CONSTRAINTS ON THE ORIGIN AND THE GENESIS OF GRAPHITE-BEARING ROCKS FROM THE VARIEGATED SEQUENCE OF THE BOHEMIAN MASSIF (AUSTRIA)

Transitions between graphitic gneisses, graphite-bearing calcsilicate rocks and marbles of the Variegated Sequence (Bunte Serie) in the Mold anubian zone of the Bohemian massif were examined for their primary se dimentary signatures. The bulk chemistry of the graphitic gneisses is similar to those of common black shales. Chondrite-normalized rare ear th element (REE) distribution patterns of graphite-bearing rocks resem ble those of average pelitic sediments. Graphite delta(13)C-values aro und - 22 parts per thousand (PDB) show primary organic isotopic charac teristics in the silicate rocks and confirm their organogenic origin. Traces of metasomatic mobilizations were found for a number of element s such as Ba, Rb, Cs, the REEs and Mn. Carbon isotope thermometry usin g the calibration of Scheele and Hoefs (1992) suggests a ''peak''-temp erature of 640-780 degrees C for the graphite-bearing calcsilicates an d marbles. Small scale isotopic studies at the interface between gneis ses and marbles argue against a pervasive streaming CO2-rich fluid der ived from an external source and imply that the fluid was controlled m ore locally by lithology. Ore petrographic investigations showed iron- rich alabandite as an abundant phase within the calcsilicate rocks and marbles. The occurrence and metamorphic origin of alabandite within t hese rocks indicates high S- and low CO2-, O-2-fugacities in local lay ers of the Variegated Sequence.