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Significance of graphite occurrences in the Aracena Metamorphic Belt, Iberian Massif

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Crespo Feo, Elena and Luque del Villar, Francisco Javier and Fernández Rodríguez, Carlos and Rodas, Magdalena and Díaz Azpiroz, Manuel and Fernández Caliani, J.C. and Fernández Barrenechea, José María (2004) Significance of graphite occurrences in the Aracena Metamorphic Belt, Iberian Massif. Geological magazine, 141 (6). pp. 687-697. ISSN 0016-7568

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Official URL: http://journals.cambridge.org/action/displayJournal?jid=GEO



Abstract

The significance of syngenetic and epigenetic graphite occurrences from the Variscan hightemperature/
low-pressure Aracena Metamorphic Belt is discussed in the framework of the tectonothermal
evolution of this southern zone of the Iberian Massif. Syngenetic graphite is associated with
both low- tomedium-grademetamorphic rocks (LaUmbr´ıa series, Precambrian in age) and high-grade,
granulite facies rocks (the Precambrian Fuente del Oro series and a Cambrian calc-silicate series).
Epigenetic, fluid-deposited occurrences correspond to overgrowths on existing metamorphic graphite
grains and vein-type mineralization. Two types of graphitized particles with remarkable differences in
reflectance, anisotropy and size can be distinguished in the Precambrian metapelites of the La Umbr´ıa
series. Large, >150 μm length, platy crystals with high reflectance and anisotropy are interpreted as
detrital and are considered indirect evidence of an old orogenic cycle prior to the Cadomian Orogeny,
during which metamorphism exceeded greenschist facies. The coexistence of two types of particles
explains the scattering of values of the c parameter of graphite determined by XRD (c=6.72–6.74 A°),
and the anomalously high temperatures of the DTA exothermic peak (close to 600 ◦C) of graphite
with respect to that inferred from mineral assemblages in these rocks. The presence of graphite-rich
quartzites and gneisses within the Fuente del Oro series and the calc-silicate series is evidence of
sedimentation under reducing conditions in a continental shelf. The characteristics of graphite reflect
the high-grade metamorphic conditions attained in the southern area of the Aracena Metamorphic
Belt. Pervasive flow of fluids related to a major Variscan extensional event resulted in overgrowths
on the pre-existing graphite in the gneisses and quartzites of the calc-silicate series, as evidenced
by the heterogeneous isotopic composition of graphite single crystals in these rocks. A later stage of
graphite precipitation is represented by scarce vein-type occurrences in mafic granulites that document
channelled flow of fluids.


Item Type:Article
Uncontrolled Keywords:Graphite, Metamorphism, P–T conditions, Aracena Spain
Subjects:Sciences > Geology > Mineralogy
ID Code:17598
Deposited On:09 Jan 2013 12:41
Last Modified:11 Dec 2018 08:46

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