Using as a case study a granulite from the Kerala Khondalite Belt, India, we show that a former anatectic melt can be preserved as tiny (<25 μm) droplets within refractory minerals,in this case garnet. The melt is either fully crystallized as a Qtz-Ab-Kfs-Bt cryptocrystalline aggregate (“nanogranite”), or completely glassy in inclusions <15 μm. Both nanogranite and glassy inclusions have a peraluminous, ultrapotassic granitic composition that, in this case, does not correspond to a “minimum melt” and points to high melting temperatures, in agreement with the ultrahigh-temperature origin of the rock. This discovery indicates that peritectic minerals, growing during incongruent melting reactions, act as hosts for inclusions of anatectic melt, and that in the general case of slow cooling of the crust these inclusions will occur as nanogranite. Exceptionally, in the smallest inclusions, glass may be present due to inhibition of crystallization. Our results extend the frontiers of petrological and geochemical research in crustal melting, as the composition of natural anatectic melts can be directly analyzed rather than assumed

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

This review presents a compositional database of primary anatectic granitoid magmas, entirely based on melt inclusions (MI) in high-grade metamorphic rocks. Although MI are well known to igneous petrologists and have been extensively studied in intrusive and extrusive rocks, MI in crustal rocks that have undergone anatexis (migmatites and granulites) are a novel subject of research. They are generally trapped along the heating path by peritectic phases produced by incongruent melting reactions. Primary MI in high-grade metamorphic rocks are small, commonly 5–10 μm in diameter, and their most common mineral host is peritectic garnet. In most cases inclusions have crystallized into a cryptocrystalline aggregate and contain a granitoid phase assemblage (nanogranitoid inclusions) with quartz, K-feldspar, plagioclase, and one or two mica depending on the particular circumstances. After their experimental remelting under high-confining pressure, nanogranitoid MI can be analyzed combining several techniques (EMP, LA-ICP-MS, NanoSIMS, Raman). The trapped melt is granitic and metaluminous to peraluminous, and sometimes granodioritic, tonalitic, and trondhjemitic in composition, in agreement with the different $P-T-aH2O$ conditions of melting and protolith composition, and overlap the composition of experimental glasses produced at similar conditions. Being trapped along the up-temperature trajectory—as opposed to classic MI in igneous rocks formed during down-temperature magma crystallization—fundamental information provided by nanogranitoid MI is the pristine composition of the natural primary anatectic melt for the specific rock under investigation. So far ~600 nanogranitoid MI, coming from several occurrences from different geologic and geodynamic settings and ages, have been characterized. Although the compiled MI database should be expanded to other potential sources of crustal magmas, MI data collected so far can be already used as natural “starting-point” compositions to track the processes involved in formation and evolution of granitoid magmas.