Contributions to the petrography, geochemistry and geochronology (U-Pb and Sm-Nd) of the paleoproterozoic effusive rocks from Iricoumé group, amazonian craton, Brazil

The southernmost region of the Guyana shield, Amazonian craton, hosts large record of Paleoproterozoic effusive rocks of the Iricoume Group. They present remarkably well-preserved igneous textures and structures. The SiO2 contents reveal a bimodal association marked by a compositional gap between acid (SiO2 > 67 wt%) and intermediate (SiO2 < 57.7 wt%) rocks. The acid effusive rocks are rhyolites to rhyodacites with high SiO2, alkali, Rb, Zr, Nb + Ta, La + Ce and 104 Ga/Al content and low Fe2O3tot, TiO2, CaO, Sr and Co content. They exhibit subalkaline, metaluminous-to-peraluminous compositions, and geochemically compatible to A-type magmatism emplaced in post-collisional to within-plate tectonic settings. The intermediate rocks are andesitic/basalt to andesite relatively high contents of TiO2, Al2O3, Fe2O3total, MgO, CaO, Sr and Co; low SiO2, K2O, Rb, Zr, Nb + Ta, La + Ce. They have subalkaline and metaluminous geochemical composition and plot on within-plate basalt field. The acid rocks crystallized at 1882 +/- 11 Ma in U-Pb analyses for LA-MC-ICPMS zircon data. The Sm-Nd isotopic data on all rocks reveal a Nd TDM model ages between 2.59 and 2.16 Ga and epsilon(Nd)(t) values between -5.78 and 0.03, indicate that the magmatic evolution was related to the reworking of older Paleoproterozoic at the Rhyacian-Siderian period, continental crust (Transamazonian crust-forming event) with some mixing with a limited amount mantle-derived magmas or with contamination by Archean crust. The petrographic, geochemical and geochronological data presented in this paper suggest a within-plate to post-collisional tectonic setting for the Iricoume volcanism, involving lower crust uplift and generation of basalt magma in an extensional regime.