gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1992.056.385.04 Geochemistry and Petrology Relation Between Graphitization of Organic Matter and Clay Mineralogy, Silurian Black Shales in Central Spain Barrenechea J. F. Rodas M. Departamento de Cristalografía y Mineralogía, Facultad de Geología, U.C.M., 28040 Madrid, Spain Arche A. Instituto de Geología Económica, C.S.I.C.-U.C.M., Facultad de Geología, 28040 Madrid, Spain 12 1992 56 385 477 485 5 8 1991 15 1 1992 Copyright © 1992, The Mineralogical Society 1992 Abstract

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 chlorite AlIV content and the graphite co parameter, the conditions of metamorphism were calculated to be 300 ± 20°C and 1.5–2 kbar.

 Keywords graphite Early Silurian Spanish Central System anchi-epimetamorphism chlorite illite crystallinity