Geology, petrology and U-Pb geochronology of metavolcanic rocks in the Mundo Novo greenstone belt, eastern S?o Francisco Craton, NE Brazil: considerations about its tectonic setting

摘要

Field, petrological and LA-ICP-MS U-Pb geochronological data of metavolcanic rocks were used to interpret the petrological processes and to propose the tectonic setting for the Mundo Novo greenstone belt (MNGB) in the eastern S?o Francisco Craton. The metavolcanic rocks studied are metakomatiite, eastern and western metabasalts, and metadacite with subordinate metarhyolite, which host ocean floor hydrothermal alteration zones and are covered by ocean floor lithological associations composed of chemical metasedimentary rocks. Fractional crystallization and heterogeneous intraoceanic contaminations explain the mineralogical differences between the two metabasalts and the high (La/Yb)N ratio values of metakomatiite and metadacite. The metakomatiite and the eastern and western metabasalts feature a vector from the MORB-OIB array to the volcanic arc array in the Nb/Yb-Th/Yb diagram, similar to the Archean intraoceanic arc-basin systems. The geochemical pattern of the eastern and western metabasalts in the Zr-Zr/Y diagram suggests volcanism in nearby island arc and back-arc basin settings, respectively. The 2595 ± 21 Ma U-Pb zircon crystallization age of the metadacite allowed the determination of the timing of volcanism in the MNGB. Therefore, an intraoceanic provenance in an arc-basin system is proposed for the MNGB in the Neoarchean, which was later compressed between cratonic blocks during the Rhyacian-Orosirian event.