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Previous issue date: 2017-07-03 === Os dep?sitos silicicl?sticos da Forma??o Tombador, unidade basal do Grupo Chapada
Diamantina, de idade mesoproteroz?ica, ocorrem numa faixa aproximadamente Norte-Sul ao
longo da Serra do Sincor?, por??o central do Cr?ton S?o Francisco (CSF), Nordeste do Brasil.
Esses dep?sitos s?o constitu?dos por uma espessa sequ?ncia de rochas silicicl?sticas.
Realizou-se um estudo petrogr?fico e petrol?gico de se??es delgadas de rochas aflorantes da
Fm. Tombador, juntamente com o aux?lio de t?cnicas especiais como a Fluoresc?ncia de
Raios X (FRX), Difratometria de Raios X (DRX), Espectroscopia de Raios X por Dispers?o
de Energia (EDS) e Microscopia Eletr?nica de Varredura (MEV). Com base nas an?lises,
verificou-se que essas rochas s?o constitu?das predominantemente por metarenitos. Os
seguintes eventos diagen?ticos foram identificados: infiltra??o mec?nica de argilas,
compacta??o mec?nica, compacta??o qu?mica, sobrecrescimento de quartzo,
sobrecrescimento de turmalina, fraturamento de gr?os, precipita??o de ?xido/hidr?xidos de
ferro, dissolu??o de gr?os, cauliniza??o e ilitiza??o. Foram identificados atrav?s da FRX em
amostra total, os seguintes elementos qu?micos em ordem decrescente, sil?cio (Si), alum?nio
(Al), pot?ssio (K), ferro (Fe), magn?sio (Mg), tit?nio (Ti), zirc?nio (Zr), estr?ncio (Sr) e ?trio
(Y). A alta concentra??o do sil?cio (Si), acima de 90% ? decorrente da abund?ncia do quartzo
(Q95F3FR2, segundo Folk, 1974), sendo importante indicador da alta maturidade mineral?gica
das rochas estudadas. Atrav?s da t?cnica de DRX, foram identificados os seguintes grupos de
argilominerais: ilita, caulinita, esmectita e interestratificados de ilita/esmectita. Embora as
caracter?sticas das amostras reflitam as condi??es pret?ritas de ambientes diagen?ticos, as
medidas do ?ndice de cristalinidade da ilita ou ??ndice de Kubler? (KI), e a presen?a do
mineral pirofilita s?o suficientes para demonstrar que as rochas da ?rea de estudo foram
afetadas pelo anquimetamorfismo, indicativo da transi??o entre a f?cies prehnita-pumpelita
superior e a f?cies xisto verde inferior, e que as condi??es de soterramento atingiram a
profundidade da ordem de 10-12 km e temperaturas da ordem de ?300 0C. A ocorr?ncia de
determinados argilominerais (ilita, caulinita e esmectita), o alto teor de quartzo, os baixos
teores de feldspato e fragmentos de rochas est?o diretamente relacionado aos sistemas
deposicionais, mostrando que a composi??o do arcabou?o ? diretamente respons?vel pela
evolu??o diagen?tica dessas rochas. === 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 Sincor?,
central portion of the S?o 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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