Geochemistry and geochronology of the Paciência Suite: implications for the final stage of the Itacambira-Monte Azul block Rhyacian evolution

Abstract

Several high potassium-rich alkaline plutons from the Paleoproterozoic occur at the basement of the São Francisco craton, as well as at the basement of its surrounding orogens. Located in the western edge of the São Francisco craton, the Itacambira-Monte Azul block integrates the Araçuaí orogen basement rocks, which were partially reworked during the Brasiliano tectono-metamorphic event. In this block, alkaline rocks from the Paciência, Morro do Quilombo and Serra Branca plutonites are included in the Paciência Suite. Petrographic studies within the Paciência Suite show predominance of monzonites and syenites with equigranular to porphyritic textures and mafic enclaves. Chemical data indicate high concentrations of alkalis and enrichment in ferromagnesian oxides, which result in a high Mg#. These rocks are predominantly metaluminous, alkaline to alkaline-calcic and magnesian, enriched in Ba, Sr, Rb and depleted in Nb, P, Ti and other highfield strength element (HFSEs). Nonetheless, the contents of rare earth elements (REEs) are fractionated towards heavy rare earth elements (HREEs), showing higher concentrations of LREEs and negative to weakly positive Eu anomalies. The geochemical characteristics of these rocks are similar to potassic granitoids derived from ongoing subduction related to orogenic events, as the result of partial melting of a metasomatized mantle source enriched in incompatible elements and the input from an Archean crust. The crystallization age at 2053 ± 6.3 Ma indicates that the rocks from the Paciência Suite are related to late stages of the Transamazonic orogenic event, associated with the evolution of an extensive arc magmatism developed during the end of the Rhyacian. Thus, the Paciência Suite, as well as the other paleoproterozoic alkaline suites that occur in the basement of the Araçuaí orogen, may represent the final stages of the stabilization of the São Francisco-Congo paleocontinent, in the Rhyacian–Orosirian transition.