U-Pb and Hf isotope study on detrital zircons from the Paranoá Group, Brasília Belt Brazil: Constraints on depositional age at Mesoproterozoic - Neoproterozoic transition and tectono-magmatic events in the S?o Francisco craton

摘要

The eastern portion of the Brasilia belt, part of the Neoproterozoic Tocantins Province in central Brazil, is mainly constituted by metasedimentary rocks that partially represent the evolution of a passive margin developed during the Meso-Neoproterozoic in the western margin of the S?o Francisco paleocontinent. New in situ U-Pb and Lu-Hf isotope analyses on detrital zircons by LA-MC-ICP-MS were obtained for clastic sedimentary rocks of the Paranoá Group, considered to be the oldest unit of the this sedimentary sequence. The U-Pb data define a maximum depositional age of 1.54Ga and a minimum depositional age of ~1.04Ga, the latter obtained on diagenetic xenotime overgrowth on zircon grains. Combined U-Pb and Lu-Hf data permitted to reconstruct the tectonomagmatic evolution of the potential source areas identified in the S?o Francisco craton. Two zircon populations at 1.54 and 1.78Ga, with e{open} _(Hf)(t) values from +5.1 to -5.5, are possibly related with the Mesoproterozoic extensional magmatism registered in the Arai and Espinha?o rifts, located in the Brasilia belt and in the S?o Francisco craton respectively. The most important Paleoproterozoic zircon population, at 2.0-2.25Ga, shows positive e{open} _(Hf)(t) (0 to +8) and Paleoproterozoic T DM values indicating a continuous long-period of magmatism with generation of juvenile crust during the Paleoproterozoic. Two minor zircon populations, at 2.1 and 2.15Ga, showing negative e{open} _(Hf)(t) values, 0 to -10.1, and T _(DM) values between 2.65 and 3.22Ga may represent arc related magmatism, with reworking of Archean crust, during the evolution of an active margin of the S?o Francisco craton. A third minor zircons population, at 2.05Ga, with strongly negative e{open} _(Hf)(t) values, between -8.6 and -17.7, and T _(DM) values between 2.93 and 3.46Ga, may represent a collisional magmatism that characterized the final stage of an active margin evolution with reworking of Early Archean crust. Several minor zircon populations of Archean ages (2.7, 2.85, 3.1 and 3.42Ga), derived from the Archean nuclei in the interior of the S?o Francisco craton, suggest the presence of different magmatic pulses that repeatedly reworked a juvenile Paleoarchean crust. The oldest event of generation of juvenile crust is registered by zircons with U-Pb age of 3.42Ga, positive e{open} _(Hf)(t) values of 3.0 and T _(DM) of 3.54Ga.