P-T-Fluid evolution and graphite deposition during retrograde metamorphism in Ribeira Fold Belt, SE Brazil: Oxygen fugacity, fluid inclusions and C-O-H isotopic evidence

Combined fluid inclusion (FI) microthermometry, Raman spectroscopy, X-ray diffraction, C-O-H isotopes and oxygen fugacities of granulites from central Ribeira Fold Belt, SE Brazil, provided the following results: i) Magnetite-Hematite ƒO2 estimates range from 10−11.5 bar (QFM +1) to 10−18.3 bar (QFM −1) for the temperature range of 896 °C-656 °C, implying ƒO2 decrease from metamorphic peak temperatures to retrograde conditions; ii) 5 main types of fluid inclusions were observed: a) CO2 and CO2-N2 (0-11 mol%) high to medium density (1.01-0.59 g/cm3) FI; b) CO2 and CO2-N2 (0-36 mol%) low density (0.19-0.29 g/cm3) FI; c) CO2 (94-95 mol%)-N2 (3 mol%)-CH4 (2-3 mol%)-H2O (water φv (25 °C) = 0.1) FI; d) low-salinity H2O-CO2 FI; and e) late low-salinity H2O FI; iii) Raman analyses evidence two graphite types in khondalites: an early highly ordered graphite (T∼450 °C) overgrown by a disordered kind (T∼330 °C); iv) δ18O quartz results of 10.3-10.7‰ imply high-temperature CO2 δ18O values of 14.4-14.8‰, 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 10-11‰ 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 ƒO2 decrease that promoted graphite deposition in retrograde granulites, simultaneous with low density CO2, CO2-N2 and CO2-N2-CH4-H2O fluid inclusions at T = 450-330 °C. Graphite δ13C results of −10.9 to −11.4‰ imply CO2 δ13C 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 CO2-N2 fluids during granulitic metamorphism at high ƒO2, followed by rapid pressure drop at T∼400-450 °C during late exhumation that caused ƒO2 reduction induced by temperature decrease and water influx, turning carbonic fluids into CO2-H2O (depleting biotite δ18O and δD values), and progressively into H2O. When ƒO2 decreased substantially by mixture of carbonic and aqueous fluids, graphite deposited forming khondalites.