The continental crust has differentiated throughout geologic time, and crustal anatexis (i.e. partial melting of the crust), accompanied by melt extraction and ascent of magma to upper levels, constitutes one the most important mechanisms of differentiation. However, the details of processes connecting granitic magmas in their source region with granitic intrusions, or their volcanic equivalents, are far from clear. As a consequence, we still have a limited understanding of the nature of crustal differentiation associated with the geodynamic settings where crustal granitic magmas are produced, including the ratio of crustal growth to crustal reworking.
Melt inclusions (MI) are small droplets of liquid, commonly a few to tens of micrometers across, trapped by minerals that grow in the presence of melt. In addition to the “classic” glassy MI described in volcanic rocks and formed upon cooling of the magma, MI have been recently reported in crystalline crustal anatectic rocks, where they form during melting, and commonly appear partially to totally crystallized to a granitic assemblage with grain size ≤ 1 micrometer (nanogranites). Preliminary studies of nanogranites show that they represent a new and powerful approach to the study of crustal anatexis, as they can provide precise information on the composition of primary anatectic melts, and on the mechanisms of anatexis.