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 fO(2) estimates range from 10(-11.5) bar (QFM + 1) to 10(-18.3) bar (QFM - 1) for the temperature range of 896 degrees C-656 degrees C, implying fO(2) decrease from metamorphic peak temperatures to retrograde conditions; ii) 5 main types of fluid inclusions were observed: a) CO2 and CO2-N-2 (0-11 mol%) high to medium density (1.01-0.59 g/cm(3)) FI; b) CO2 and CO2-N-2 (0-36 mol%) low density (0.19-0.29 g/cm(3)) FI; c) CO2 (94-95 mol%)-N-2 (3 mol%)-CH4 (2-3 mol%)-H2O (water phi(v) (25 degrees 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 similar to 450 degrees C) overgrown by a disordered kind (T similar to 330 degrees C); iv) delta O-18 quartz results of 10.3-10.7 parts per thousand, imply high-temperature CO2 delta O-18 values of 14.4-14.8 parts per thousand, suggesting the involvement of a metamorphic fluid, whereas lower temperature biotite delta O-18 and delta D results of 7.5-8.5 parts per thousand and -54 to -67 parts per thousand respectively imply H2O delta O-18 values of 10-11 parts per thousand and delta D-H2O of -23 to -36 parts per thousand suggesting delta O-18 depletion and increasing fluid/rock ratio from metamorphic peak to retrograde conditions. Isotopic results are compatible with low-temperature H2O influx and fO(2) decrease that promoted graphite deposition in retrograde granulites, simultaneous with low density CO2, CO2-N-2 and CO2-N-2-CH4-H2O fluid inclusions at T = 450-330 degrees C. Graphite delta C-13 results of -10.9 to -11.4 parts per thousand imply CO2 delta C-13 values of -0.8 to -1.3 parts per thousand 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-N-2 fluids during granulitic metamorphism at high fO(2), followed by rapid pressure drop at T similar to 400-450 degrees C during late exhumation that caused fO(2) reduction induced by temperature decrease and water influx, turning carbonic fluids into CO2-H2O (depleting biotite delta O-18 and delta D values), and progressively into H2O. When fO(2) decreased substantially by mixture of carbonic and aqueous fluids, graphite deposited forming khondalites. (C) 2010 Elsevier Ltd. All rights reserved.