Caracterização cristalográfica e isotópica do mineral zircão através do método de traços de fissão, U-Pb e U-Th/He: geocronologia da província Borborema, RN, Brasil
Data
2021-07-22
Autores
Azevedo, Murilo Candido de
Título da Revista
ISSN da Revista
Título de Volume
Editor
Universidade Estadual Paulista (Unesp)
Resumo
Este estudo analisa a geocronologia e cristalografia de amostras de zircão coletadas em diferentes domínios tectônicos na parte Norte da Província Borborema: São José do Campestre, Rio Piranhas-Seridó, Jaguaribeano, Ceará Central e Médio Coreaú; localizados no estado do Rio Grande do Norte. Ao todo foram datadas 17 amostras por diferentes métodos: Método de Traços de Fissão por Detector Externo (MTF-MDE), MTF via LA-ICP-MS, cronometria U-Pb e cronometria U-Th/He. Na caracterização cristalográfica, técnicas como espectroscopia micro-Raman, Microscopia Eletrônica de Varredura (MEV) com Energy dispersive spectroscopy (EDS) e backscattering (BDS) e Microscopia Óptica (MO) foram realizadas em diferentes tipos de grãos de zircão para se estudar a anisotropia na revelação de traços, que pode apresentar várias causas. Estudos de espectrometria Raman e MEV foram feitos de forma a analisar o efeito de inclusões minerais na estrutura cristalina do zircão, que influenciam na revelação de traços e na intensidade dos picos de espectro Raman. A partir da análise por Microscopia Óptica (MO) de mais de 1000 grãos, uma nova classificação de grãos em relação à revelação de traços é proposta como complemento da clássica (homogêneo, heterogêneo, híbrido e anômalo), na qual se considera fatores como integridade e danificação da superfície, irregularidade de traços e camadas de crescimento do grão. Foi analisada a porcentagem de cada tipo de grão em amostras padrão em comparação com as amostras da Província Borborema. Os avanços metodológicos explicados acima foram utilizados para determinar as idades via MTF e U-Th/He. Esses dados junto com os dados de idades U-Pb e Ar-Ar da literatura foram utilizados para determinar a história térmica da Província Borborema. As idades obtidas via MTF indicam que em alguns domínios (mais enfaticamente, em algumas rochas plutônicas) da Província Borborema podem ter mais do que uma população de idades. Isso poderia ser explicado a partir do metamorfismo dessa região ou a partir da fusão de plutões com diferentes taxas de resfriamento após o choque dos microcontinentes. Os resultados tanto das idades como das histórias térmicas poderiam explicar a anomalia topográfica da Província Borborema.
This study analyzes the geochronology and crystallography of zircon collected in different tectonic domains in the northern part of the Borborema Province: São José do Campestre, Rio Piranhas-Seridó, Jaguaribeano, Ceará Central and Médio Coreaú; located in the state of Rio Grande do Norte. Altogether, 17 samples were dated by different methods: Fission Track Method by External Detector (MTF-MDE), MTF via LA-ICP-MS, U-Pb chronometry and U-Th/He chronometry. In crystallographic characterization, techniques such as micro-Raman spectroscopy, Scanning Electron Microscopy (SEM) with energy dispersive spectroscopy (EDS) and backscattering (BDS) and Optical Microscopy (OM) were grouped into different types of zircon grains to study anisotropy in revealing of tracks, which can have several causes. Raman spectrometry and SEM studies were carried out in order to analyze the effect of mineral inclusions on the crystal structure of zircon, which influences the development of tracks and the intensity of the Raman spectrum peaks. From optical microscopy analysis of more than 1000 grains, a new classification of grains in relation to the revelation of tracks is proposed as a complement to the classic one (homogeneous, heterogeneous, hybrid and anomalous), which considers factors such as integrity and damage to the surface, irregularity of tracks and layers of grain growth. The percentage of each type of grain in standard samples was analyzed in comparison with that of Borborema Province. The methodological advances explained above were used to determine ages via MTF and U-Th/He. These data together with the U-Pb and Ar-Ar age data from the literature were used to determine the thermal history of Borborema Province. Ages defined via MTF indicate that in some domains (most emphatically, in some plutonic rocks) of Borborema Province may have more than one population of ages. This could be explained from the metamorphism of this region or from the fusion of plutos with different cooling rates after the impact of microcontinents. The results of both ages and thermal histories could explain the topographical anomaly of Borborema Province.
This study analyzes the geochronology and crystallography of zircon collected in different tectonic domains in the northern part of the Borborema Province: São José do Campestre, Rio Piranhas-Seridó, Jaguaribeano, Ceará Central and Médio Coreaú; located in the state of Rio Grande do Norte. Altogether, 17 samples were dated by different methods: Fission Track Method by External Detector (MTF-MDE), MTF via LA-ICP-MS, U-Pb chronometry and U-Th/He chronometry. In crystallographic characterization, techniques such as micro-Raman spectroscopy, Scanning Electron Microscopy (SEM) with energy dispersive spectroscopy (EDS) and backscattering (BDS) and Optical Microscopy (OM) were grouped into different types of zircon grains to study anisotropy in revealing of tracks, which can have several causes. Raman spectrometry and SEM studies were carried out in order to analyze the effect of mineral inclusions on the crystal structure of zircon, which influences the development of tracks and the intensity of the Raman spectrum peaks. From optical microscopy analysis of more than 1000 grains, a new classification of grains in relation to the revelation of tracks is proposed as a complement to the classic one (homogeneous, heterogeneous, hybrid and anomalous), which considers factors such as integrity and damage to the surface, irregularity of tracks and layers of grain growth. The percentage of each type of grain in standard samples was analyzed in comparison with that of Borborema Province. The methodological advances explained above were used to determine ages via MTF and U-Th/He. These data together with the U-Pb and Ar-Ar age data from the literature were used to determine the thermal history of Borborema Province. Ages defined via MTF indicate that in some domains (most emphatically, in some plutonic rocks) of Borborema Province may have more than one population of ages. This could be explained from the metamorphism of this region or from the fusion of plutos with different cooling rates after the impact of microcontinents. The results of both ages and thermal histories could explain the topographical anomaly of Borborema Province.
Descrição
Palavras-chave
Caracterização do Mineral Zircão, Método de Traços de Fissão, Sistemática U-Pb, Geocronologia, Província Borborema, Zircon Mineral Characterization, Fission Track Method, U-Pb systematics, Geochronology, Borborema Province