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Cordierite-bearing anatectic rocks inform our understanding of low-pressure anatectic processes in the continental crust. This article focuses on cordierite-bearing lithologies occurring at the upper structural levels of the Higher Himalayan Crystallines (eastern Nepal Himalaya). Three cordierite-bearing gneisses from different geological transects (from Mt Everest to Kangchenjunga) have been studied, in which cordierite is spectacularly well preserved. The three samples differ in terms of bulk composition likely reflecting different sedimentary protoliths, although they all consist of quartz, alkali feldspar, plagioclase, biotite, cordierite and sillimanite in different modal percentages. Analysis of the microstructures related to melt production and/or melt consumption allows the distinction to be made between peritectic and cotectic cordierite. The melt productivity of different prograde assemblages (from two-mica metapelite/metagreywacke to biotite-metapelite) has been investigated at low-pressure conditions, evaluating the effects of muscovite v. biotite dehydration melting on both mineral assemblages and microstructures. The results of the thermodynamic modelling suggest that the mode and type of the micaceous minerals in the prograde assemblage is a very important parameter controlling the melt productivity at low-pressure conditions, the two-mica protoliths being significantly more fertile at any given temperature than biotite gneisses over the same temperature interval. Furthermore, the cordierite preservation is promoted by melt crystallization at a dry solidus and by exhumation along P-T paths with a peculiar dP/dT slope of about 15–18 bar °C−1. Overall, our results provide a key for the interpretation of cordierite petrogenesis in migmatites from any low-P regional anatectic terrane. The cordierite-bearing migmatites may well represent the source rocks for the Miocene andalusite-bearing leucogranites occurring at the upper structural levels of the Himalayan belt, and low-P isobaric heating rather than decompression melting may be the triggering process of this peculiar peraluminous magmatism.

The metamorphic evolution of a granulitized eclogite from Punta de li Tulchi, NE Sardinia, Italy, reconstructed utilizing a combined microstructural (symplectitic, coronitic and kelyphytic features) and thermodynamic approach, involved a complex metamorphic history with equilibrium attained only at a domainal scale. Microstructural analysis and mineral zoning allow recognition of reactants and products involved in successive balanced mineral reactions. The P–T conditions at which each microstructure was formed are constrained by calculating isochemical phase diagrams (pseudosections) for the composition of effectively reacting domains. A pre-symplectite stage developed during prograde metamorphism under conditions ranging from 660–680 °C, 1.6–1.8 GPa to 660–700 °C at 1.7–2.1 GPa. Pseudosections calculated for subsequent clinopyroxene + plagioclase and orthopyroxene + plagioclase symplectitic coronae using the composition of effectively reacting microdomains suggest temperature in excess of 800 °C and pressures of 1.0–1.3 GPa. Modelling the development of later plagioclase + amphibole coronae around garnet during decompression yields conditions of 730–830 °C and 0.8–1.1 GPa. H2O (wt%) isomodes indicate that the granulitized eclogites were H2O-undersaturated at peak-P conditions and during most of the subsequent heating and decompression. This allowed the preservation of prograde garnet zoning in spite of the strong granulite facies overprint. The P–T evolution of Punta de li Tulchi granulitized eclogite is very similar in shape to that registered by other NE Sardinia retrogressed eclogites thus suggesting a common tectonic scenario for their evolution.

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 μm, and only rarely reaching 30 μm; 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 μm) 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.

In Alpine Corsica (France), deeply subducted metabasalts are well preserved as lawsonite-bearing eclogite (Law-Ecl), occurrence of which is restricted to similar to 10 localities worldwide. The Corsican Law-Ecl, consisting of omphacite + lawsonite + garnet + phengite + titanite, occurs as both single undeformed metabasaltic pillows surrounded by lawsonite blueschist (Law-Bs), and carbonate-bearing eclogitic veins. Law-Bs are found as variably deformed metabasaltic pillows locally cross-cut by eclogitic veins and consist of glaucophane + actinolite + lawsonite + garnet + phengite + titanite. Field evidence and microstructures reveal that both Law-Ecl and Law-Bs are stable at the metamorphic peak in the lawsonite-eclogite stability field. Isochemical phase diagrams (pseudosections) calculated for representative Law-Ecl and Law-Bs samples indicate that both lithologies equilibrated at the same conditions of similar to 520 +/- 20 degrees C and 2.3 +/- 0.1 GPa. Therefore, the coexistence at the same peak metamorphic conditions of Law-Ecl and Law-Bs implies that different portions of deeply subducted oceanic crust may store significantly different H2O contents, depending on bulk-rock chemical composition. In addition, thermodynamic modelling of phase equilibria indicates that the occurring progressive dehydration reactions, which are significantly depending on bulk-rock chemical composition, strongly influence rock densification and eclogite formation in subducting slabs.

The Lago di Cignana ultra-high-pressure unit (LCU), which consists of coesite–eclogite facies metabasics and metasediments, preserves the most deeply subducted oceanic rocks worldwide. New constraints on the prograde and early retrograde evolution of this ultra-high pressure unit and adjoining units provide important insights into the evolution of the Piemontese–Ligurian palaeo-subduction zone, active in Paleocene–Eocene times. In the LCU, a first prograde metamorphic assemblage, consisting of omphacite + Ca-amphibole + epidote + rare biotite + ilmenite, formed during burial at estimated P 3.2 GPa. The prograde path was characterized by a gradual decrease in the thermal gradient from ∼9–10 to ∼5–6 °C km−1. This variation is interpreted as the evidence of an increase in the rate of subduction of the Piemonte–Ligurian oceanic slab in the Eocene. Accretion of the Piemontese oceanic rocks to the Alpine orogen and thermal relaxation were probably related to the arrival of more buoyant continental crust at the subduction zone. Subsequent deformation of the orogenic wedge is responsible for the present position of the LCU, sandwiched between two tectonic slices of meta-ophiolites, named the Lower and Upper Units, which experienced peak pressures of 2.7–2.8 and

The El Arenal metagabbros preserve coronitic shells of orthopyroxene ± Fe-oxide around olivine, as well as three different types of symplectite consisting of amphibole + spinel, clinopyroxene + spinel and, more rarely, orthopyroxene + spinel. The textural features of the metagabbros can be explained by the breakdown of the olivine + plagioclase pair, producing orthopyroxene coronas and clinopyroxene + spinel symplectites, followed by the formation of amphibole + spinel symplectites, reflecting a decrease in temperature and, possibly, an increase in water activity with respect to the previous stage. The metagabbros underwent a complex P–T history consisting of an igneous stage followed by cooling in granulite, amphibolite and greenschist facies conditions. Although the P–T conditions of emplacement of the igneous protolith are still doubtful, the magmatic assemblage suggests that igneous crystallization occurred at a pressure lower than 6 kbar and at 900–1100 °C. Granulitic P–T conditions have been estimated at about 900 °C and 7–8 kbar combining conventional thermobarometry and pseudosection analysis. Pseudosection calculation has also shown that the formation of the amphibole + spinel symplectite could have been favoured by an increase in water activity during the amphibolite stage, as the temperature of formation of this symplectite strongly depends on aH2O (

Two impure ultrahigh-pressure (UHP) marbles, a calcite marble with the peak assemblage Grt + Phe + Cpx + Rt + (Arg) and a dolomite marble with the peak assemblage Crn + Chl + Rt + Dol (±Arg), from the same lens from the polymetamorphic complex of the Brossasco-Isasca Unit (BIU) (southern Dora-Maira Massif) have been petrologically investigated and modelled by calculating P–T phase-diagram projections for H2O–CO2 mixed-volatile systems. Thermobarometric data obtained from the calcite marble suggest Alpine peak conditions in the diamond stability field (4.0 GPa at 730 °C), and allow reconstruction of the earlier portion of the Alpine retrograde P–T path, which is characterized by a significant decompression coupled with a moderate and continuous cooling to 650 °C at 2.50 GPa. The modelled fluid compositions at peak conditions point to 0.025 ≤ X(CO2) ≤ 0.10 and X(CO2) ≤ 0.0012 in the calcite marble and dolomite marble, respectively, suggesting fluid heterogeneity at the local scale and an internally buffered fluid evolution of the studied impure marbles. The lack of micro-diamond in the BIU marbles is explained by the very-low X(CO2) values, which favoured relatively high fO2-conditions, preventing the formation of diamond at the UHP peak metamorphic conditions.