Estudo petrográfico e petrológico da Formação Tombador, Mesoproterozóico da Chapada Diamantina, Bahia
The siliciclastic deposits of the Tombador Formation, basal unit of the Chapada Diamantina Group, of mesoproterozoic age, occur in a north-south range along the Serra do Sincorá, central portion of the São Francisco Craton (CSF), Northeast Brazil. These deposits consist of a thick sequence of sil...
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| Formato: | Dissertação |
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| Endereço do item: | https://repositorio.ufrn.br/jspui/handle/123456789/23876 |
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| Resumo: | The siliciclastic deposits of the Tombador Formation, basal unit of the Chapada Diamantina
Group, of mesoproterozoic age, occur in a north-south range along the Serra do Sincorá,
central portion of the São Francisco Craton (CSF), Northeast Brazil. These deposits consist of
a thick sequence of siliciclastic rocks. A petrographic and petrological study of thin sections
of outcropping rocks of Fm. Tombador was carried out, along with the help of special
techniques such as X-Ray Fluorescence (FRX), X-ray Diffraction (XRD), X-ray Spectroscopy
by Energy Dispersion (EDS) and Scanning Electron Microscopy (SEM). Based on the
analyzes, it was verified that these rocks are constituted predominantly by meta-sandstones.
The following diagenetic events were identified: mechanical infiltration of clays, mechanical
compaction, chemical compaction, quartz overgrowth, tourmaline overgrowth, grain
fracturing, iron oxide/hydroxide precipitation, grain dissolution, kaolinization, ilitization and
mineral pyrophyllite that reflects the transition stages between diagenesis and
anchemimorphism. The following chemical elements in descending order, Si, Al, K, Fe, Mg,
Ti, Zr, Sr and Y were identified by X - ray fluorescence in total sample, with values above
85%, together with the high amount of Quartz (Q95F3FR2, according to Folk, 1974), indicate
the high mineralogical maturity of these rocks. Through the XRD technique, the following
groups of clay minerals were identified: illite, kaolinite, smectite, interstratified of
illite/smectite and the mineral pyrophyllite. Although the characteristics of the samples reflect
the previous conditions of diagenetic environments, measurements of the illite crystallinity
index or "Kubler Index" (KI), and the presence of the mineral pyrophyllite are sufficient to
demonstrate that the rocks of the study area were affected by anchemimorphism, indicative of
the transition between the prehnita-pumpellyite facies and the below greenschist facies, and
that burial conditions reached a depth of the order of 10-12 km and temperatures in the order
of ± 300 ° C. The occurrence of certain clay minerals is directly related to the depositional
system, showing that the composition of the framework are directly responsible for the
diagenetic evolution of these rocks. |
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