The occurrence of crystallized and glassy melt inclusions (MI) in high-grade, partially melted metapelites and metagraywackes has opened up new possibilities to investigate anatectic processes. The present study focuses on three case studies: khondalites from the Kerala Khondalite Belt (India), the Ronda migmatites (Spain), and the Barun Gneiss (Nepal Himalaya). The results of a detailed microstructural investigation are reported, along with some new microchemical data on the bulk composition of MI. These inclusions were trapped within peritectic garnet and ilmenite during crystal growth and are therefore primary inclusions. They are generally isometric and very small in size, mostly £15 lm, and only rarely reaching 30 lm; they occur in clusters. In most cases inclusions are crystallized (nanogranites) and contain a granitic phase assemblage with quartz, feldspar and one or two mica depending on the particular case study, commonly with accessory phases (mainly zircon, apatite, rutile). In many cases the polycrystalline aggregates that make up the nanogranites show igneous microstructures, e.g. granophyric intergrowths, micrographic quartz in K-feldspar and cuneiform rods of quartz in plagioclase. Further evidence for the former presence of melt within the investigated inclusions consists of melt pseudomorphs, similar to those recognized at larger scale in the host migmatites. Moreover, partially crystallized inclusions are locally abundant and together with very small (£8 lm) glassy inclusions may occur in the same clusters. Both crystallized and partially crystallized inclusions often display a diffuse nanoporosity, which may contain fluids, depending on the case study. After entrapment, inclusions underwent limited microstructural modifications, such as shape maturation, local necking down processes, and decrepitation (mainly in the Barun Gneiss), which did not influence their bulk composition. Re-homogenized nanogranites and glassy inclusions show a leucogranitic and peraluminous composition, consistent with the results of partial melting experiments on metapelites and metagraywackes. Anatectic MI should therefore be considered as a new and important opportunity to understand the partial melting processes

With less than two decades of activity, research on melt inclusions (MI) in crystals from rocks that have undergone\ud crustal anatexis – migmatites and granulites – is a recent addition to crustal petrology and geochemistry.\ud Studies on this subject started with glassy inclusions in anatectic crustal enclaves in lavas, and then progressed\ud to regionally metamorphosed and partially melted crustal rocks, where melt inclusions are normally crystallized\ud into a cryptocrystalline aggregate (nanogranitoid).\ud Since the first paper on melt inclusions in the granulites of the Kerala Khondalite Belt in 2009, reported and\ud studied occurrences are already a few tens. Melt inclusions in migmatites and granulites show many\ud analogieswith theirmore common and long studied counterparts in igneous rocks, but also display very important\ud differences and peculiarities,which are the subject of this review. Microstructurally, melt inclusions\ud in anatectic rocks are small, commonly 10 μm in diameter, and their main mineral host is peritectic garnet,\ud although several other hosts have been observed. Inclusion contents vary from glass in enclaves\ud that were cooled very rapidly from supersolidus temperatures, to completely crystallized material in\ud slowly cooled regional migmatites. The chemical composition of the inclusions can be analyzed combining\ud several techniques (SEM, EMP, NanoSIMS, LA–ICP–MS), but in the case of crystallized inclusions the\ud experimental remelting under confining pressure in a piston cylinder is a prerequisite. The melt is\ud generally granitic and peraluminous, although granodioritic to trondhjemitic compositions have also\ud been found.\ud Being mostly primary in origin, inclusions attest for the growth of their peritectic host in the presence of\ud melt. As a consequence, the inclusions have the unique ability of preserving information on the composition\ud of primary anatectic crustal melts, before they undergo any of the common following changes in their way\ud to produce crustal magmas. For these peculiar features, melt inclusions in migmatites and granulites, largely\ud overlooked so far, have the potential to become a fundamental tool for the study of crustal melting,\ud crustal differentiation, and even the generation of the continental crust

Using a metatexite from the Spanish Betic Cordillera as an example, we show that in situ and otherwise impossible to retrieve compositional information on natural anatectic melts can be reliably gained from experimentally rehomogenized melt inclusions in peritectic garnets. Experiments were conducted on single garnet crystals in a piston cylinder apparatus until the complete homogenization of crystal-bearing melt inclusions at the conditions inferred for the anatexis. The compositions of quenched glasses, representative of the early anatectic melts, are leucogranitic and peraluminous, and differ from those of leucosomes in the host rock. The H2O contents in the glasses suggest that melts formed at low temperature (~700 °C) may not be as hydrous and mobile as thought. Providing for the fi rst time the precise melt composition (including the volatile components) in the specifi c anatectic rock under study, our approach improves our understanding of crustal melting and generation of S-type granites

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Important advances have been made during the last 15 years in the study of melt inclusions in minerals from migmatites and granulites. Pioneer work on high temperature metapelitic anatectic enclaves in peraluminous dacites from SE Spain has shown that droplets of granitic melt can be trapped by minerals growing during incongruent melting reactions, and that the composition of such trapped melts can be representative of that of the bulk melt in the system during the anatexis of the rock. Therefore melt inclusions may represent samples of embryionic anatectic granite. In most cases, these melt inclusions define microstructures that are typical of primary entrapment, and show little or no evidence of melt crystallization upon cooling. Rather, the melt solidified to glass due to very fast cooling associated with the submarine extrusion of the dacites. Hence inclusions can readily be analyzed for major and trace elements by conventional methods such as the electron microprobe or by laser ablation-inductively coupled plasmamass spectrometry.Based on the results from these quite unusual anatectic enclaves, one would expect similar melt inclusions to be present also in more conventional, slowly cooled, regionally metamorphosed migmatite and granulite terranes. As a matter of fact, recent investigations confirm this hypothesis. Tiny (<25 μm) inclusions containing a cryptocrystalline aggregate of quartz, feldpars, biotite and muscovite have been found in garnet from the metapelitic granulites of the Keraka Khondalite Belt, as well as in garnet and ilmenite from metapelitic and quartzo-feldspathic migmatites from the Alps, Ronda and the Himalayas. Due to the grain-size, texture and chemical/mineralogical composition, these inclusions are called "nanogranites" and are interpreted to represent a crystallized inclusion of anatectic melt. Exceptionally and spatially associated with the nanogranites, inclusions containing glass have also been observed. In general, the preparation of the samples and analysis of these inclusions in migmatites and granulites require more sophisticated techniques than those applied to inclusions in xenoliths and enclaves, but the information on the composition of crustal anatectic melts can also be obtained.Since its discovery, new occurrences of nanogranite are being reported, or can be inferred from re-assessment of literature data, from migmatites and granulites worldwide. These former melt inclusions open new perspectives both for the microstructural approach to partially melted rocks and for the chemical characterization of natural crustal melts.Citation: 2011. Melt inclusions in migmatites and granulites. In: (Eds.)

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Nanogranites represent totally crystallized inclusions of anatectic melt trapped within peritectic minerals of\ud migmatites and granulites. They have recently been discovered in several locations. This discovery opens new\ud possibilities for investigating crustal melting processes, provided that an appropriate method for retrieving the\ud information contained within nanogranite inclusions is available. Here, we describe a series of remelting experiments\ud that have been performed at different temperatures and under dry, and H2O-added, conditions on nanogranite\ud inclusions hosted in migmatitic garnet, using a piston-cylinder apparatus. The glasses obtained by\ud quenching the sample from temperature that approaches the trapping temperature have compositions very similar\ud to those of preserved glassy inclusions coexisting with nanogranites in the same cluster. No significant differences\ud in H2O contents were observed for nanogranites rehomogenized under dry and wet conditions. Higher\ud (50–100°C) experimental temperatures resulted in dissolution of the host into the melt and inclusion decrepitation\ud with the loss of volatiles. Therefore, piston-cylinder remelting experiments may eliminate inclusion decrepitation,\ud maintaining the primary fluid contents in the originally trapped melt. These volatiles would otherwise be lost during\ud remelting experiments at ambient pressure. By preventing volatile loss, the inclusion does not have to be\ud overheated to achieve homogenization, and the compositions of quenched glasses so obtained can be assumed\ud to be those of melts produced (and trapped as inclusions) during crustal anatexis. The experimental approach\ud described here represents a promising technique for the successful rehomogenization of crystallized melt inclusions\ud from high-pressure environments, such as the mafic continental crust