Microstructures of melt inclusions in anatectic metasedimentary rocks

Abstract

The occurrence of crystallized and glassy melt inclusions (MI) in high-grade, partially melted metapelites and metagraywackes has opened up new possibilities to investigate anatectic processes. The present study focuses on three case studies: khondalites from the Kerala Khondalite Belt (India), the Ronda migmatites (Spain), and the Barun Gneiss (Nepal Himalaya). The results of a detailed microstructural investigation are reported, along with some new microchemical data on the bulk composition of MI. These inclusions were trapped within peritectic garnet and ilmenite during crystal growth and are therefore primary inclusions. They are generally isometric and very small in size, mostly £15 lm, and only rarely reaching 30 lm; they occur in clusters. In most cases inclusions are crystallized (nanogranites) and contain a granitic phase assemblage with quartz, feldspar and one or two mica depending on the particular case study, commonly with accessory phases (mainly zircon, apatite, rutile). In many cases the polycrystalline aggregates that make up the nanogranites show igneous microstructures, e.g. granophyric intergrowths, micrographic quartz in K-feldspar and cuneiform rods of quartz in plagioclase. Further evidence for the former presence of melt within the investigated inclusions consists of melt pseudomorphs, similar to those recognized at larger scale in the host migmatites. Moreover, partially crystallized inclusions are locally abundant and together with very small (£8 lm) glassy inclusions may occur in the same clusters. Both crystallized and partially crystallized inclusions often display a diffuse nanoporosity, which may contain fluids, depending on the case study. After entrapment, inclusions underwent limited microstructural modifications, such as shape maturation, local necking down processes, and decrepitation (mainly in the Barun Gneiss), which did not influence their bulk composition. Re-homogenized nanogranites and glassy inclusions show a leucogranitic and peraluminous composition, consistent with the results of partial melting experiments on metapelites and metagraywackes. Anatectic MI should therefore be considered as a new and important opportunity to understand the partial melting processes

Oriented multiphase needles in garnet from ultrahigh-temperature granulites, Connecticut, U.S.A.

“…(3) Multiphase inclusions in garnets within ultrahigh-temperature (UHT) granulites of the Barun Gneiss (Himalayas, Nepal) contain mostly quartz, biotite, muscovite, and plagioclase, together with accessory minerals such as zircon (Ferrero et al 2012). The inclusions are crystallized anatectic melt trapped within the garnet host; similar inclusions from other localities may even preserve glassy quenched melt (Ferrero et al 2012).…”

“…Garnets and other refractory minerals in mantle rocks, ultrahigh-pressure (UHP) rocks, and high-grade gneisses may contain multiphase solid inclusions that are relicts of fluids entrapped during garnet growth (e.g., Stöckhert et al 2001;van Roermund et al 2002;Carswell and van Roermund 2005;Ferrando et al 2005;Malaspina et al 2006;Frezzotti et al 2007;Perchuk 2008;Mposkos et al 2009;Ferrero et al 2012;Hermann et al 2013). Common inclusion phases are quartz/coesite, feldspars, micas, chlorite, carbonates, apatite, rutile, graphite, and zircon.…”

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

“…Sawyer 1996Sawyer , 2008; and references therein), (ii) glass analyses from the relatively few reported studies of glassy MI in metasedimentary/metagranitic anatectic enclaves and rehomogenized nanogranitoids in migmatites and granulites (Cesare et al 1997;2009;Acosta-Vigil et al 2007;Ferrero et al 2012Ferrero et al , 2014Ferrero et al , 2015Bartoli et al 2013aBartoli et al , 2013b press), and (iii) Helz 1976;Rutter and Wyllie 1988;Hacker 1990;Rushmer 1991;Rapp et al 1991;Wolf and Wyllie 1994;Rapp and Watson 1995;Skjerlie and Patiño Douce 2002;Schmidt et al 2004;Laurie and Stevens 2012;Quian and Hermann 2013).…”

Chemical diffusion of major components in granitic liquids: Implications for the rates of homogenization of crustal melts

“…The above model calculations depend on overstepping only to the extent that they have utilized results from overstepped melting experiments to evaluate the extent of heterogeneity of crustal melts at the source. One way to check for the presence of overstepping during crustal anatexis is by comparing the compositional patterns of melt obtained in experiments conducted and several degrees of overstepping (Acosta-Vigil et al, 2006b) with the compositions of crustal melts produced at the source area, recorded by melt inclusions in anatectic enclaves and migmatites (Acosta-Vigil et al, 2007;Cesare et al, 2011;Ferrero et al, 2012).…”