POORLY CRYSTALLINE CARBONACEOUS MATTER IN HIGH-GRADE METASEDIMENTS - IMPLICATIONS FOR GRAPHITIZATION AND METAMORPHIC FLUID COMPOSITIONS

Poorly crystalline carbonaceous matter was observed in chlorite to sil limanite grade metasediments from the Trois Seigneurs Massif, in contr ast to other studies of carbon crystallinity which observed well cryst allised graphite under upper greenschist facies conditions. Using tran smission electron microscopy four types of carbon particle were identi fied; globular carbon, composite flakes, homogeneous flakes and crysta lline graphite. Globular carbon and composite flakes are poorly crysta lline microporous carbon. Homogeneous flakes decompose in the electron beam and are probably composed of heavy volatile hydrocarbons. Graphi te is confined to samples from retrograde shear zones and often occurs with globular carbon. The lack of graphitisation in metasediments is probably a consequence of the microporous structure of the carbonaceou s matter combined with low f(O2). The preservation of carbonaceous mat ter in the Trois Seigneurs metasediments is not compatible with the me tasediments having been externally buffered by a high X(H2O) fluid syn -metamorphism. An alternative hypothesis of internal buffering is pref erred to explain the carbonaceous matter in the Trois Seigneurs metase diments.