MIGMATITES, GARNET AND MELT INCLUSIONS: RECENT INNOVATIONS IN CRUSTAL PETROLOGY FOLLOWING LINCOLN'S FOOTSTEPS

Lincoln Hollister’s scientific approach has always tried to relate small-scale observations with crustal-scale processes. We have applied a similar approach to the understanding of the processes by which crustal melting occurs in Nature, by uncovering the occurrence of melt inclusions (MI) in migmatites and granulites and their importance for crustal petrology.

Studies on this subject started with glassy inclusions in anatectic crustal enclaves in lavas, and then progressed to regionally metamorphosed and partially melted crustal rocks, where MI are normally crystallized into a cryptocrystalline aggregate (nanogranitoid).

Microstructurally, melt inclusions in anatectic rocks are small, commonly 10 mm in diameter, and their main mineral host is peritectic garnet, although several other hosts have been observed.

Since the first paper in 2009, reported and studied occurrences are already a few tens. Inclusion contents vary from glass in enclaves that were cooled very rapidly from supersolidus temperatures, to completely crystallized material in slowly cooled regional migmatites. Although MI are small, their chemical composition can be successfully investigated combining several analytical techniques. In the case of crystallized inclusions the experimental remelting under confining pressure is a prerequisite. The melt is generally granitic and peraluminous, although granodioritic to trondhjemitic compositions have also been found.

Melt inclusions in migmatites and granulites show many analogies with their more common and long studied counterparts in igneous rocks, but also display very important differences and peculiarities. The fundamental one is that MI form during incongruent melting reactions along the prograde path of heating and anatexis. MI attest for the growth of their host garnet in the presence of melt. As a consequence, the inclusions store and preserve information on the composition of primary anatectic crustal melts, before these undergo changes in their way to produce crustal magmas. For these peculiar features, melt inclusions in migmatites represent an important innovation for the study of crustal melting, crustal differentiation, and even the generation of the continental crust.