Melt inclusions in migmatites and granulites

Abstract

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 plasmamass 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.Citation: 2011. Melt inclusions in migmatites and granulites. In: (Eds.)

Charnockite microstructures: From magmatic to metamorphic

“…2 and 4 in Cesare et al, 2009 or Fig. 7 in Cesare et al, 2011) in peritectic garnet in xenolithic granulites and migmatites (Cesare et al, 2009.…”

“…The glass phase in the melt inclusion represents a peraluminous rhyodacitic Cl-enriched melt phase that was CO 2 saturated in the lower crust (Chupin et al, 2001) indicating the presence of a high-salinity aqueous and a CO 2 -rich fluid. Another example of melt inclusions found in granulite xenoliths includes the ones described by Cesare et al (2009Cesare et al ( , 2011. They identified extremely small glass e and (larger) so-called "nanogranite" inclusions (see Figs.…”

When the Continental Crust Melts

“…The melt inclusion has a typical polyhedric shape ("negative crystal"; see Cesare et al 2011) and crystallized into a fi negrained aggregate of quartz (Qtz), biotite (Bt), K-feldspar (Kfs), apatite (Ap) and plagioclase (not visible in this image). IMAGE COURTESY OF OMAR BARTOLI, UNIVERSITY OF PARMA, ITALY…”

The composition of nanogranitoids in migmatites overlying the Ronda peridotites (Betic Cordillera, S Spain): the anatectic history of a polymetamorphic basement

“…Recent studies have demonstrated that melt inclusions (MI) constitute a new and powerful tool to investigate crustal anatexis (Cesare 2008;Cesare et al 2009Cesare et al , 2011Cesare et al , 2015. Studies of MI in crustal anatectic enclaves found within peraluminous dacites of SE Spain have shown that these trapped droplets of melt, now solidified to glass due to rapid ascent and extrusion, can provide precise information on the composition of primary anatectic melts during, and on the mechanisms of, crustal anatexis (Cesare et al 1997(Cesare et al , 2003Acosta-Vigil et al 2007, 2012a.…”