Navegando por Autor "PONTES, Gabriel Silva de Araujo"

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Geocronologia e geoquímica isotópica Pb--Sr-Nd das rochas vulcano-plutônicas paleoproterozóicas, porção sul do Cráton Amazônico, região de Vila Mandi (PA).
(Universidade Federal do Pará, 2024-06-01) PONTES, Gabriel Silva de Araujo; FERNANDES, Carlos Marcello Dias; http://lattes.cnpq.br/0614680098407362; https://orcid.org/0000-0001-5799-2694; GALARZA TORO, Marco Antonio; http://lattes.cnpq.br/8979250766799749; https://orcid.org/0000-0002-7271-4737
The Amazonian Craton is in the South American Platform and is one of the most important Precambrian terrains in the world. This vast tectonic unit is covered by Paleoproterozoic volcanic rocks, both effusive and explosive, which extend over an area of approximately 1,500,000 km2. Near the Vila Mandi district, in the southernmost part of São Félix do Xingu municipality (PA), this activity is represented by predominantly felsic volcanicplutonic rocks, with some intermediate lithotypes, and associated sediments belonging to the Cinco Estrelas and Vila Mandi formations. These relatively understudied sequences can be compared to rocks from other parts of the Amazonian Craton and placed within the geological context of the Uatumã event, which is present in various regions of the craton. Therefore, the purpose of this study is to provide information on the petrographic, geochemical, and geochronological characteristics of the volcanic-plutonic system present in the region. This study combines petrographic and isotopic geochemical analyses of Pb-Sr-Nd in the Paleoproterozoic volcanic-plutonic units in the southern region of the Amazonian Craton, particularly in the Vila Mandi area (PA). Among the studied units, the lower unit known as the Cinco Estrelas Formation consists of volcanic rocks ranging from basic to intermediate nature and is divided into two distinct facies: lava flows and volcaniclastic. Petrographic and chemical analysis of the lava flow facies reveals a variation in composition, ranging from andesitic to rhyolitic, with porphyritic textures in some cases. The volcaniclastic facies is characterized as an ash tuff composed of angular to subangular fragments of plagioclase, quartz, opaque minerals, and alkali feldspar, enveloped by a fine matrix composed of quartz-feldspar and opaque minerals. The upper unit, called the Vila Mandi Formation, consists of more evolved rocks formed in a tectonic environment like the lower unit. These rocks vary in composition from rhyodacites to rhyolites, with only one sample showing an alkaline-rhyolite composition. The Vila Mandi Formation is divided into four distinct facies: lava flows, ignimbrites, inequigranular granitoids, and granitic porphyritic dikes. Petrographic and geochemical analyses indicate that the rocks from both formations exhibit compositional similarities with other magmatic events belonging to the Uatumã event present throughout the Amazonian Craton. The Rb-Sr ages of the rhyolites from the Vila Mandi and Cinco Estrelas formations are approximately 1871±47 Ma and 1880±43 Ma, respectively. The initial 87Sr/86Sr ratio of the Cinco Estrelas Formation is 0.7054, while that of the Vila Mandi Formation is 0.7034. Additionally, the rhyolites from the Vila Mandi Formation exhibit an isochronous Pb-Pb age of 1875±17 Ma. The (87Sr/86Sr)i values of the rocks from both units range from 0.7026 to 0.7059, while the µ values range from 10.6 to 11.0, indicating a contribution from the upper crust with U enrichment. The positive εSr(t) values range from +5 to +53, suggesting a strong crustal contribution to the formation of the units, supported by the negative εNd(t) values ranging from –5.35 to –7.86. The Nd-TDM model ages range from 2.62 to 2.86 Ga, indicating a Neoarchean source for both formations. The Pb-Pb isotopic compositions of the Vila Mandi Formation indicate a Pb source in the upper crust with a significant orogenic Pb component. When comparing the Pb-Sr-Nd isotopic signatures of the Vila Mandi and Cinco Estrelas formations in the Vila Mandi region with other formations inserted in the Uatumã magmatism, a strong correlation between these units in these regions is suggested. The ages obtained in this study can also be correlated with the volcanic-plutonic rocks of the Uatumã event in the regions of São Félix do Xingu, Tapajós, and Pitinga, providing data that can support a solid correlation and a similar origin between the units in the Vila Mandi region and those present in the Amazonian Craton.