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Mineralogical Magazine; October 2000; v. 64; no. 5; p. 801-814
© 2000 Mineralogical Society of Great Britain and Ireland
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Graphite occurrences in the low-pressure/high-temperature metamorphic belt of the Sierra de Aracena (southern Iberian Massif)

M. Rodas1, F. J. Luque1,*, J. F. Barrenechea1, J. C. Fernández-Caliani2, A. Miras3 and C. Fernández-Rodríguez2

1 Departamento de Cristalografía y Mineralogía, Facultad de Geología, Universidad Complutense de Madrid, 28040 Madrid, Spain
2 Departamento de Geología, Facultad de Ciencias Experimentales, Universidad de Huelva, 21819 Palos de la Frontera, Huelva, Spain
3 Departamento de Cristalografía y Mineralogía, Facultad de Química, Universidad de Sevilla, 41071 Sevilla, Spain

* E-mail: jluque{at}eucmax.sim.ucm.es

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 ~800°C. The thermal properties of graphite are also typical of well-ordered graphite and provide DTA exothermic maxima ranging from 810 to 858°C 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 {delta}13C values in the range from –31.6 to –21.4{per thousand} for syngenetic graphite of types I, II and III attributable to biogenically-derived carbon. The heavier signatures for graphite in vein occurrences ({delta}13C= –17.7 to –18.3{per thousand} ) 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.

KEYWORDS: graphite, calc-silicate rocks, low-P/high-T metamorphism, southern Spain




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 Geological MagazineHome page
E. CRESPO, J. LUQUE, C. FERNANDEZ-RODRIGUEZ, M. RODAS, M. DIAZ-AZPIROZ, J. C. FERNANDEZ-CALIANI, and J. F. BARRENECHEA
Significance of graphite occurrences in the Aracena Metamorphic Belt, Iberian Massif
Geological Magazine, November 1, 2004; 141(6): 687 - 697.
[Abstract] [Full Text] [PDF]


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