C. Dias Neto

Laboratório Nacional de Energia e Geologia, Amadora, Lisbon, Portugal

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Publications (6)6.59 Total impact

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    ABSTRACT: Pseudosections, geothermobarometric estimates and careful petrographic observations of gneissic migmatites and granulites from Neoproterozoic central Ribeira Fold Belt (SE Brazil) were performed in order to quantify the metamorphic P-T conditions during prograde and retrograde evolution of the Brasiliano Orogeny. Results establish a prograde metamorphic trajectory from amphibolite facies conditions to metamorphic peak (T = 850 ± 50 °C; P = 8 ± 1 kbar) that promoted widespread dehydrationmelting of 30 to 40% of the gneisses and high-grade granitization. After the metamorphic peak, migmatites evolved with cooling and decompression to T ≈ 500 °C and P ≈ 5 kbar coupled with aH2O increase, replacing the high-grade paragenesis plagioclase-quartz-K-feldspar-garnet by quartz-biotite-sillimanite-(muscovite). Cordierite absence, microtextural observations and P-T results constrain the migmatite metamorphic evolution in the pseudosections as a clockwise P-T path with retrograde cooling and decompression. High-temperature conditions further dehydrated the lower crust with biotite and amphibole-dehydration melting and granulite formation coupled with 10% melt generation. Granulites can thus be envisaged as middle to lower crust dehydrated restites. Granulites were slowly (nearly isobarically) cooled, followed by late exhumation/retrograde rapid decompression and cooling, reflecting a two step P-T path. This retrograde evolution, coupled with water influx, chemically reequilibrated the rocks from granulite to amphibolite/greenschist facies, promoting the replacement of the plagioclase-quartz-garnet-hypersthene peak assemblage by quartz-biotite- K-feldspar symplectites. Key wordsRibeira Fold Belt–granulite–geothermobarometry–pseudosection–P-T path
    Geosciences Journal 10/2011; 15(1):27-51. · 0.62 Impact Factor
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    ABSTRACT: Combined fluid inclusion (FI) microthermometry, Raman spectroscopy, X-ray diffraction, CeOeH isotopes and oxygen fugacities of granulites from central Ribeira Fold Belt, SE Brazil, provided the following results: i) MagnetiteeHematite fO2 estimates range from 10�11.5 bar (QFM þ1) to 10�18.3 bar (QFM �1) for the temperature range of 896 �Ce656 �C, implying fO2 decrease from metamorphic peak temperatures to retrograde conditions; ii) 5 main types of fluid inclusions were observed: a) CO2 and CO2eN2 (0e11 mol%) high to medium density (1.01e0.59 g/cm3) FI; b) CO2 and CO2eN2 (0e36 mol%) low density (0.19e0.29 g/cm3) FI; c) CO2 (94e95 mol%)eN2 (3 mol%)eCH4 (2e3 mol%)eH2O (water 4v (25 �C) ¼ 0.1) FI; d) low-salinity H2OeCO2 FI; and e) late low-salinity H2O FI; iii) Raman analyses evidence two graphite types in khondalites: an early highly ordered graphite (Tw450 �C) overgrown by a disordered kind (Tw330 �C); iv) d18O quartz results of 10.3e10.7& imply high-temperature CO2 d18O values of 14.4e14.8&, suggesting the involvement of a metamorphic fluid, whereas lower temperature biotite d18O and dD results of 7.5e8.5& and �54 to �67&, respectively imply H2O d18O values of 10e11& and dDH2O of �23 to �36&, suggesting d18O depletion and increasing fluid/rock ratio from metamorphic peak to retrograde conditions. Isotopic results are compatible with low-temperature H2O influx and fO2 decrease that promoted graphite deposition in retrograde granulites, simultaneous with low density CO2, CO2eN2 and CO2eN2eCH4eH2O fluid inclusions at T ¼ 450e330 �C. Graphite d13C results of �10.9 to �11.4& imply CO2 d13C values of �0.8 to �1.3&, suggesting decarbonation of Cambrian marine carbonates with small admixture of lighter biogenic or mantle derived fluids. Based on these results, it is suggested that metamorphic fluids from the central segment of Ribeira Fold Belt evolved to CO2eN2 fluids during granulitic metamorphism at high fO2, followed by rapid pressure drop at Tw400e450 �C during late exhumation that caused fO2 reduction induced by temperature decrease and water influx, turning carbonic fluids into CO2eH2O (depleting biotite d18O and dD values), and progressively into H2O. When fO2 decreased substantially by mixture of carbonic and aqueous fluids, graphite deposited forming khondalites.
    Journal of South American Earth Sciences 05/2011; 31(1):93-109. · 1.53 Impact Factor
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    ABSTRACT: The studied sector of the central Ribeira Fold Belt (SE Brazil) comprises metatexites, diatexites, charnockites and blastomylonites. This study integrates petrological and thermochronological data in order to constrain the thermotectonic and geodynamic evolution of this Neoproterozoic–Ordovician mobile belt during Western Gondwana amalgamation. New data indicate that after an earlier collision stage at ∼610Ma (zircon, U–Pb age), peak metamorphism and lower crust partial melting, coeval with the main regional high grade D1 thrust deformation, occurred at 572–562Ma (zircon, U–Pb ages). The overall average cooling rate was low (<5 ◦C/Ma) from 750 to 250 ◦C (at ∼455 Ma; biotite–WR Rb–Sr age), but disparate cooling paths indicate differential uplift between distinct lithotypes: (a) metatexites and blastomylonites show a overall stable 3–5 ◦C/Ma cooling rate; (b) charnockites and associated rocks remained at T > 650 ◦C during sub-horizontal D2 shearing until ∼510–470Ma (garnet–WR Sm–Nd ages) (1–2 ◦C/Ma), being then rapidly exhumed/cooled (8–30 ◦C/Ma) during post-orogenic D3 deformation with late granite emplacement at ∼490Ma (zircon, U–Pb age). Cooling rates based on garnet–biotite Fe–Mg diffusion are broadly consistent with the geochronological cooling rates: (a) metatexites were cooled faster at high temperatures (6 ◦C/Ma) and slowly at low temperatures (0.1 ◦C/Ma), decreasing cooling rates with time; (b) charnockites show low cooling rates (2 ◦C/Ma) near metamorphic peak conditions and high cooling rates (120 ◦C/Ma) at lower temperatures, increasing cooling rates during retrogression. The charnockite thermal evolution and the extensive production of granitoid melts in the area imply that high geothermal gradients were sustained for a long period of time (50–90 Ma). This thermal anomaly most likely reflects upwelling of asthenospheric mantle andmagmaunderplating coupled with long-term generation of high HPE (heat producing elements) granitoids. These factors must have sustained elevated crustal geotherms for ∼100 Ma, promoting widespread charnockite generation at middle to lower crustal levels.
    Precambrian Research 01/2010; 180:285-298. · 4.44 Impact Factor
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    ABSTRACT: New structural analysis and petrological results on granulites from the central segment of Ribeira Fold Belt indicate that the regional thrusts D1 (245-260°, 55-70° NW) was coeval with the high grade peak metamorphism at 565 – 575 Ma, post-dating earlier collision related nappe thrusts at 630 – 610 Ma (D0). These were mostly erased by D2 thrusting and long-term dextral transpressional shearing (50-65°, 70-85°NW), simultaneous with very slow cooling of the orogen until 480 – 520 Ma. Brittle, extensional, tectonic event D3 (290-320°, subvertical) is associated with late granite emplacement in the area, being followed by tectonic collapse that resulted in rapid exhumation/cooling of the high grade rocks at ~ 470 Ma. Results suggest that a > 35 Ma period of nearly orthogonal shortening between the San Francisco and West Congo cratons occurred until 565 Ma with development of the D1 flower thrust system. Afterwards, when rocks could not absorb further orthogonal shortening, D2 dextral transpressive regime became dominant, turning the flower structure asymmetric. Specific positioning within the flower structure and partition of deformation induced “local” antithetical sinistral kinematics within the main regional dextral regime and differential exhumation: granulites in the central axis were rapidly exhumed, whereas along the lateral branches exhumation was much slower resulting in very slow cooling that lasted for almost 100 Ma.
    Comunicações Geológicas. 01/2009; 96:101-122.
  • Geochmica et Cosmochimica Acta 07/2008; 72(12).
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    Cadernos do Laboratorio Xeolóxico de Laxe: Revista de xeoloxía galega e do hercínico peninsular, ISSN 0213-4497, Nº 31, 2006, pags. 105-126. 10/2006;