“NANOGRANITE” INCLUSIONS IN PERITECTIC MINERALS: FINDING THE ANATECTIC MELT IN MIGMATITES AND GRANULITES

Theoretical considerations suggest that peritectic minerals growing during incongruent melting reactions may act as hosts for inclusions of anatectic melt. We have recently verified this hypothesis in a granulite from the Kerala Khondalite Belt, India, discovering tiny inclusions within garnet; those between 15-25 μm are fully crystallized to a Qtz-Ab-Kfs-Bt cryptocrystalline aggregate (nanogranite), whereas inclusions <15 μm are completely glassy (Cesare et al., 2009). In these rocks, both re-melted nanogranite and glassy inclusions have a peraluminous, ultrapotassic granitic composition which does not correspond to a “minimum melt” and points to higher melting temperatures, in agreement with the UHT origin of the rock.

Nanogranite inclusions have been also found in migmatites from other geological settings, such as the HP migmatitic gneisses of the Ulten Zone (Italy) and the LP migmatites of Ronda (Spain). Inclusions are hosted in garnet at Ulten, and in both garnet and ilmenite at Ronda.

Our studies support the view that nanogranite inclusions are witness to the melt that was trapped by peritectic minerals growing during anatexis. Depending on P-T-X conditions, other peritectic phases such as orthopyroxene, spinel, kyanite and cordierite may be potential hosts to melt inclusions, and should be carefully (re)investigated.

Entrapment of melt inclusions during anatexis is not guaranteed and depends on several parameters like the amount of melt, the stress field acting on the rock, the growth rate of peritectic minerals, and the presence of impurities. Afterwards, the preservation of the inclusions depends on the extent of chemical interaction with the host mineral, and on the mechanical behavior of the host during the subsequent history, as microfracturing would allow the entrance of, and interaction with, fluids and alteration of the primary melt composition or nanogranite assemblage. Therefore, melt inclusions should be targeted in strong minerals from the least deformed rock domains.

Our results extend the frontiers of petrological and geochemical research in crustal melting, as the composition of anatectic melts can be directly analyzed rather than assumed.

References

Cesare B., Ferrero S., Salvioli-Mariani E., Pedron D. e Cavallo A. (2009) Nanogranite and glassy inclusions: the anatectic melt in migmatites and granulites. Geology, 37, 627-630