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Graphite occurrences in the low-pressure/high-temperature metamorphic belt of the Sierra de Aracena (southern Iberian Massif)

Published online by Cambridge University Press:  05 July 2018

M. Rodas ,
F. J. Luque ,
J. F. Barrenechea ,
J. C. Fernández-Caliani ,
A. Miras  and
C. Fernández-Rodríguez
M. Rodas
Affiliation:
Departamento de Cristalografía y Mineralogía, Facultad de Geología, Universidad Complutense de Madrid, 28040 Madrid, Spain
F. J. Luque*
Affiliation:
Departamento de Cristalografía y Mineralogía, Facultad de Geología, Universidad Complutense de Madrid, 28040 Madrid, Spain
J. F. Barrenechea
Affiliation:
Departamento de Cristalografía y Mineralogía, Facultad de Geología, Universidad Complutense de Madrid, 28040 Madrid, Spain
J. C. Fernández-Caliani
Affiliation:
Departamento de Geología, Facultad de Ciencias Experimentales, Universidad de Huelva, 21819 Palos de la Frontera, Huelva, Spain
A. Miras
Affiliation:
Departamento de Cristalografía y Mineralogía, Facultad de Química, Universidad de Sevilla, 41071 Sevilla, Spain
C. Fernández-Rodríguez
Affiliation:
Departamento de Geología, Facultad de Ciencias Experimentales, Universidad de Huelva, 21819 Palos de la Frontera, Huelva, Spain
*
*E-mail: [email protected]
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Abstract

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 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.

Keywords

graphitecalc-silicate rockslow-P/high-Tmetamorphismsouthern Spain
Type
Research Article
Information
Mineralogical Magazine , Volume 64 , Issue 5 , October 2000 , pp. 801 - 814
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2000

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