Gondwana Research, zyxwvutsrqpo % zyxwvutsrqponmlk 3, No. 3,.pp. 405-413. zyxwvuts 0 2000 International Association for Gondwana Research, Japan. ISSN: 1342-937X zyxwvutsrqpo Biogenic Graphite as a Potential Geomarker - Application to Continental Reconstructions of Pan - African Gondwana Terrains C.B. Dissanayakel, Rohana Chandrajith*and J.P. Boudou3 ' Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka ' Department of Natural Resources, Facdty of Applied Sciences, Sabaragamuwa, University of Sri Lanka, Buttala, Sri hnka Labaratoire de Geochimie et Metallogenie, case 124, Universite Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris cedex 05, France (Manuscript received November zyxwvutsrq 5,1999; accepted February 2,2000) zyxwvut Abstract Graphite is present in nature in several forms. Genetically they may be broadly classified as biogenic and abiogenic. The biogenic forms are those that are clearly derived from an organic precursor while the abiogenic or inorganic 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 recognized and shows different degrees of crystallinity depending on the relative grades of metamorphism it had undergone. Once it attains a certain degree of crystalline order, it does not revert to a lower state even under changing metamorphic conditions, thereby making it a good mineral geothcrrnonieter 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 effectively used to trace sites of sedimentary basins and subsequent ocean closures that may have resulted in geosutures. Studies of the Pan-African terrains of the Gondwana crustal fragments as exemplified by the sutures of the Mozambique Belt running through East Africa, Madagascar, Sri Lanka and Antarctica 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 Key words: Biogenic graphite, geomarker, Pan-African, Gondwana fragments. Introduction The search for geochemical evidence of very early life on Earth has focused greater attention on graphite. It is now being increasingly realised that graphite, an end product of a process culminating in the formation of pure carbon, may still harbour hidden clues indicative of the source of carbon as well as the geological processes that preceded such an event. Particular attention is being devoted to potential biomarkers expected to be present in graphite and which, with recent advanced techniques of analysis are now amenable for closer scrutiny. Among these are the trace metals such as zyxwvut V, Cu, Zn, and Mo that are known for their biophilic associations and which may be present in graphite (van Zuillen et al., 1999), carbon and nitrogen isotopic signatures (Hahn-Weinheimer and Hirner, 1981; Javoy et al., 1986), as well as presence of certain volatiles and light elements (Douthitt, 1985; Duke, 1995). Detailed investigations on biomarkers are necessary in view of the fact that high-grade metamorphic overprints obliterate much of the direct evidence for the existence of early life. An important by-product of this intensive search for the origin of early life (Schidlowski, 1995; Mojzsis et al., 1996) is the information on the geological setting of the sedimentary basins in which the most ancient forms of life had existed. Such geological conditions and the locations of the sedimentary basins could be traced using the distribution of biogenic graphite and this paper aims to highlight the significance of biogenic graphite as a geomarker, particularly of belts and geosutures within Gondwana fragments, bearing in mind that tracing such geological features is an essential part of the reconstruction of the linkages of the ancient land masses. Geological