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Título

INTEGRATED OPHIOLITE AND JUVENILE ARC EVOLUTION IN THE EARLY BRASILIANO OROGENY: AN EXAMPLE FROM THE SUL-RIOGRANDENSE SHIELD, BRAZIL

Texto do resumo

The rearrangement of continents resulted in opening and closure of oceans, subduction related to lithosphere consumption and crustal growth by emplacement onto continents. The integrated establishment of magmatic, metamorphic and metasomatic mechanisms provides a qualitative basis to understand the geological evolution of ophiolitic fragments and intra-oceanic arcs. Neoproterozoic rocks from the São Gabriel arc entrained within the Dom Feliciano Belt evolved similarly to modern-style plate tectonics. The studied ophiolites include the Cambaizinho, Palma and Palma Leste fragments, São Gabriel Terrane, Sul-Riograndense Shield. LA-ICP-MS U-Pb isotopes were obtained from zircon, garnet and rutile; Lu-Hf isotopes and trace element chemistry from zircon, and boron isotopes from dravite. Petrography, mineral chemistry and thermodynamic modeling allowed the interpretation of metamorphic conditions. Magmatic ages of 724.6 ± 3.2 Ma (Vila Nova Suite metatonalite), 731.6 ± 0.5 Ma (São Luís metagabbro), 698.9 ± 4.2 Ma and 673.9 ± 6.8 Ma (Sanga do Jobim Suite monzogranite) were recorded in samples representative of the infrastructure of the São Gabriel arc. Positive zircon εHf (t) values from metatonalite (+10.1 and +7.4), metagabbro (+11.1 and +7.1) and monzogranite (+9.2 and +2.5) indicate a depleted mantle source. Ages of the superstructure arc were acquired in zircon from metasedimentary rock samples, ranging from 817.0 to 702.5 Ma, with two peaks at 769.1 ± 4.6 Ma and 731.6 ± 2.2 Ma. The maximum depositional age of 739.04 ± 4.47 Ma was calculated from schist samples. Age, values of εHf(t), signature of rare earth elements of detrital zircon and positive δ11B in dravite (+0.51 to +3.39) suggest that the source of metasedimentary rocks was from the arc. The low amphibolite metamorphic facies is represented by the assemblage garnet + staurolite + biotite + muscovite + chlorite + plagioclase + ilmenite + quartz, formed under temperatures of 536-555 ºC and pressures of 4.5-5.1 kbar. U-Pb dating in garnet (721 ± 14 Ma, metasediment) and rutile (706 ±11 Ma, metagabbro) is considered a metamorphic age. U-Pb dating in rutile (787.6 ± 2.6 Ma, ultramafic rock) and zircon (889.2 ± 2.3 Ma, metasomatic chloritite), with positive εHf(t)zircon (+17.5 and +9.9) indicate metasomatism of the seafloor in the ophiolite mantle section. The complex multiphase geotectonic processes included the formation and metasomatism of the oceanic lithosphere precursor of the ophiolite and its obduction at ~887 and ~787 Ma, construction of the São Gabriel Terrane at ~817-725 Ma and metamorphism related to the orogeny at ~721-706 Ma. The present integrated ophiolite and arc evolution interpretation using specific features allows the perception of a key point in geological evolution. This study contributes with the understanding of the global mechanisms involving Rodinia break-up and the Brasiliano orogeny in Gondwana.

Palavras Chave

ophiolite; São Gabriel juvenile arc; U-Pb geochronology; boron isotope; Brasiliano orogeny