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

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.