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Volcanic arcs above subduction zones are thought to be the principal locations where juvenile magmatic crust forms and is refined to become continental crust with an andesitic composition. During this refinement mechanism, the formation of dense garnet pyroxenites (arclogites), represented by high-pressure cumulates and restites after partial melting, leads to the delamination of the lower arc crust. The Mercaderes-Río Mayo area in southern Colombia is the only known locality in an active volcanic arc where arclogitic xenoliths have been recovered. These xenoliths are entrained in the Granatifera Tuff, a late Cenozoic volcanic vent, and they mainly consist of garnet, clinopyroxene, amphibole, plagioclase, rarely scapolite, and accessory mineral inclusions of rutile, apatite, zircons, and quartz. Moreover, the arclogites are also characterized by the presence of melt inclusions (MI), which are mainly found within garnet, but can be also observed in amphibole, plagioclase, clinopyroxene, and scapolite. The glasses measured for the MI in garnet and scapolite typically have SiO2 contents >57 wt.%, ranging from andesite to rhyolite in composition. Petrographic and geochemical investigations allowed to discriminate between cumulitic and restitic arclogites, with the latter showing the concomitant presence of primary MI and quartz inclusions within the peritectic garnets. Therefore, our study provides for the first time a strong evidence, at the microscale, for the anatectic origin of some arclogitic xenoliths. Pressure and temperature conditions for the studied arclogites were estimated by intracrystalline geothermometry, elastic geothermobarometry, phase equilibria modelling and classical Fe–Mg exchange between garnet and clinopyroxene. Results fall within the range of 960°C to 1150°C and 1.6 to 1.9 GPa for most samples. We suggest that the investigated arclogites derive from the root of the active Colombian volcanic arc, where differentiation processes from mantle-derived melts and lower crust anatexis occur in close association. [ABSTRACT FROM AUTHOR]

Partial melting up to ultra high temperature (UHT) conditions is one of the major processes for the geochemical differentiation and reworking of the mid‐ to lower continental crust, with relevant implications on its rheological behaviour. UHT granulites from the Gruf Complex (European Central Alps) display garnet and sapphirine porphyroblasts containing a variety of primary melt inclusions (MI). Typically, MI in garnet occur as glassy and polycrystalline inclusions (i.e. nanogranitoids), the latter commonly organized in mm‐sized clusters associated with primary fluid inclusions (FI). Nanogranitoids are characterized by an elliptical faceted shape, with variable sizes ranging from 2 to 115 µm, while glassy inclusions show negative crystal shapes that usually never exceed 15 µm in diameter and present CO2‐rich shrinkage bubbles. The characteristic mineral assemblage observed in nanogranitoids consists of quartz, biotite, muscovite, plagioclase, K‐feldspar, kokchetavite and rarely aluminosilicates. Glassy and re‐homogenized MI are peraluminous and rhyolitic in composition, with SiO2 = 69 − 80 wt% and Na2O + K2O = 5 − 12 wt%. Commonly, the analysed MI have very high K2O (>6 wt%) and very low Na2O (<2 wt%) contents, indicative for potassic to ultrapotassic melts. Measured H2O contents of the melts range from 2.9 to 8.8 wt%, whereas CO2 concentrations are between 160 and 1738 ppm. Accordingly, calculated viscosities for re‐homogenized MI vary between 104 and 105 Pa·s. Related primary FI mainly contain CO2, with rare occurrence of CO and N2, and are commonly associated with quartz, as well as different carbonates and phyllosilicates. It is assumed that the source for the carbonic fluid was external and probably related to the degassing lithospheric mantle. Consequently, it is argued that anatexis was initially triggered by incongruent dehydration melting reactions involving biotite breakdown and proceeded in the presence of an externally derived COH‐bearing fluid. The coexistence of COH‐bearing fluid and MI indicates that partial melting occurred under conditions of fluid − melt immiscibility. Potassic to ultrapotassic MI in UHT granulites suggests that lower crustal anatexis may play a significant role in the redistribution of heat‐producing elements (such as K2O), potentially influencing the thermal structure of the continental crust. [ABSTRACT FROM AUTHOR]

We investigate the inclusions hosted in peritectic garnet from metapelitic migmatites of the Kinzigite Formation (Ivrea Zone, NW Italy) to evaluate the starting composition of the anatectic melt and fluid regime during anatexis throughout the upper amphibolite facies, transition, and granulite facies zones. Inclusions have negative crystal shapes, sizes from 2 to 10 μm and are regularly distributed in the core of the garnet. Microstructural and micro‐Raman investigations indicate the presence of two types of inclusions: crystallized silicate melt inclusions (i.e., nanogranitoids, NI), and fluid inclusions (FI). Microstructural evidence suggests that FI and NI coexist in the same cluster and are primary (i.e., were trapped simultaneously during garnet growth). FI have similar compositions in the three zones and comprise variable proportions of CO2, CH4, and N2, commonly with siderite, pyrophyllite, and kaolinite, suggesting a COHN composition of the trapped fluid. The mineral assemblage in the NI contains K‐feldspar, plagioclase, quartz, biotite, muscovite, chlorite, graphite and, rarely, calcite. Polymorphs such as kumdykolite, cristobalite, tridymite, and less commonly kokchetavite, were also found. Rehomogenized NI from the different zones show that all the melts are leucogranitic but have slightly different compositions. In samples from the upper amphibolite facies, melts are less mafic (FeO + MgO = 2.0–3.4 wt%), contain 860–1700 ppm CO2 and reach the highest H2O contents (6.5–10 wt%). In the transition zone melts have intermediate H2O (4.8–8.5 wt%), CO2 (457–1534 ppm) and maficity (FeO + MgO = 2.3–3.9 wt%). In contrast, melts at granulite facies reach highest CaO, FeO + MgO (3.2–4.7 wt%), and CO2 (up to 2,400 ppm), with H2O contents comparable (5.4–8.3 wt%) to the other two zones. Our results represent the first clear evidence for carbonic fluid‐present melting in the Ivrea Zone. Anatexis of metapelites occurred through muscovite and biotite breakdown melting in the presence of a COH fluid, in a situation of fluid–melt immiscibility. The fluid is assumed to have been internally derived, produced initially by devolatilization of hydrous silicates in the graphitic protolith, then as a result of oxidation of carbon by consumption of Fe3+‐bearing biotite during melting. Variations in the compositions of the melts are interpreted to result from higher T of melting. The H2O contents of the melts throughout the three zones are higher than usually assumed for initial H2O contents of anatectic melts. The CO2 contents are highest at granulite facies, and show that carbon‐contents of crustal magmas are not negligible at high T. The activity of H2O of the fluid dissolved in granitic melts decreases with increasing metamorphic grade. Carbonic fluid‐present melting of the deep continental crust represents, together with hydrate‐breakdown melting reactions, an important process in the origin of crustal anatectic granitoids. [ABSTRACT FROM AUTHOR]

The combination of compressional and shear wave velocity is an important tool in discriminating rock types and identifying residing melts within the continental crust. Here we report the laboratory measurements for Vp and Vs obtained in varying conditions up to those exceeding the beginning of melting (950 °C at 500 MPa confining pressure) on two residual garnet–biotite–sillimanite–cordierite–spinel metapelitic enclaves recovered from the Neogene dacites of Mazarrón (SE Spain). The enclaves preserve widespread interstitial rhyolitic glass as evidence of primary melt extraction. At 500 MPa, the experimental Vp ranges from 7.21 ÷ 7.46 km s − 1 at room temperature to 5.44 km/s at 950 °C. The mean Vs is 4.5 km/s at room temperature with shear-wave splitting of 0.25 ÷ 0.3 km/s, measured along three mutually orthogonal directions. When temperature increases, the Vs evidences a reversible slope change at about 650 °C, and the shear-wave splitting reduces to zero (isotropic material) at 850–950 °C, where the sample Vs is ~ 3.0 km/s. The Vp anisotropy is 7–10% up to 700 °C increasing to ~ 20% at 950 °C, while the Vs anisotropy continuously decreases with temperature from 5% to 7% at room temperature to zero at 950 °C. No mineral reactions are observed up to 650–700 °C; however, microstructure equilibrates due to the relaxation of the primary glass at the glass transition temperature. Between 850 °C and 950 °C, a new melting reaction is observed producing biotite + spinel + ilmenite + plagioclase + melt. At melting, the rock becomes isotropic for Vs, and Poisson's ratio increases to 0.30. Our seismic data show that the seismic signature of high grade metapelitic rocks is similar to that of mafic materials. The evolution of Vp and Vs at melting conditions is compatible with the geophysical data of the area, supporting the hypothesis of the current existence of anatectic melts at intermediate depths of the Alborán domain. [ABSTRACT FROM AUTHOR]

In recent years, melt inclusions (MI) or nanogranitoid inclusions (NI) found in peritectic minerals have become fundamental tools in crustal petrology and geochemistry. Despite their small size, which poses analytical challenges, a thorough microstructural and geochemical investigation of such inclusions may provide unprecedent in situ information to unravel anatexis, fluid regime of high-grade terranes and initial volatile contents of granitic magmas. The study of inclusions hosted in peritectic garnet from metapelitic migmatites of the Kinzigite Formation (Ivrea Zone, NW Italy) throughout the upper amphibolite-facies, transition and granulite-facies zones has revealed the coexistence of primary melt and fluid inclusions [Carvalho et al., 2018]. Inclusions have negative crystal shapes, size from 2 to 10 μm and are regularly distributed in the core of the garnet, and therefore were trapped in the same anatectic event. Melt inclusions in this case are NI characterized by the presence of cryptocrystalline aggregates of K-feldspar, plagioclase, quartz, biotite, muscovite, chlorite, graphite and, rarely, calcite. Polymorphs such as kumdykolite, cristobalite, tridymite are also present, and indicate that the original composition of the melt is preserved. FI have similar composition in the three zones and comprise variable proportions of CO2, CH4 and N2, commonly with siderite, magnesite, calcite, pyrophyllite and kaolinite, suggesting an initial COHN composition of the trapped fluid, later modified during cooling. These results represent the first and clear evidence of carbonic fluid-present melting in the Ivrea Zone. Anatexis of metapelites occurred through muscovite and biotite breakdown melting in the presence of a COH fluid, in a situation of fluid-melt immiscibility that is to be expected as a rule in graphitic protoliths. Our findings suggest that carbonic fluid-present melting of the deep continental crust represents, together with breakdown melting reactions, an important key process in the origin of crustal anatectic granitoids.[Carvalho, B.B., Bartoli, O., Ferri, F., Cesare, B., Ferrero, S., Remusat, L., Capizzi, L.S. and Poli, S., 2018. Anatexis and fluid regime of the deep continental crust: new clues from melt and fluid inclusions in metapelitic migmatites from Ivrea Zone (NW Italy). Journal of Metamorphic Geology. DOI: 10.1111/jmg.12463] [ABSTRACT FROM AUTHOR]