U-Pb LA-MC-ICPMS geochronology of Cambro-Ordovician post-collisional granites of the Ribeira belt, southeast Brazil: Terminal Brasiliano magmatism in central Gondwana supercontinent

Abstract

Abstract New U-Pb ages for zircon and titanite obtained by LA-MC-ICPMS are reported for post-collisional granites from the central Ribeira belt (Rio de Janeiro State, southeast Brazil). These post-collisional, I-type, megaporphyritic and equigranular leucogranite plutons and dykes intrude high-grade metasedimentary units, orthogneisses, and migmatites within the root zone of the deeply eroded Neoproterozoic-Cambrian Ribeira belt. The ages obtained are: 511.2 ± 6.9 Ma (zircon) for the Surui Granite; 490.3 ± 8.7 Ma (zircon) for a cross-cutting dyke of the pegmatitic facies of the Andorinha Granite from the same outcrop; 480.7 ± 6.1 Ma (zircon) for the Frades Granite; 488.7 ± 4.2 Ma (titanite) for the Nova Friburgo Granite; and 490.9 ± 9.8 Ma (zircon) for the Sana Granite. These new U-Pb ages and those compiled from the literature for post-orogenic intrusions distributed ∼400 km along the strike of the orogen (Rio de Janeiro and Espirito Santo States), define two separate intervals for magmatic activity, which are consistent with mineralogical and structural signatures. The magmatic intervals consist of an older Cambrian magmatic pulse occurred at ca.512 Ma (Pedra Branca, Surui and Buarama plutons), and a younger Ordovician event at ca. 486 Ma (Mangaratiba, Favela, Andorinha, Frades, Nova Friburgo and Sana granites). The Cambrian pulse post-dates the end of the first and main collisional phase by ca. 35 m.y. It also post-dates the onset of the second collisional episode by ca. 20 m.y. The late-Ordovician magmatic pulse post-dates the end of the second collisional episode by ca. 25 m.y. In map view, the alignment of the post-collisional plutons and stocks depicts a sinuous belt running along the eroded roots of central/northern Ribeira and Aracuai belts. This granite belt probably marks the zone where preferential heating and melting of lower continental crust took place, either caused by breaking off of subducted slab, or by the extensional collapse of hot, overthickened continental crust.