ALBERT

Description: 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.

Description: Published

Description: 303-322

Description: 3.5. Geologia e storia dei vulcani ed evoluzione dei magmi

Description: JCR Journal

Description: restricted

Description: 〈span〉〈div〉Abstract〈/div〉The garnet-biotite-sillimanite enclaves from El Hoyazo are quenched anatectic metapelites found within peraluminous dacites (Betic Cordillera, SE Spain), representing a residual lower crust in the area after 40–60% of melt extraction. Anatexis occurred concomitantly with deformation in a regional metamorphic setting during the Upper Miocene at the base of the continental crust. Previous studies have provided detailed information on the pressure-temperature evolution, the sequence of melting reactions, and associated melt proportions and compositions. They show that enclaves mostly record peak metamorphic assemblages, mineral compositions, and, likely, microstructures, with minor changes upon entrapment within the dacite magma and rapid ascent and extrusion. The enclaves still preserve a proportion of the primary melt, that solidified to glass in abundant melt inclusions (MI) and matrix melt, permitting the study of the microstructural relationships between melt and residue. This study focuses on the geometry of the glass network at the microscale that, combined with the previously reported anatectic history, helps shed light on the mechanisms and history of melt drainage from these rocks.A representative sample of the enclaves was investigated by synchrotron μ-XRF and scanning electron microscopy to map the distribution of glass and minerals on three thin sections cut perpendicularly to the foliation. The combination of major and trace element μ-XRF distribution maps and detailed backscattered electron images evidence the presence of a pervasive and mostly interconnected glass network through the studied centimeter-scale sections. Interconnection is due to the crosscutting of films and glass-rich domains oriented parallel and at high angle with foliation. Although enclaves lost ∼40–60% of melt, they still contain ∼10–15% of glass, with a considerable proportion of it stored within the Mix, which is an aggregate of micrometer-sized fibrolitic sillimanite and glass. The distribution of glass (former melt) is not in textural equilibrium with the solid residue and resembles the interconnected network of deformation bands observed in migmatites of anatectic terranes at the mesoscale.Microstructural studies of melt pseudomorphs in migmatites and granulites of anatectic terranes are scarce, but the following remarkable interpretations can be made combining our observations of these enclaves: melt formed an interconnected network during anatexis that permitted melt segregation and extraction, though melt-residue textural disequilibrium is the rule rather than the exception. The proportion of melt present in residual migmatites can be much higher than the permeability threshold for crustal protoliths; in this particular study, two reasons for this might be that (1) melt was still being produced and flowing through the residual migmatite right before disaggregation and inclusion within the host dacite, where additional melt drainage was impeded by the hydrostatic stress field, and (2) a particular microstructure produced at the onset of anatexis, such as the Mix, acted as a trap for melt impeding or delaying melt segregation.〈/span〉

Description: This review presents a compositional database of primary anatectic granitoid magmas, entirely based on melt inclusions (MI) in high-grade metamorphic rocks. Although MI are well known to igneous petrologists and have been extensively studied in intrusive and extrusive rocks, MI in crustal rocks that have undergone anatexis (migmatites and granulites) are a novel subject of research. They are generally trapped along the heating path by peritectic phases produced by incongruent melting reactions. Primary MI in high-grade metamorphic rocks are small, commonly 5–10 μm in diameter, and their most common mineral host is peritectic garnet. In most cases inclusions have crystallized into a cryptocrystalline aggregate and contain a granitoid phase assemblage (nanogranitoid inclusions) with quartz, K-feldspar, plagioclase, and one or two mica depending on the particular circumstances. After their experimental remelting under high-confining pressure, nanogranitoid MI can be analyzed combining several techniques (EMP, LA-ICP-MS, NanoSIMS, Raman). The trapped melt is granitic and metaluminous to peraluminous, and sometimes granodioritic, tonalitic, and trondhjemitic in composition, in agreement with the different $$P-T-{a}_{{\mathrm{H}}_{2}\mathrm{O}}$$ conditions of melting and protolith composition, and overlap the composition of experimental glasses produced at similar conditions. Being trapped along the up-temperature trajectory—as opposed to classic MI in igneous rocks formed during down-temperature magma crystallization—fundamental information provided by nanogranitoid MI is the pristine composition of the natural primary anatectic melt for the specific rock under investigation. So far ~600 nanogranitoid MI, coming from several occurrences from different geologic and geodynamic settings and ages, have been characterized. Although the compiled MI database should be expanded to other potential sources of crustal magmas, MI data collected so far can be already used as natural "starting-point" compositions to track the processes involved in formation and evolution of granitoid magmas.

Description: This study presents the first precise evaluation of the extent of chemical equilibrium between felsic melt and crystalline residuum during the crustal anatexis of metasediments. The high precision of these results stems from the fact that, in this case, the melts are represented by pristine glasses that occur in the matrices of partially melted metapelitic enclaves, and as abundant glassy melt inclusions trapped by different minerals in the same enclaves. In previous studies of anatexis, the composition of the melt has been approximated by that of leucosomes in migmatites, which do not necessarily represent the composition of the melt very well. The present study is based on laser ablation analyses of 35 trace elements in the glass and principal minerals of enclaves included within Neogene peraluminous dacites exposed at El Hoyazo, SE Spain. The enclaves, which contain the assemblage plagioclase + biotite + sillimanite + garnet + glass + ilmenite + graphite ± K-feldspar ± cordierite ± quartz, represent fragments of metapelitic continental crust that was partially melted at pressures of 5–7 kbar during a regional metamorphic event. Matrix melt and melt inclusions within the enclaves were quenched to glass upon eruption of the dacite. The glasses in the melt inclusions are interpreted as the remains of melt produced by an initial melting reaction involving a muscovite-rich assemblage at 700–750°C, whereas the matrix glasses are consistent with continued melting and/or a later melt dominated by the incipient melting of biotite at 800–850°C. The distribution of trace elements between residuum and melt during anatexis of the enclaves is different from that predicted by disequilibrium models that consider effective distribution coefficients close to unity. Instead, the bulk melt and the major minerals (plagioclase, biotite, alkali feldspar, cordierite) were close to equilibrium (except for the case of garnet) during the generation of the melt inclusions and matrix glasses. The quenched melts (melt inclusions and particularly the matrix glasses) were not in equilibrium with accessory zircon and monazite, and are depleted in zirconium and the light rare earth elements. Recrystallization of minerals and diffusion in the melt were important controls during the redistribution of trace elements between the solids and the bulk melt. Local disequilibrium processes also took place at mineral–melt interfaces during the crystallization of peritectic minerals and the recrystallization of residual phases, as shown by the depletion of compatible elements in the melt inclusions.

Description: Using a metatexite from the Spanish Betic Cordillera as an example, we show that in situ and otherwise impossible to retrieve compositional information on natural anatectic melts can be reliably gained from experimentally rehomogenized melt inclusions in peritectic garnets. Experiments were conducted on single garnet crystals in a piston cylinder apparatus until the complete homogenization of crystal-bearing melt inclusions at the conditions inferred for the anatexis. The compositions of quenched glasses, representative of the early anatectic melts, are leucogranitic and peraluminous, and differ from those of leucosomes in the host rock. The H 2 O contents in the glasses suggest that melts formed at low temperature (~700 °C) may not be as hydrous and mobile as thought. Providing for the first time the precise melt composition (including the volatile components) in the specific anatectic rock under study, our approach improves our understanding of crustal melting and generation of S-type granites.