Browsing by Author "Araujo, Cristina Santos"

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    Zircon in emplacement borders of post-collisional plutons compared to country rocks : a study on morphology, internal texture, U–Th–Pb geochronology and Hf isotopes (Araçuaí orogen, SE Brazil).
    (2020) Araujo, Cristina Santos; Soares, Antônio Carlos Pedrosa; Lana, Cristiano de Carvalho; Dussin, Ivo Antonio; Queiroga, Gláucia Nascimento; Serrano, Paula; Medeiros Júnior, Edgar Batista de
    Zircon is a powerful tool to study the internal evolution of igneous bodies and their interactions with country rocks. At pluton borders, zircon may record the emplacement history from the crystallization onset to deuteric processes, as well as inheritance from country rocks. We present a detailed morphology and internal structure study coupled with isotopic analyses (UeThePb and LueHf) on a great number of zircon grains extracted from samples collected at the borders of three distinct post-collisional intrusions of the Araçuaí orogen: granites from the Arace^ e Pedra Azul and Vitoria plutons, and a tonalite from the Mestre Alvaro pluton. For comparison, we also present mineral and bulk-rock chemistry data from these samples of post-collisional intrusions, as well as zircon UePb-Hf data from their country rocks (the Nova Venecia migmatitic paragneisses and Atal eia granites) and a wide dataset compilation. Zircon saturation geothermometry suggests igneous temperatures above 800 C for pluton borders. Zircon geochronology resulted in crystallization ages for borders of the plutons at 523 ± 2 Ma (Arace^ e Pedra Azul), 505 ± 1 Ma (Vitoria), and 527 ± 2 Ma (Mestre Alvaro). Lu eHf data (Arace^ e Pedra Azul pluton: εHf(t) 18.6 to 23.8, TDM ages from 2.25 to 2.47 Ga; Vitoria pluton: εHf(t) 7.4 to 10.3, TDM ages from 1.58 to 1.71 Ga; Mestre Alvaro pluton: εHf(t) 0.7 to 8.8, TDM ages from 1.27 to 1.66 Ga; Nova Venecia migmatitic paragneiss: εHf(t) þ4.1 to 39.2, TDM ages from 1.20 to 3.47 Ga; and Ataleia granite: εHf(t) 3.2 to 8.1, TDM ages from 1.42 to 1.64 Ga) indicate involvement of country rocks in the petrogenesis of post-collisional intrusions. Together, new and compiled data suggest: i) magma hybridization at high temperature, involving country rocks; ii) rapid growth of zircon crystals probably at rapid cooling rates; and iii) in situ dissolutionerecrystallization and overgrowth processes in zircon crystals in response to interactions with residual (late-stage) melts and/or deuteric fluids.
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    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 Pedrosa
    Unraveling 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.