Person:
Fernández Barrenechea, José María

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First Name
José María
Last Name
Fernández Barrenechea
URI
https://hdl.handle.net/20.500.14352/75219
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Geológicas
Department
Mineralogía y Petrología
Area
Cristalografía y Mineralogía
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Author: Fernández Caliani, J.C. × Subject: 549.2 ×

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    Graphite occurrences in the low-pressure/high-temperature metamorphic belt of the Sierra de Aracena (southern Iberian Massif)
    (Mineralogical magazine, 2000) Rodas González, Magdalena; Luque Del Villar, Francisco Javier; Fernández Barrenechea, José María; Fernández Caliani, J.C.; Miras Ruiz, Adolfo; Fernández Rodríguez, Carlos
    Four distinct associations of graphite have been identified in the low-pressure, high-temperature belt of the Sierra de Aracena (SW Spain). Syngenetic occurrences include: (1) stratiform graphite mineralization 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 . Such c values correspond to temperatures of formation of ~8008C. The thermal properties of graphite are also typical of well-ordered graphite and provide DTA exothermic maxima ranging from 810 to 8588C depending on the mode of occurrence. The differences among the temperatures of formation estimated by graphite geothermometry, the position of the exothermic maximum in the DTA curves, and petrologic geothermometers are discussed in terms of the applicability of graphite geothermometry to granulite-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 heavier signatures for graphite in vein occurrences (δ13C= 17.7 to 18.3%) with respect to syngenetic graphites suggest that isotopically heavy carbonic species were incorporated into the metamorphic fluids (probably as a consequence of decarbonation reactions of the calc-silicate rocks) from which graphite precipitated into the veins. These fluids were strongly channelled through structural pathways.