Estudo petrográfico, geoquímico e isotópico de granitos da porção Leste do Domínio Tapajós, nas folhas São Domingos e Jardim do Ouro, Pará

Abstract

The Tapajós Domain, located in Brazil Central shield (southern portion of the Amazonian Craton) makes part of the Tapajós-Parima (or Ventuari-Tapajós), Province corresponding to a Paleoproterozoic orogenic belt of NW-SE direction, extending from southwestern Pará state to southern Venezuela. Some authors describe this province as been formed by development of two distinct orogenies that accreted four magmatic arcs, followed by a post-orogenic alkaline magmatism. This post-orogenic Orosirian magmatism, occurred around 1880 Ma, is represented by the Maloquinha Intrusive Suite (MIS) and Iriri Group that have occured postdating Tropas Orogeny which give rise to Parauari Intrusive Suite (PIS). The MIS comprises alkali granites generated in post-collisional anorogenic environment that is spatially associated with Iriri Group volcanic rocks. This suite consists of alkali feldspar granite, syenogranite and monzogranite leucocratic bodies with predominant pertitic orthoclase and rare microcline. These rocks crop out as elliptical to round stocks and batholiths along NW-SE regional lineaments in the Tapajós Domain. Many bodies belonging to SIM have been dated by both methods, Pb-evaporation and U-Pb on zircon, that yielded ages between 1882±4 and 1864±18 Ma. In the eastern portion of the Tapajós Domain, the Nd isotope signature for these suite granites suggests paleoproterozoic sources (TDM Nd of 2.28 to 2.23 Ga, and εNd(t) of -0.72 to -2.45). Many plutons of SIP were dated by zircon Pb-evaporation method, and the ages found were between 1891±3 e 1879±11 Ma, its isotopic signature of Nd are εNd(t) -5.21 to -1.82 and TDM Nd of 2.43 to 2.32 Ga. This work was carried out in the eastern part of the Tapajós Domain, in the SB.21-Z-A-II (São Domingos) and SB.21-Z-A-III (Jardim do Ouro) 1:100,000 sheets, where five granitic bodies were studied (Igarapé Tabuleiro, Dalpaiz, Mamoal, Serra Alta and Igarapé Salustiano). The Igarapé Salustiano body shows a calc-alkaline affinity, meta to peraluminous and syncolisional I-Type granites nature, showing close relationship with the Parauari Intrusive Suite rocks of this Domain. Other bodies could be related to MIS and they are basically represented by hololeucocratics alkali feldspar granites, syenogranites and monzogranites. These rocks have high SiO2 (>70%), FeOt/(FeOt+MgO) (>0.80) and K2O/Na2O, low CaO, Al2O3, MgO and Sr contents, and post-collisional granites affinity, alkalic-calcic to alkalic character and high REE content with Eu negative anomalies, own from A-type granites. This work, as others made in the Tapajós-Parima and Amazônia Central provinces, identified TDM Nd contradictory ages to the ones assigned these two provinces for geochronological subdivision between them.