Rhyacian evolution of the eastern São Luís Craton: petrography, geochemistry and geochronology of the Rosário Suite

Abstract

ABSTRACT:The São Luís Cráton comprises an area between northeast Pará state and northwest Maranhão that exposes Paleoproterozoic granitic suites and meta-volcanosedimentary sequences. In the east of this geotectonic unit, about 70 km south of São Luís, there is a portion of the São Luís Craton, represented by the intrusive Rosario Suite (RS). This work is focused on rocks of this suite, including petrographic, lithochemical and geochronological studies to understand the crustal evolution of these granitoid rocks. The rock spectrum varies from tonalitic to granodioritic, quartz dioritic and granitic compositions, and there are partial structural and mineralogical changes related to deformation along transcurrent shear zones. The geochemical studies show granitic metaluminous compositions of the calc-alkaline series with I-type affinity typical of magmatic arc. Rare earth elements show marked fractionation and slight Eu positive or negative anomalies (Eu/Eu* = 0.82 to 1.1). Zircon U-Pb data provided consistent ages of 2165 ± 7 Ma, 2170 ± 7 Ma, 2170 ± 7 Ma, 2161 ± 4 Ma and 2175 ± 8 Ma, dating emplacement of these granitoids as Paleoproterozoic (Rhyacian). Sm-Nd isotopic data provided model ages (T DM ) of 2.21 to 2.31 Ga with positive values of εNd +1.9 to +3.2 (t = 2.17 Ga), indicating predominantly Rhyacian crustal sources for the parental magmas, similar to those ones found in other areas of the São Luís Craton. The data, integrated with published geological and geochronological information, indicate the occurrence of an important continental crust formation event in this area. The Paleoproterozoic evolution between 2.17 and 2.15 Ga is related to the Transamazonian orogeny. The granitoids of the Rosario Suite represent the main phase of continental arc magmatism that has continuity in other parts of the São Luís Craton and can be correlated with Rhyacian accretionary magmatism in the northwestern portion of the Amazonian Craton that amalgamated Archean terrains during the Transamazonian orogeny.KEYWORDS: Rosário Suite; São Luís Craton; Petrology; U-Pb geochronology; Sm-Nd T DM . RESUMO: O Cráton INTRODUCTIONIn models of global supercontinent reconstruction, the São Luís Craton and the northeastern portion of the Amazonian Craton (to the east of the Guayana Shield) have been considered remnants of the West African Craton preserved in the northern South American Platform after the breakup of the Pangea Supercontinent (Hurley et al. 1967, Torquato & Cordani 1981, Lesquer et al. 1984, Brito Neves et al. 2000.On the African side, several studies based on structural, geochemical, geophysical and geochronological data demonstrate the existence of Archean and dominantly juvenile Paleoproterozoic crust (Abouchami et al. 1990, Boher et al. 1992, Gasquet et al. 2003. On the Brazilian side, the northern part of the Amazonian Craton and São Luís Craton have demonstrated geochronological and evolutionary similarities. In this part of the Amazonian Craton, the ancient continental crust stabilized in the Archean is bordered by...