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Telmo Bento dos Santos, Centro/Departamento de Geologia, Universidade de Lisboa (Portugal)
José Munhá, Centro/Departamento de Geologia, Universidade de Lisboa (Portugal)
Colombo Tassinari, Instituto de Geociências, Universidade de São Paulo (Brazil)
Fernando Noronha, Centro de Geologia, Universidade do Porto (Portugal)
Alexandra Guedes, Centro de Geologia, Universidade do Porto (Portugal)
Paulo Fonseca, Centro/Departamento de Geologia, Universidade de Lisboa (Portugal)
Coriolano Dias Neto, Instituto de Geociências, Universidade de São Paulo (Brazil)
Armanda Dória, Centro de Geologia, Universidade do Porto (Portugal)
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This work combines new C-O-H isotopic data with fluid inclusion microthermometry, Raman spectroscopy and ?O2 results in order to constrain fluid and geodynamic evolution of Ribeira Fold Belt (SE Brazil) granulites during the Panafricano - Braziliano Orogeny.
Integration of the data reveals that metamorphic peak fluids evolved under high ?O2 conditions (QFM +1) coeval with fluid inclusion generation of CO2 and CO2-N2 (0 to 11 mol%) high to medium density (1.01 - 0.59 g/cm3) at T ∼ 800 °C [1] and XH2O < 0.05 [2], whereas metamorphic retrograde (T ∼ 600 °C [1]) low density (0.19-0.29 g/cm3) CO2 and CO2-N2 (0 to 36 mol%), CO2 (94 to 95 mol%) - N2 (3 mol%) - CH4 (2 to 3 mol%) - H2O (Flw = 0.1) (in graphitic granulites), N2 (95 mol%) - CH4 (5 mol%), H2O-CO2 and late H2O fluids were reduced ?O2 (QFM -1 to -3). δ18O quartz results of 10.3 - 10.7‰ imply high-temperature CO2 δ18O values of 14.4 to 14.8‰, suggesting the involvement of a metamorphic fluid, whereas lower temperature biotite δ18O and δD results of 7.5 - 8.5‰ and -54 to -67‰, respectively imply H2O δ18O values of 10 to 11‰ and δDH2O of -23 to -36‰, suggesting δ18O depletion and increasing fluid/rock ratio from metamorphic peak to retrograde conditions. Isotopic results are compatible with low-temperature H2O influx and ?O2decrease that promoted graphitic deposition in retrograde granulites, simultaneous with low density CO2, CO2-N2and CO2-N2-CH4-H2O fluid inclusions at T = 450 - 330 °C. Graphite δ13C results of -10.9 to -11.4, imply CO2δ13C values of -0.8 to -1.3‰ suggesting decarbonation of Cambrian marine carbonates [3] with small admixture of lighter biogenic or mantle derived fluids.
Results suggest that peak fluids were probably 18O enriched metamorphic fluids derived from deep-seated carbonated sources. Rapid pressure and temperature drop during retrograde metamorphism induced ?O2 decrease by fluid admixture with shallower waters, turning peak carbonic fluids into CO2-H2O and depleting biotite δ18O and δD values, and as low-salinity H2O fluids progressively became dominant, late-graphite deposited at shallower crustal levels.
FAPESP, POCA-PETROLOG (CEGUL, UI: 263; POCTI/FEDER) and SFRH/BD/17014/2004 FCT PhD scholarship co-financed by FEDER provided support for field and analytical work.
[1] Bento dos Santos et al., 2007. Geochimica et Cosmochimica Acta, 71, 15, Sup. 1, A79.
[2] Bento dos Santos et al., 2008. Geophysical Research Abstracts, 10, EGU2008-A-00262.
[3] Veizer et al., 1999. Chemical Geology, 161, 59-88.
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