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Geocronologia Pb-Pb por evaporação em monocristal de zircão do magmatismo da região de Tucumã, SE do estado do Pará, Amazônia Oriental
(Universidade Federal do Pará, 1996-10-16) AVELAR, Valter Gama de; LAFON, Jean Michel; http://lattes.cnpq.br/4507815620234645
The region of Tucumã, in the southeast of the State of Pará (Eastern Amazon-Brazil), constitutes a continuity to the southwest of the Mineral Province of Carajás-PMC. The Archean compartmentalization of this region is represented by a preserved granite-greenstone terrain (TGG), including Archean granitoids (Granodiorite Rio Maria) and supracrustal sequences (Tucumã Group), and by the Itacaiúnas Shear Belt (CCI) that includes granodiorite and tonalitics (Xingu Complex), granulites (Pium Complex), and stratoid granitoids (Plaquê Suite). In the Proterozoic, the region is marked by the intrusion of several anorogenic bodies (Seringa and Velho Guilherme Granites), which cut through all Archean units. The geochronological results, presented here for Archean and Proterozoic rocks from the Tucumã region, were obtained through the Pb/Pb method by evaporation in zircon microcrystals separated from saprolite and fresh rocks. The zircons from a granodiorite orthogneiss from the Xingu Complex provided a Pb/Pb age of 2972 ± 16 Ma (2σ), while the zircons from the Granodiorite Rio Maria presented an age of 2850 ± 17 Ma (2σ). The zircons of the Plaque Suite Stratoid Granitoid defined a Pb/Pb age of 2729 ± 29 Ma (2σ). Among the anorogenic granite bodies, only the zircons from the Granite Seringa allowed the determination of Pb/Pb defined around 1893 ± 15 Ma (2σ). The zircons of Granito Velho Guilherme did not reveal sufficient amounts of Pb to allow the calculation of the age of this rock. The Pb/Pb ages by evaporation obtained in this work are similar to the ages given by the U/Pb method in zircons of correlated rocks in the PMC, particularly in the Rio Maria region, and are interpreted in terms of crystallization ages. The age obtained for the Plaque Suite Stratoid Granitoid rock (2972 ± 29 Ma) should be understood as a minimum age for crystallization of this rock, given the absence of other U/Pb geochronological information in this type of rocks throughout the PMC. The Rb/Sr ages between 2.57 Ga and 2.74 Ga, previously obtained respectively for the gneiss rocks (Xingu Complex) and for the Archean granites of the TGG, as well as the Rb/Sr age of 1.75 Ga obtained for the Granite Syringe, should be considered as rejuvenated ages. The geochronological data presented here indicate the existence of at least 4 distinct magmatic events for the Tucumã region, confirming that this portion of the PMC had a similar evolution to the granite-greenstone terrains of the Rio Maria region, where the same events were recognized. The Pb/Pb age by evaporation obtained for the granodiorite orthogneiss of the Xingu Complex at 2972 ± 16 Ma, represents the oldest data found so far for the arquane rocks of Tucumã. The Pb/Pb age around 2850 ± 17 Ma of the Rio Maria Granodiorite confirms the age of the TTG magmatism associated with the greenstone-belts occurring at about 2.85-2.87 Ga, in the south-central portion of the PMC. The 2729 ± 29 Ma age of the Plaque Suite Stratoid Granitoid provides an age for the structuring of the Itacaiúnas shear belt. The age of 1893 ± 15 Ma obtained for the Seringa Granite confirms that the formation of anorogenic granites in the PMC occurred in a short period of time during the Lower Proterozoic (1.87-1.89 Ga). The similarities of the 207Pb/206Pb ages provided by the zircons of saprolitic material and fresh rock from the same rocks demonstrated the effectiveness of the use of saprolite for the geochronological study of Pb/Pb in zircons. Finally, the Pb/Pb ages in zircons of Archean and Proterozoic rocks from the Tucumã region obtained in this work demonstrate the potential of the method for determining the age of crystallization of ancient rocks in the Amazon region.