Geoquímica e geocronologia U-Pb, Sm-Nd e Rb-Sr dos granitoides orosirianos do domínio Erepecuru-Trombetas, Província Amazônia Central, Noroeste do Pará

Abstract

The granitic rocks of the northwestern Pará State are part of two large volcano- plutonic associations that marked the central region of the Amazonian Craton during Orosirian. The area of this study is located in the southwest portion of the Erepecuru- Trombetas Domain, south of the Guyana Shield. The oldest association recently mapped and dated at about 2.0-1.95 Ga, is formed by volcanic rock of Igarapé Paboca formation and granitoids of Caxipacoré Suite. The youngest association (1.90-1.86 Ga) is related to the Água Branca and Mapuera suites and the effusive and pyroclastic rocks of Iricoumé Group. Petrographic studies in the Caxipacoré, Água Branca and Mapuera granitoids allowed the definition of five granitoids varieties (quartz-monzonite, monzonite, monzogranites, syenogranites and ácali-feldspar granites) with variable content of hornblende and biotite, the main mafic phases. In general, they are isotropic rocks, leucocratic, inequigranular, medium- to coarse-grained with hypidiomorphic granular, rapakivi, granophyric and micro-porphyritic textures. The geochemical characterization allowed the distinction of two rock groups with different signatures. The first group formed by Caxipacoré and Água Branca suites, shows high-K to shoshonitic, magnesian, metaluminous to peraluminous calc-alkaline trends. LILE- enrichment (e.g. K, Rb, Ba and Sr), strong negative Nb anomalies, moderate heavy REE- fractionation and weak Eu anomalies are common characteristics of these granites. The second lithological group formed by Mapuera Suite, comprises acid rocks (71.29<SiO2<78.03%), alkaline nature and affinity with high-K, peraluminous to metaluminous, ferroan granitoids. These rocks exhibit HFSE-enrichment (e.g. Zr, Hf, Th,), high content of REE and pronounced negative Eu anomalies. In tectonic environment classification diagrams, the Caxipacoré and Água Branca granitoids show geochemical affinities with calc-alkaline volcanic arc granites (VAG), related to subduction zones, while the Mapuera granitoids resemble with A2-type post-collisional granites, related to anarogenic intraplate environments. Zircon U-Pb dating by LA-ICP-MS provided ages of 1991±5.9 and 2005±7.2 Ma for the Caxipacoré granitoids and 1886.5±4.8 Ma in a hornblende-biotite monzogranite of Água Branca suite. For Mapuera Suite, an age of 1870±14 Ma was obtained in a biotite alkali- feldspar granite. These ages were interpreted as crystallization ages of these granitoids. A gap of approximately 100 Ma years was observed between the Caxipacoré Suite Suite and Água Branca and Mapuera suites. The geochemical characteristics allied to geochronological data allowed to set a geodynamic evolution involving an orogenic context related to a subduction environment at 2.0-1.95 Ga, which favored the formation of the Igarapé Paboca volcanicn formation and the Caxipacoré Suite, followed by a transitional environment (1.90-1.86 Ga) between a convergent framework and a extensional intracontinental context, marked by the coexistence of calc-alkaline and alkaline rocks of the Água Branca and Mapuera suites, respectively. Nd-TDM model ages between 1.95 and 2.30 Ga, and Sr-TUR between 1.84 and 2.02 Ga and positive to slightly negative εNd values (+2.29 to -1.96) for Caxipacoré, Água Branca and Mapuera granitoids indicate that the parental magmas were derived from melting of an older Paleoproterozoic sialic crust with minor mantle contribution. In addition, the lack of Archean ages led to discard the existence of an Archaean basement in the western Erepecuru-Trombetas Domain, differently from proposals of tectonic evolution of the Amazonian Craton.