Microstructures of melt inclusions in anatectic metasedimentary

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.