A horizon containing graphite in the Early Silurian slates of the Eastern Central System, Central Spain, has been studied. The carbonaceous slates contain quartz, muscovite, graphitized organic matter, plagioclase, pyrite and apatite; and the slates above the graphite horizon contain quartz, phengite, plagioclase and chlorite. The origin of the graphite is related to a period of very slow accumulation rate, anoxic seafloors deep-water upwellings and plankton blooms. The chlorite and graphite are mutually exclusive and by' using the chlori te AlIV content and the graphite Co parameter, the conditions of metamorphism were calculated to be 300 ± 20°C and 1.5-2 kbar.
