We report a new occurrence of melt inclusions in polymetamorphic granulitic gneisses of the Jubrique unit, a\ud complete though strongly thinned crustal section located above the Ronda peridotite slab (Betic Cordillera, S\ud Spain). The gneissic sequence is composed of mylonitic gneisses at the bottomand in contactwith the peridotites,\ud and porphyroblastic gneisses on top. Mylonitic gneisses are strongly deformed rocks with abundant garnet and\ud rare biotite. Except for the presence of melt inclusions, microstructures indicating the former presence of melt\ud are rare or absent. Upwards in the sequence, garnet decreases whereas biotite increases in modal proportion.\ud Melt inclusions are present from cores to rims of garnets throughout the entire sequence. Most of the former\ud melt inclusions are now totally crystallized and correspond to nanogranites, whereas some of them are partially\ud made of glass or, more rarely, are totally glassy. They show negative crystal shapes and range in size from ≈5\ud to 200 μm, with a mean size of ≈30–40 μm. Daughter phases in nanogranites and partially crystallized melt\ud inclusions include quartz, feldspars, biotite and muscovite; accidental minerals include kyanite, graphite, zircon,\ud monazite, rutile and ilmenite; glass has a granitic composition. Melt inclusions are mostly similar throughout\ud all the gneissic sequence. Some fluid inclusions, of possible primary origin, are spatially associated with melt\ud inclusions, indicating that at some point during the suprasolidus history of these rocks granitic melt and fluid\ud coexisted. Thermodynamic modeling and conventional thermobarometry of mylonitic gneisses provide peak\ud conditions of ≈850 °C and 12–14 kbar, corresponding to cores of large garnets with inclusions of kyanite and\ud rutile. Post-peak conditions of ≈800–850 °C and 5–6 kbar are represented by rim regions of large garnets with\ud inclusions of sillimanite and ilmenite, cordierite-quartz-biotite coronas replacing garnet rims, and the matrix\ud with oriented sillimanite. Previous conventional petrologic studies on these strongly deformed rocks have\ud proposed that anatexis started during decompression from peak to post-peak conditions and in the field of\ud sillimanite. The study of melt inclusions shows, however, that melt was already present in the system at peak\ud conditions, and that most garnet grew in the presence of melt

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The study of the composition of primary melts during anatexis of high-pressure granulitic migmatites is relevant to understand the generation and differentiation of continental crust.Peritectic minerals in migmatites can trap dropless of melt that forms via incongruent melting reactions during crustal anatexis. These melt inclusions commonly crystallize and form nanogranitoids upon slow cooling of the anatectic terrane. To obtain the primary compositions of crustal melts recorded in these nanogranitoids, including volatile concentrations and information on fluid regimes, they must be remelted and rehomogenize before analysis. A new occurrence of nanogranitoids was recently reported in garnets of mylonitic metapelitic gneisses (former high pressure granulitic migmatites) at the bottom of the prograde metamorphic sequence of Jubrique, located on top of the Ronda peridotite slab (Betic Cordillera, S Spain). Nanogranitoids within separated chips of cores and rims of large garnets from these former migmatites were remelted at 15 kbar and 850, 825 or 800 ºC and dry (without added H 2 O), during 24 hours, using a piston cylinder apparatus. Although all experiments show glass (former melt) within melt inclusions, the extent of rehomogenization depends on the experimental temperature. Experiments at 850-825 ºC show abundant disequilibrium microstructures, whereas those at 800 ºC show a relatively high proportion of rehomogenized nanogranitoids, indicating that anatexis and entrapment of melt inclusions in these rocks was likely close to 800 ºC. Electron microprobe and NanoSIMS analyses show that experimental glasses are leucogranitoid and peraluminous, though define two distinct compositional groups. Type I corresponds to K-rich, Ca-and H 2 O-poor leucogranitic melts, whereas type II represents K-poor, Ca-and H 2 O-rich granodioritic to tonalitic melts. Type I and II melt inclusions are found in most cases at the cores and rims of large garnets, respectively. We tentatively suggest that these former migmatites underwent two melting events under contrasting fluid regimes, possibly during two different orogenic periods. This 3 study demonstrates the strong potential of melt inclusions studies in migmatites and granulites in order to unravel their anatectic history, particularly in strongly deformed rocks where most of the classical anatectic microstructures have been erased during deformation.