Fluid-present anatexis of metapelites at El Joyazo (SE Spain): constraintsfrom Raman spectroscopy of graphite

The garnet-biotite-sillimanite anatectic xenoliths in the Neogene dacite do me 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 type s: only type III is fully consistent with the granulite-facies conditions r eached by the xenoliths during partial melting, the others indicate metamor phic temperatures covering amphibolite-facies conditions, with only a few e xamples of granulite-grade crystallinity. All graphite crystallized before or during the anatectic event, indicating that a large fraction of the grap hite 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 explaine d in terms of fluid-melt-graphite interaction during the anatectic event. D isequilibrium behaviour during high-temperature metamorphism and anatexis i s typical of types I, II and particularly of IV, and is attributed to slugg ish kinetics of solid-state graphitization, mainly owing to the limited tim e of the process and carbon saturation of the intergranular fluid. The coex isting, well-ordered type III graphite is the product of melting in the pre sence of a graphite-saturated fluid, a process that would account for the d eposition of new graphite. The LRS results, together with petrologic observ ations, suggest that it is possible that high melt fractions can be generat ed by fluid-present melting of a metasedimentary protolith also in a closed system. Although this contradicts the commonly accepted hypothesis that, d ue 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.