Fluid-present anatexis of metapelites at El Joyazo (SE Spain): constraints from Raman spectroscopy of graphite |
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Authors: | Bernardo Cesare Cinzia Maineri |
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Institution: | (1) Dipartimento di Mineralogia e Petrologia, Università di Padova, Corso Garibaldi, 37, I-35137 Padova, Italy E-mail: bernardo@dmp.unipd.it, IT;(2) CNR-Centro Studio Minerogenesi e Geochimica Applicata Via La Pira, 4, I-50121 Firenze E-mail: cmaineri@steno.geo.unifi.it, |
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Abstract: | The garnet-biotite-sillimanite anatectic xenoliths in the Neogene dacite dome of El Joyazo (also called Cerro de Hoyazo,
SE Spain) contain four types of graphite (I to IV), distinguished on the basis of grain size and texture. Structural characterization
of graphite by Laser Raman spectroscopy (LRS) shows systematic differences in the degree of ordering among the four types:
only type III is fully consistent with the granulite-facies conditions reached by the xenoliths during partial melting, the
others indicate metamorphic temperatures covering amphibolite-facies conditions, with only a few examples of granulite-grade
crystallinity. All graphite crystallized before or during the anatectic event, indicating that a large fraction of the graphite
did not equilibrate at peak temperatures. The mm-scale coexistence of different types and degrees of ordering in the graphite
suggests different origins, i.e. of biogenic derivation and “fluid-deposited”, and is explained in terms of fluid-melt-graphite
interaction during the anatectic event. Disequilibrium behaviour during high-temperature metamorphism and anatexis is typical
of types I, II and particularly of IV, and is attributed to sluggish kinetics of solid-state graphitization, mainly owing
to the limited time of the process and carbon saturation of the intergranular fluid. The coexisting, well-ordered type III
graphite is the product of melting in the presence of a graphite-saturated fluid, a process that would account for the deposition
of new graphite. The LRS results, together with petrologic observations, suggest that it is possible that high melt fractions
can be generated by fluid-present melting of a metasedimentary protolith also in a closed system. Although this contradicts
the commonly accepted hypothesis that, due to limited rock porosities, extensive fluid-present melting is precluded unless
infiltration occurs, it is a possible end-member model in anatectic settings characterized by rapid heating rates and low-grade
source rocks.
Received: 14 July 1998 / Accepted: 16 November 1998 |
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