic activity were defined, ranging from Archaean to Proterpzoic times: 1. Archaean granitoids (> 2.500 Ma) of tonalitic, trondhjemitic and granodipritic compositions, generally metamorphosed and associated to granite-greenstone sequences; 2. Granitoids of Transamazonian age (c.a. 2.000 Ma) possibly related to the Transamazonian event; and 3. Middle Proterozoic anorogenic granitoids (1.600 Ma - 1.750 Ma). Archaean and Transamazonian granitoids show isotopic characteristics of the mantle or of a low time crusted residence material. The initial strontium ratios of the Middle Proterozoic anorogenic granitoids are higher man the typical mantle ones and range from 0,7068 to more than 0,715. Detailed Rb-Sr isotopic studies for the latter rocks show in a same body an increase of the initial isotopic ratios and a decrease of the corresponding age from the less to the more evolved fades. Comparison with recent U-Pb zircon ages from the same or similar bodies led us to think mat the observed spreading and lower values of the Rb-Sr age of these granitoids could be explained by effect of late magmatic alteration processes, differences in the magmatic source or by crustal contamination phenomena during the crystallization