Produção Nacional
Titanita De Granitos Arqueianos E Paleoproterozóicos Da Província De Carajás, Cráton Amazônico: Variações De Composição E Sua Relação Com a Fugacidade Do Oxigênio
2023; RELX Group (Netherlands); Linguagem: Inglês
10.2139/ssrn.4348863
ISSN1556-5068
AutoresIngrid Roberta Viana da Cunha, Roberto Dall’Agnol, Bruno Scaillet, Caio José Soares Mesquita, Arthur Santos da Silva Neri,
Tópico(s)Paleontology and Stratigraphy of Fossils
ResumoChemomineralogical studies on titanite of reduced to oxidized Neoarchean granites of the Planalto and Vila Jussara suites and oxidized Paleoproterozoic rocks of the Jamon Suite of the Carajás Province in the Amazonian Craton, were performed via optical microscopy, scanning electron microscopy and electron microprobe, revealing remarkable textural and compositional variations in this mineral. The Fe/Al ratios of titanite are quite variable in the granites studied. Three groups of titanites were distinguished: 1) high Fe/Al ratio (Fe/Al>0.5); 2) moderate Fe/Al ratio (0.5≤Fe/Al≥0.25); and 3) low Fe/Al ratio (Fe/Al<0.25). The titanites of the granites of the Vila Jussara Suite show marked variations in the Fe/Al ratio and are distributed inall three groups, while those of the Planalto Suite are located only in the groups with moderate and low Fe/Al ratio. In turn, the titanites of the Jamon Suite granites show great uniformity and are marked by high Fe/Al ratios, except in the associated dikes that show variations for in the moderate ratio group. A clear relationship was observed between the nature of the titanium-bearing minerals, textural and compositional variations in the titanite and oxygen fugacity. The granites of the Vila Jussara and Planalto suites formed under strongly reduced conditions exhibit a predominance of titanites involving ilmenites, with a corona texture and irregular zoning. In these same suites, with a slight increase in oxygen fugacity for moderately reduced conditions, in addition to the corona texture, the titanites form isolated crystals that differ because they are subhedral and without marked zoning in the Vila Jussara Suite, while in the Planalto Suite, they are euhedral with regular oscillatory zoning. Neoarchean granites formed under oxidizing conditions occur only in the Vila Jussara suite and in its oxidized ferroan variety, titanite appears as isolated subhedral crystals or in the form of fine-grained aggregates, while in the magnesian varieties, it forms subhedral to anhedral crystals, rarely fractured and may locally exhibit a corona texture involving magnetite. The titanites of the granites of the Jamon Suite, formed under oxidizing conditions, exhibit a textural aspect similar to that observed in the magnesian varieties of the Vila Jussara Suite. In general, there is a clear decrease in the Fe/Al ratio from the center to the edge of the crystals, although the zones are not regular. The titanites of the Archean magnesian granites and Paleoproterozoic ferroan granites formed under oxidizing conditions exhibit predominantly high Fe/Al ratios and correspond perfectly to the pattern generally observed in magmatic titanites. On the other hand, the reduced to moderately reduced varieties of Archean granites have moderate to low Fe/Al ratios in titanite, even in euhedral titanites with regular oscillatory zoning. This textural aspect indicates that the titanite present there is also of magmatic origin and not metamorphic or hydrothermal. A geobarometer of Al in titanite was tested, which was more coherent for granites formed under oxidizing conditions, providing pressures of ~300 MPa in the titanites of the magnesian varieties of Vila Jussara and in the oxidized ferroan Jamon granite. However, the pressures obtained using this geobarometer were inconsistent in the titanites of the Neoarchean granites formed under reduced conditions. In general, the data obtained corroborate the trend of a greater stability
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