MICROSTRUCTURES AND COMPOSITION OF ANATECTIC MELT INCLUSIONS IN METASEDIMENTARY ROCKS

Melt inclusions (MI) investigation represents a widely used approach to gain information on a number of igneous processes on the Earth system. The recent finding of crystallized and glassy MI in high-grade, partially melted metapelites and metagraywackes opened up the possibility to investigate also anatectic processes with the same method. The present work expands the study of Cesare et al. (2009; 2011) providing a detailed microstructural and microchemical investigation of three occurrences: Khondalites (India), Ronda migmatites (Spain) and Barun gneisses (Nepal). These are only a part of a continuously-growing list of very recent findings, suggesting that these MI are widespread in migmatites, despite they went (almost) unnoticed until now. Anatectic MI are so far reported in peritectic garnet and ilmenite, although they may be expected in other peritectic phases, more likely in hardest minerals, e.g. spinel. They preferentially form clusters with different degree of packing, a spatial arrangement that testify for their primary entrapment, i.e. during host mineral growth. Inclusions are generally isometric, more rarely tubular, and very small in size, mostly <=15 ?m, rarely up to 30 ?m. In most cases inclusions are crystallized, and contain a granitic phase assemblage with quartz, feldspars and one or two micas (depending on the case study), often along with accessory phases (mainly zircon, apatite, rutile). Because of these features, crystallized inclusions have been named "nanogranites" (Cesare et al., 2009). Besides fully crystallized nanogranites, partially crystallized inclusions are locally abundant, and also glassy inclusions, generally very small (<=8 ?m) may occur in the same cluster. After entrapment, inclusions underwent limited microstructural modifications, such as shape maturation and, locally, necking-down processes, that however should not have modified their original bulk composition. Decrepitation evidence are present, and locally very widespread (Barun gneisses), but also in this case no assemblage differences are recognizable in between decrepitated and preserved inclusions. Obviously the most important aim of these research is to gain representative data of the original anatectic melt in the targeted rock, that is likely to be represented by the bulk composition of the anatectic MI. To obtain a larger and reliable dataset, nanogranites have been homogenized and analyzed by electron microprobe (EMP), since workable glassy inclusions, which would not need any treatment before analyses, are generally very rare. Several problems must be considered, and overcome, in order to obtain reliable analyses, such as Na migration, size of the melt droplets close to EMP analytical limits, interaction with the host garnet on re-melting and possible occurrence of boundary layer effects. The obtained data show a leucogranitic and peraluminous composition, similar to those expected from literature and experiments on partial melting of natural rocks (Montel, Vielzeuf, 1997; Stevens et al. 2007). When plotted in a ternary CIPW diagram, they are at variable distances from the minimum melt composition, consistently with the inferred partial melting T of each melt, e.g. more K-rich melt from Khondalites form at T >=900°C (Johannes, Holtz, 1996). Bulk compositions of anatectic MI may become in the future the strongest constraint to the composition of the anatectic melt obtained by other means, pushing forward our understanding of how crustal rocks undergo partial melting.