Mineralogia de amígdalas de basaltos toleíticos mesozoicos do extremo Norte da Província Borborema, NE do Brasil

• Ano de defesa: 2020
• Autor(a) principal: Araújo, Lidyane Mayara Lima de
• Tipo de documento: Dissertação
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal do Rio Grande do Norte; Brasil; UFRN; PROGRAMA DE PÓS-GRADUAÇÃO EM GEODINÂMICA E GEOFÍSICA
• Palavras-chave em Português: Diques Rio Ceará-Mirim; Basaltos Serra do Cuó; Zeólitas; Fluidos meteóricos
• Link de acesso: https://repositorio.ufrn.br/handle/123456789/31689

Volcanic rocks, especially basalts, commonly have vesicles generated by the escape (expansion) of gases during the cooling process. These vesicles can be filled (turning into amygdales) by different mineral assemblages through the circulation of hydrothermal or meteoric fluids. The identification and characterization of these secondary assemblages, as well as their processes and formation conditions can provide important insights on the subsolidus history of volcanic rocks, such as the evolution and compositional and physical-chemical factors of the fluids involved, chemical balance and mineralizing processes. The northern portion of the Borborema Province was affected during the Mesozoic by an important continental tholeiitic volcanism related to the Gondwana breakup and the opening of the Equatorial Atlantic Ocean. This magmatism manifests itself mainly as the Rio Ceará-Mirim dike swarm (CMD) and the Serra do Cuó basaltic flow (SCB), whose basalts and diabases are often vesicular and amygdaloidal. The detailed characterization of the secondary/hydrothermal mineralogy in these rocks is still an unraveled research topic, therefore this work aims to fill this gap with a systematic mineralogical study of amygdales from selected samples of CMD and SCB volcanics, based on petrographic, X-ray diffraction (XRD), thermogravimetric (TGA/DTA) and mineral chemical (EDS) data. The results show that the CMD amygdales are predominantly monomineralic, filled with laumontite and, locally, quartz or calcite. Laumontite was probably formed at temperatures of ~125oC after a higher temperature (up to ~300oC) hydrothermal event that changed the primary mineralogy to mafic phyllosilicates. Infillings in the SCB amygdales are polymineralic, defined by the initial low-temperature (~150oC) formation of nontronite/saponite on the cavity walls, followed by the precipitation of natrolite and/or Na-Ca phillipsite, reflecting changes in compositional and temperatures (up to ~250oC) of the meteoric fluids from which these phases have precipitated. A late low-temperature stage is marked by the alteration of zeolites to clay minerals. It is inferred that the needed chemical components for the formation of these mineral assemblages result from the destabilization and alteration of interstitial volcanic glass, primary mafic minerals and plagioclase caused by the percolation of meteoric fluids within the rocks during their cooling. The observed mineralogical variability in the CMD and SCB amygdales points to contrasting compositions of these fluids which ultimately reflect the compositional (chemical and mineralogical) differences between the two volcanisms, as well as external contributions.