Biogenic graphite as a potential geomarker - Application to continental reconstructions of Pan-African Gondwana terrains

Graphite is present in nature in several forms. Genetically they may be bro adly classified as biogenic and abiogenic. The biogenic forms are those tha t are clearly derived from an organic precursor while the abiogenic or inor ganic forms are more complex from the point of view of their origin, nature and geological relations. As a geomarker, biogenic graphite has certain advantages. It is easily reco gnized and shows different degrees of crystallinity depending on the relati ve grades of metamorphism it had undergone. Once it attains a certain degre e of crystalline order, it does not revert to a lower state even under chan ging metamorphic conditions, thereby making it a good mineral geothermomete r. It is also found in specific, restricted geological environments and is therefore useful as a boundary marker of ancient sedimentary terrains. These special characteristics of the biogenic type of graphite can be effec tively used to trace sites of sedimentary basins and subsequent ocean closu res that may have resulted in geosutures. Studies of the Pan-African terrai ns of the Gondwana crustal fragments as exemplified by the sutures of the M ozambique Belt running through East Africa, Madagascar, Sri Lanka and Antar ctica illustrate this point. A further example comes from the Mashan Group of East China, one of the most productive graphite - bearing regions of the world.