Melt inclusions in migmatites and granulites

Bernardo Cesare, Antonio Acosta-Vigil, Silvio Ferrero, and Omar Bartoli

Important advances have been made during the last 15 years in the study of melt inclusions in minerals from migmatites and granulites. Pioneer work on high temperature metapelitic anatectic enclaves in peraluminous dacites from SE Spain has shown that droplets of granitic melt can be trapped by minerals growing during incongruent melting reactions, and that the composition of such trapped melts can be representative of that of the bulk melt in the system during the anatexis of the rock. Therefore melt inclusions may represent samples of embryionic anatectic granite. In most cases, these melt inclusions define microstructures that are typical of primary entrapment, and show little or no evidence of melt crystallization upon cooling. Rather, the melt solidified to glass due to very fast cooling associated with the submarine extrusion of the dacites. Hence inclusions can readily be analyzed for major and trace elements by conventional methods such as the electron microprobe or by laser ablation-inductively coupled plasma-mass spectrometry.

Based on the results from these quite unusual anatectic enclaves, one would expect similar melt inclusions to be present also in more conventional, slowly cooled, regionally metamorphosed migmatite and granulite terranes. As a matter of fact, recent investigations confirm this hypothesis. Tiny (<25 μm) inclusions containing a cryptocrystalline aggregate of quartz, feldpars, biotite and muscovite have been found in garnet from the metapelitic granulites of the Keraka Khondalite Belt, as well as in garnet and ilmenite from metapelitic and quartzo-feldspathic migmatites from the Alps, Ronda and the Himalayas. Due to the grain-size, texture and chemical/mineralogical composition, these inclusions are called "nanogranites" and are interpreted to represent a crystallized inclusion of anatectic melt. Exceptionally and spatially associated with the nanogranites, inclusions containing glass have also been observed. In general, the preparation of the samples and analysis of these inclusions in migmatites and granulites require more sophisticated techniques than those applied to inclusions in xenoliths and enclaves, but the information on the composition of crustal anatectic melts can also be obtained.

Since its discovery, new occurrences of nanogranite are being reported, or can be inferred from re-assessment of literature data, from migmatites and granulites worldwide. These former melt inclusions open new perspectives both for the microstructural approach to partially melted rocks and for the chemical characterization of natural crustal melts.