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

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Rodas, Magdalena and Luque del Villar, Francisco Javier and Fernández Barrenechea, José María and Fernández Caliani, J.C. and Miras Ruiz, Adolfo and Fernández Rodríguez, Carlos (2000) Graphite occurrences in the low-pressure/high-temperature metamorphic belt of the Sierra de Aracena (southern Iberian Massif). Mineralogical magazine, 64 (5). pp. 801-814. ISSN 0026-461X

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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 ~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.


Item Type:Article
Uncontrolled Keywords:Graphite, Calc-silicate rocks, Low-P/high-T metamorphism, Southern Spain
Subjects:Sciences > Geology > Mineralogy
ID Code:17600
Deposited On:09 Jan 2013 13:35
Last Modified:11 Dec 2018 08:47

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