%0 Journal Article
%A Rodas, Magdalena
%A Luque del Villar, Francisco Javier
%A Fernández Barrenechea, José María
%A Fernández Caliani, J.C.
%A Miras Ruiz, Adolfo
%A Fernández Rodríguez, Carlos
%T Graphite occurrences in thelow-pressure/high-temperature metamorphicbelt of the Sierra de Aracena (southern Iberian Massif)
%D 2000
%@ 0026-461X
%U https://hdl.handle.net/20.500.14352/57787
%X Four distinct associations of graphite have been identified in the low-pressure, high-temperature belt ofthe Sierra de Aracena (SW Spain). Syngenetic occurrences include: (1) stratiform graphitemineralization within a calc-silicate series; (2) disseminated graphite within a terrigenous sequence;and (3) ‘restitic’ graphite within anatectic tonalites and their enclaves. Epigenetic graphite occurs as (4)veins cross-cutting mafic granulites.Graphite in all types of occurrences is highly crystalline, with the c parameter close to 6.70 Å . Suchc values correspond to temperatures of formation of ~8008C. The thermal properties of graphite arealso typical of well-ordered graphite and provide DTA exothermic maxima ranging from 810 to 8588Cdepending on the mode of occurrence. The differences among the temperatures of formation estimatedby graphite geothermometry, the position of the exothermic maximum in the DTA curves, andpetrologic geothermometers are discussed in terms of the applicability of graphite geothermometry togranulite-facies rocks. Carbon isotope analysis yields δ13C values in the range from 31.6 to 21.4%for syngenetic graphite of types I, II and III attributable to biogenically-derived carbon. The heaviersignatures for graphite in vein occurrences (δ13C= 17.7 to 18.3%) with respect to syngeneticgraphites suggest that isotopically heavy carbonic species were incorporated into the metamorphicfluids (probably as a consequence of decarbonation reactions of the calc-silicate rocks) from whichgraphite precipitated into the veins. These fluids were strongly channelled through structural pathways.
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