Browsing by Author "Faleiros, Frederico Meira"

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    Late Permian siliceous hot springs developed on the margin of a restricted epeiric sea : insights into strata-confined silicification in mixed siliciclastic‐carbonate successions.
    (2022) Varejão, Filipe Giovanini; Warren, Lucas Veríssimo; Alessandretti, Luciano; Rodrigues, Mariza Gomes; Riccomini, Claudio; Assine, Mario Luis; Cury, Leonardo Fadel; Faleiros, Frederico Meira; Simões, Marcello Guimarães
    Hot springs are sources of carbonate minerals in modern settings; however, few fossil structures are recorded in successions older than the Quaternary due to their enhanced erosional potential. >4500 siliceous mounds are recognized in a well-defined level from the upper part of the Permian Teresina Formation (Parana ́ Basin, SE Brazil). Additionally, a new mound level is here reported for the first time about 15 m below the main occur- rence. Remarkable lithological, mineralogical, paleontological and geochemical features characterize the mounds of both levels, pointing to a hydrothermal origin. Therefore, these structures are here called as hot springs. These were originated subaerially, on the margins of a closing epeiric sea developed on a large intra- continental sag basin. Hydrothermal fluids were sourced from deep circulating basin waters that erupted through intraplate deep-rooted faults. Our data suggests that these hot springs were active during distinct cycles of base level variations. Strata-confined silicification was enhanced during periods of high evaporation and hydrother- mal exudation. Finally, the vertical and lateral facies associations of the Permian hot spring succession are compared with modern sites, and implications for the recognition of syn-depositional hydrothermally precipitated silica in ancient sedimentary basins are discussed.
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    Strain localization and fluid-assisted deformation in apatite and its influence on trace elements and U–Pb systematics.
    (2020) Ribeiro, Bruno Vieira; Lagoeiro, Leonardo Evangelista; Faleiros, Frederico Meira; Hunter, Nicholas J. R.; Queiroga, Gláucia Nascimento; Raveggi, M.; Cawood, Peter Anthony; Finch, M.; Campanha, Ginaldo Ademar da Cruz
    This paper presents electron backscatter diffraction (EBSD), trace element and U–Pb data of apatite grains from a granitic mylonite from the Taxaquara Shear Zone (SE Brazil). The mylonite recrystallized under upper-greenschist facies and presents two types of apatite with distinct microstructures. Type1 apatite appears in quartz-rich layers and does not exhibit any microstructural, crystallographic, or chemical evidence of deformation/recrystallization, and resembles the original igneous apatite. Type2 apatite appears in mica-rich layers and exhibits core-and-mantle microstructures, and intragranular subgrain development, suggesting that they have undergone dynamic recrystallization. Recrystallized tails of type-2 apatite grains exhibit a strong c-axis crystallographic preferred orientation parallel to the X-direction (stretching lineation), and lack evidence of dislocation density. This evidence from type-2 apatite grains, combined with REE depletion, high La and a negative Ce anomaly compared to type-1 grains, suggests that type-2 apatite tails underwent recrystallization via dissolution-precipitation creep, whereas parental grains underwent crystal-plastic deformation and subgrain formation through dynamic recrystallization. Phase-equilibrium modelling and quartz CPO opening-angle thermometry are consistent with recrystallization at ∼480 – 530◦C and 2.2 – 5.0 kbar. We were not able to determine precise deformation ages from type-2 apatite because fluid-assisted recrystallization appears to have substantially decreased the U/Pb ratio. We find that preferential fluid flow along high-strain, biotite-rich layers in the mylonite caused type-2 apatite to recrystallise, whereas type-1 apatite in low strain layers was unaffected and retained the characteristics of the protolith.