O granito de duas micas Ney Peixoto nordeste do Pará: aspectos petrológicos e significado tectônico

Abstract (English)

The Ney Peixoto granite crops out on the left side of the Guamá river, between the cities of Ourém and São Miguel do Guamá, NE of Pará state. It is the only granitic intrusion of Brasiliano age (549 ± 5 Ma) known in the Gurupi region, which was essentially built up during the Paleoproterozoic era. Despite an outcropping area of about 6 km2, the contacts with its host rocks cannot be observed due to the alluvium and colluvium cover. This intrusion is slightly foliated and locally banded, having an average modal composition of 35.2% quartz, 32.8% microcline, 20.4% oligoclase, 5.9% muscovite  and 5.4% biotite. Its peraluminous rocks (A/CNK > 1.1), which are rich in SiO2, Al2O3, K2O and Na2O, and poor in MgO and CaO, show normative composition close to the ternary minimum of the SiO2-K2O-Na2O-H2O system. Among the trace elements, Ba, Rb, Sr and Zr are the most abundant, affording Rb/Sr and Sr/Ba ratios that range from 0.7 to 1.2 and from 0.24 to 0.37, respectively. The distribution pattern of the rare earth elements displays a moderate fraccionation of the LREE relative to the HREE (Lan/Ybn = 27 a 75,5) and a moderate negative Eu anomaly (Eun/Eu* = 0,46 a 0,62). Derived from a Paleoproterozoic crust, the Ney Peixoto granite was most likely  generated by partial melting of the Jeritequara Formation mica-schists of the Gurupi Group. No significant differentiation trend is apparent from bivariate plots using major or trace elements, although FeOt + MgO + TiO2 decreases as silica increases. This behavior may be due to the variable proportions of biotite in the rock samples. On the other hand, pegmatite pods might represent the residual liquids of the magma crystallization which became so deprived of Fe and Mg that biotite could no longer precipitate. The conditions for the magma crystallization were estimated at temperatures of 600-650°C under pressures around 5 kbar. The mineralogical, structural and chemical characteristics of the Ney Peixoto body, in particular its Nb, Y and Rb contents, allow to interpret it as a syn-collisional  intrusion, suggesting a thickening of the crust of the Gurupi region also in Neoproterozoic times.