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A combined study of melt inclusions and classical petrology on the Ronda metatexites (S Spain) Por: Omar Bartoli Dipartimento di Geoscienze, Università di Padova Viernes, 4 de Mayo de 2012, 12.30 horas Sala de Audiovisuales, Facultad de Ciencias » Resumen » Breve reseña curricular Melt inclusions hosted in peritectic minerals of migmatites represent a novel and powerful small-scale tool to investigate the anatexis of the continental crust. In this study, using as an example the Ronda metatexites from the Betic Cordillera (S Spain) and taking advantage of a new experimental approach for remelting crystallized melt inclusions, a combined study of classical petrology and melt inclusions in migmatites was performed to characterize in detail the composition and the physical properties of anatectic melts, the fluid regimes, and the melting mechanisms and conditions during the anatexis of the metasedimentary crust located below the Ronda peridotite. Doctor Omar Bartoli got his Bachelor degree in Geology (2008) at the Università di Parma (Italy). During his Master thesis he worked on the petrology and geochronology of magmatic enclaves hosted in trachytes, and on melt inclusions in migmatites from Ronda (S Spain) during his PhD thesis (Università di Parma, 2012). At present, he is a research fellow at the Università di Padova (Italy), working on the characterization of the early crustal melts produced in different anatectic terranes through the geochemical study of melt inclusions and nanogranites in migmatites and granulites. The quartzo-feldspathic metatexites are mainly composed of Qtz + Pl + Kfs + Bt + Sil + Grt and probably derived from a greywacke protolith. Muscovite is very rare. The former presence of melt is recorded by melt inclusions and melt pseudomorphs at the microscale, and by peraluminous leucogranitic leucosomes at the mesoscale. Melt inclusions have been found in peritectic garnets of the investigated quartzofeldspathic migmatites. They are very small in size, mostly ≤ 10 µm, and typically show a well-developed negative crystal shape. Three types of inclusions were identified: totally crystallized (nanogranites), partially crystallized and preserved glassy inclusions. Crystallized melt inclusions contain a granitic phase assemblage with quartz, feldspars and micas. The remelting experiments were conducted with a piston cylinder apparatus, an approach that has never been attempted before, until the complete homogenization of crystal-bearing melt inclusions at conditions (T=700 °C and P=5 kbar) close to those inferred for anatexis. The compositions of quenched glasses are leucogranitic and peraluminous. H2O content measured by Raman spectroscopy ranges from 3.1 to 7.6 wt %. The pseudosection constructed using the bulk composition of the fully rehomogenized melt inclusions constrained the melt entrapment at temperature close to the minimum re-homogenization temperature (T ~700 °C). The composition of those melt inclusions re-homogenized at 700 °C is comparable to that of preserved glassy inclusions in the same rocks. Combining information from the melt inclusions and from the classical petrology allowed a better understanding of the melting processes occurred in the crustal sequence below the Ronda peridotite. The data collected in this study suggest that the crustal melting in Ronda metatexites occurred initially and locally at the fluidsaturated solidus owing to the presence of H2O-rich intergranular fluids, and likely progressed by mica breakdown melting. Melt inclusions record the composition of the melt during the earliest stages of anatexis; on the other hand, leucosomes in the same rocks reflect the composition of melt at, or closer to, the peak metamorphic conditions. The anatectic melts at Ronda have viscosity values greater than those commonly considered for granitic melts formed at the same P-T conditions, implying much longer timescales for melt extraction and ascent through the metasedimentary crust at Ronda, as well as much greater strength of the migmatites.