DEGEO - Departamento de Geologia
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8
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Item Zircon petrochronology reveals the moderately juvenile signature of a diatexite from the boundary one between the Brasília and Ribeira orogens (SE Brazil) : relict of a Tonian arc?(2022) Costa, Fernanda Moura; Penna, João Lucas Andrade; Martins, Lorena Cristina Dias; Tedesch, Mahyra; Novo, Tiago Amâncio; Araujo, Cristina Santos; Vieira, Pedro Leonardo Rossi; Lana, Cristiano de Carvalho; Soares, Antônio Carlos PedrosaUnraveling the tectonic evolution of Precambrian high-grade rocks is often challenging owing to widespread metamorphic overprint under high to ultra-high temperature conditions. In this scenario, isotopic systems-based investigations constitute an important tool to obtain the time constrains of formation and evolution of rocks.We present a petrochronological investigation using U–Pb and Lu–Hf data of zircon grains extracted from a biotitebearing granitic leucosome from a diatexite found in the boundary zone between the Brasília and Ribeira orogens, SE Brazil. Clusters of 176Hf/177Hf(t) ratio values characterize two distinct zircon data groups, suggesting at least two different crystallization stages. 176Hf/177Hf(t) ratios from group 1 range from 0.28230 to 0.28247 with εHf (t) between +7.54 and − 1.55, representing a moderately juvenile to evolved magma. This group constrains a time interval of ca. 890-540 Ma, and the ancient zircon core (890 Ma) could potentially record the minimum age for the magmatic crystallization of a moderately juvenile protolith, with ϵHf (890) ranging from +8.66 to +0.36. Excluding this ancient zircon (since it is the only date older than 800 Ma), the oldest grain provides a minimum crystallization age of ca. 797 Ma, with ϵHf (797) between +6.73 and -1.61. Group 2, exhibiting 176Hf/177Hf(t) ratios from 0.28214 to 0.28225and negative εHf(t) values (− 3.35 to − 7.53), represents xenocrysts of crustal origin from the protolith or an influx of allochthonous crustal melt during high-grade metamorphism. The age spreading from ca. 890 Ma to ca. 540 Ma along the concordia is consistent with a Pb-loss scenario, with cores locally providing younger ages than rims. This is also consistent with prolonged metamorphism, as newly crystallized domains also present a spread of ages. Our data indicate that the leucosome is the metamorphic product of a moderately juvenile rock. When placed in the regional context, the biotite-bearing granitic leucosome may represent the relic of a magmatic arc, now located in the boundary zone between the Brasília and Ribeira orogens. Its moderately juvenile signature suggests the following tectonic setting interpretations: (i) an intra-oceanic subduction setting similar to modern island arcs or an early stage of continental subduction, formed at ca. 890–800 Ma and, thus could be potentially correlated with juvenile magmatic arcs found in both Brasília (Mara Rosa – Arenopolis ´ arc system) and Ribeira (Serra da Prata – Rio Negro arc system) orogens, suggestive of a large ocean realm to the south of the Sao ˜ Francisco paleocontinental block in Tonian time; and (ii) the most primitive relic of an early stage of the continental magmatic arc installed on the active margin of the Paranapanema paleocontinent.Item Integrated ophiolite and arc evolution, southern Brasiliano Orogen.(2020) Alves, Tiara Cerva; Hartmann, Leo Afraneo; Remus, Marcus Vinicius Dorneles; Lana, Cristiano de CarvalhoIntegrated study of Tonian ophiolite mélanges and intra-oceanic arc of the São Gabriel juvenile terrane exposes a key geotectonic unit in the evolutionary history of Gondwana during Brasiliano Orogeny. LA-ICP-MS U-Pb geochronology in rutile and zircon were combined with Lu-Hf and rare earth elements in zircon. We report the first rutile dating of ultramafic rock in the Brazilian Shield of 787.6 ± 2.6 Ma for the Cambaizinho ophiolite magnesian schist. Dating of rutile corresponds to the metamorphic age of the mantle section of the ophiolite. São Gabriel arc infrastructure is represented by the Cambaí Complex, including the Vila Nova Suite metatonalite with zircon U-Pb age of 724.6 ± 3.2 Ma and late Sanga do Jobim Suite monzogranite intrusions with two main pulses at 698.9 ± 4.2 Ma and 673.9 ± 6.8 Ma. Zircon εHf(t) values are positive for both rocks and range between +10.1 and +7.4 in metatonalite and +9.2 to +2.5 in monzogranite. Detrital zircon from metasedimentary rocks support superstructure arc investigation. Ages range from 817 to 650 Ma, with strong concentration between 750 and 730 Ma. Positive εHf(t) values and Rare Earth Elements (REE) signature point to main source of metasedimentary rocks in the arc, with ophiolitic contribution. Therefore, geotectonic processes in the Brasiliano Orogeny include the construction of São Gabriel terrane with Tonian oceanic crust evolving to intra-oceanic subduction until 650 Ma to final development along the margin of continental crust. Overthrusting of the São Gabriel terrane onto the Rio de La Plata Craton occurred at 650–620 Ma. Our data correlate with Tonian intra-oceanic arcs from Brasilia belt (Goiás arc ca. 862–630 Ma) and Ribeira belt (Serra da Prata arc – 856–838 Ma; Rio Negro arc – 790–620 Ma). Oceanic crust generation combined with subduction to form Tonian arcs along the Brasiliano Orogen were most significant for construction of West Gondwana.