DEGEO - Departamento de Geologia
URI permanente desta comunidadehttp://www.hml.repositorio.ufop.br/handle/123456789/8
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Resultados da Pesquisa
Item Evidence for change in crust formation process during the Paleoarchean in the São Francisco Craton (Gavião Block) : coupled zircon Lu-Hf and U-Pb isotopic analyses and tectonic implications.(2022) Santos, Cláudia dos; Zincone, Stéfano Albino; Queiroga, Gláucia Nascimento; Bersan, Samuel Moreira; Lana, Cristiano de Carvalho; Oliveira, Elson Paiva deThe continental crust growth/evolution processes and the tectonic regime through Eo- and Paleoarchean times are enigmatic due to the scarcity of preserved crust. The Gaviao ̃ Block, S ̃ ao Francisco Craton (Brazil), contains exposed and well-preserved Eo-Paleoarchean crust remnants, providing a rare opportunity to investigate these issues. Here, we describe new U-Pb ages and Hf isotope data of 3.51–3.4 Ga tonalites and diorites from the Gaviao ̃ Block and compare these with previously published Lu-Hf data from Hadean/Eo-Paleoarquean zircons from the Gavi ̃ ao Block and other primitive cratons. The Eo- to Paleoarchean evolution of the Gaviao ̃ Block is registered by ca. 360 Myr of continuous magmatic events from ca. 3.66 Ga to 3.30 Ga. From the available Hf data, we interpret that each of the events younger than 3.6 Ga registers a new juvenile addition that assimilated older crust, whereas the rocks older than 3.6 Ga are exclusively formed through the reworking of a Hadean, and to a less extent early Eoarchean crust. The shift in the crust generation process with the input of juvenile material into the Gavi ̃ ao Block has been documented within the ~ 3.8–3.5 Ga time in other primitive cratonic complexes including the Wyoming, Pilbara, Kaapvaal, Slave, Singhbhum, and Yilgarn. As documented in these other cra- tons, our data suggest that a shift in the Hf isotope record to rocks younger than 3.6 Ga reflects a transition from stagnant-lid to mobile-lid tectonics in the crust formation process of the S ̃ ao Francisco Craton. This change in the geodynamic regime appears to have been global at ca. ~ 3.8–3.5 Ga and facilitated the extraction of juvenile melts, crustal reworking, evolved magmatism, and the production of stabilizing melt-depleted lithospheric mantle.Item Birthplace of the São Francisco craton, Brazil : evidence from 3.60-3.64 Ga gneisses of the Mairi gneiss complex.(2020) Oliveira, Elson Paiva de; McNaughton, Neal Jesse; Zincone, Stéfano Albino; Talavera, CristinaRecords of Earth's primitive crust are scarce. Eoarchean (older than 3.6 Ga) banded mafic to felsic gneisses have been discovered in the São Francisco Craton, Brazil, pushing back by over 100 million years the oldest gneisses known to date in South America (3.5 Ga). Zircon U-Pb data yield rock ages from 3598 Ma to 3642 Ma with a few ca. 3.65 to 3.69 Ga grains suggesting even older rocks in the area. Zircon grains show significantly negative to nearly chondritic initial εHf values and two-stage model ages from 3.82 Ga to 4.33 Ga, which may indicate the existence of a recycled Hadean to early Eoarchean crust in the region. The felsic gneisses are chemically similar to the low-pressure Tonalite-Trondhjemite-Granodiorite association whereas the mafic gneisses have geochemical signatures that resemble within-plate basaltic andesite to andesite of Iceland (icelandites). The results are relevant to constrain the composition of Earth's first continental crust.Item High-K granites between the Archean Gavião and Jequié blocks, São Francisco craton, Brazil : implications for cratonization and amalgamation of the Rhyacian Atlantica continent.(2021) Zincone, Stéfano Albino; Oliveira, Elson Paiva de; Ribeiro, Bruno Pinto; Marinho, Moacyr MouraPrecambrian terrane boundaries are sites of diverse rock associations and structural complexity. Motions along the boundary zones are commonly related to the construction of ancient large continental masses. This seems to be the case in the Sao ˜ Francisco Craton, Brazil, where three major blocks (Gavião, Jequié and Serrinha) collided in the Paleoproterozoic to form the northern part of the craton. This manuscript presents zircon U-Pb ages and whole-rock geochemistry for rock associations on the western border of the Jequié Block, which is defined by the north-trending left-lateral 600-km long Contendas-Jacobina fault zone. The studied rocks are granulite to amphibolite facies Neoarchean orthopyroxene-hornblende granite gneisses of the Jequié block, which also occur tectonically imbricated within Paleoproterozoic metavolcanosedimentary rocks of the Contendas-Mirante belt. The rocks are 2.65-2.63 Ga high-K ferroan, metaluminous to slightly peraluminous syeno-monzogranite in composition, with negative ƐNd (2650) and Nd model ages of 3.05-2.88 Ga. They are depleted in Sr and Ti, enriched in Th, Ba and Pb enrichment, and have marked negative Eu/Eu*. This geochemical signature coupled with the Nd-isotope data indicate that the parental granitic magma was generated by crustal reworking of felsic Mesoarchean crust distinct from the western-sitting >3.3 Ga Gavião Block. The investigated granitic gneisses are similar to other potassic granitoids that occur in the São Francisco Craton, which mark the thermal stabilization of the Archean lithosphere. The westward thrusting of the Neoarchean hornblende granitic gneiss and associated high-Mg tremolitite (meta-basalts) onto Paleoproterozoic volcano-sedimentary sequences establishes field evidence of Paleoproterozoic nappe stacking before lateral displacement during block collision. The crustal amalgamation is related to the assembly of the São Francisco-Congo Craton, a major landmass established before the Columbia supercontinent.Item Detrital zircon U-Pb ages as evidence for deposition of the Saúde Complex in a Paleoproterozoic foreland basin, northern S~ao Francisco Craton, Brazil.(2017) Zincone, Stéfano Albino; Barbuena, Danilo; Oliveira, Elson Paiva de; Baldim, Maurício RigoniThe Archean and Paleoproterozoic terrane of the northeastern part of the S~ao Francisco Craton shows evidence of a prolonged accretionary history. A major boundary within this portion of the craton is the Contendas-Jacobina lineament, a 600 km-long north-trending left-lateral thrust fault that separates major Archean blocks amalgamated during the Paleoproterozoic orogeny. Different supracrustal sequences occur along this lineament, but the tectonic imbrication erased the original stratigraphy, precluding the reconstruction of the sedimentary and tectonic evolution of the different groups. We investigated the sediment provenance and tectonic setting of the Saúde Complex, located in the northern part of the Contendas-Jacobina lineament and previously taken as part of the Bahia Gold Belt. We found that the Saúde Complex is part of a Paleoproterozoic foreland basin and correlates with the Contendas- Mirante Basin in the south. The correlation with the western Au-bearing Jacobina Basin as a single Paleoproterozoic basin is no longer favorable. The Saúde Complex contains abundant 2.06e2.20 Ga and 2.50e2.68 Ga detrital zircons mainly derived from the eastern Itabuna-Salvador-Curaç a Orogen and reworked Neoarchean rocks within the orogen, mainly the Caraíba magmatic arc.Item Field and geochronological evidence for origin of the Contendas-Mirante supracrustal Belt, São Francisco Craton, Brazil, as a Paleoproterozoic foreland basin.(2017) Zincone, Stéfano Albino; Oliveira, Elson Paiva deThe low-grade Contendas-Mirante meta-volcano-sedimentary belt (CMB) was previously interpreted as an Archean greenstone belt with the uppermost formation deposited in the Paleoproterozoic. New zircon U-Pb LAICP- MS ages on sedimentary groups and granite sheets intrusive in the belt constrain the timing of basin development. Phyllites of the Lower Group and meta-arenites of the Upper Group yielded similar young zircon populations at ca. 2080 Ma, which are interpreted as the maximum deposition age for the entire sequence. The unique exception is the 3273–3338 Ma zircon-bearing quartzite that occurs in spatial association with the 3300 Ma rhyolite. A granite sheet intrusive into metagraywacke of the Lower Group constrains the minimum deposition age of 2045 ± 26 Ma for the CMB. Altogether four main age clusters of detrital zircon grains were recognized, namely 2075–2200 Ma, 2200–2440 Ma, 2500–2770 Ma, 3270–3380 Ma. These age intervals match the ages of igneous and metamorphic rocks in the different terranes that comprise the São Francisco craton. Maximum deposition age for both groups indicates that sedimentation occurred immediately prior the highgrade metamorphism in the adjacent Archean-Paleoproterozoic Jequié Block and Itabuna-Salvador-Curaçá Orogen. These observations, along with coarsening upwards in the sediments, indicate that the CMB represents a foreland basin developed along the eastern margin of the Gavião Block during plate convergence. Paleoarchean basement rocks were reactivated and emplaced into the CMB during basin inversion and is timely correlated with the granitic intrusion. Recognition of older supracrustal fragments is ambiguous, one main example is the 3273–3338 Ma zircon-bearing quartzite, which correlates with the northern Jacobina basin along the Contendas- Mirante lineament (e.g., Bahia gold belt) and may represent part of an Archean intracontinental rift sequence.Item 3.30 Ga high-silica intraplate volcanic–plutonic system of the Gavião Block, São Francisco Craton, Brazil : evidence of an intracontinental rift following the creation of insulating continental crust.(2016) Zincone, Stéfano Albino; Oliveira, Elson Paiva de; Laurent, Oscar; Zhang, Hong; Zhai, MingguoHigh-silica rhyolites having U–Pb zircon ages of 3303±11Ma occur along the eastern border of the Gavião Block (Brazil) associated with the Contendas-Mirante and Mundo Novo supracrustal belts. Unlike many Archean greenstone sequences, they are not interlayered with mafic to intermediate units. Instead, they belong to an inter-related plutonic–volcanic system, together with granitic massifs having similar zircon crystallization ages of ca. 3293 ± 3 Ma and 3328 ± 3 Ma and plotting along the same geochemical trends as the rhyolites. The rhyolites show well-preserved primary volcanic features such as magma flow textures and euhedral phenocrysts. High emplacement temperatures are indicated by petrographic evidence (β-quartz phenocrysts), zircon saturation temperatures (915–820 °C) and geochemical data, especially high SiO2 (74–79 wt.%) togetherwith elevated Fe2O3(T) (~3 wt.%), MgO (0.5–1.5 wt.%) and low Al2O3 (b11 wt.%). The rhyolites show homogeneous trace element ratios (La/YbN 4.8 ± 1.8; EuN/Eu* ~0.55; Sr/Y ~0.7) and negative ԐHf(3.3 Ga) from 0 to −7, indicating derivation from a single crustal source for both occurrences. Specifically, the rhyolites would have derived from extraction and eruption of highly silicic residual liquid formed by crystallization of granitic magma in a relatively shallow (b10 km) reservoir, now represented by the granite massifs. The granite magma was formed by melting or differentiation of material similar to the diorite gneiss that occurs regionally. The 3.30 Ga volcanic– plutonic systems formed after a period of crustal growth and stabilization of a thick continental lithosphere, represented by massive 3.40–3.33 Ga TTG and medium to high-K calk-alkaline magmatism in the Gavião Block. The 3.30 Ga-old rhyolites and graniteswould therefore have formed in an intracontinental tectonic setting after the formation and stabilization of newcontinental crust, and accordinglywould represent the first stages of rifting and continental break-up. Intraplate magmatism and intracrustal differentiation processes took place on Earth at 3.3 Ga and produced magmas that were distinct from Archean TTGs, questioning the reliability (or at least the uniqueness) of “intraplate models” to explain the origin of the latter.