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Carbonate-bearing sediments, containing calcite, dolomite or magnesite as major carbonate components, are important constituents of sedimentary sequences deposited on passive margins through Earth's history. When involved in collisional orogenic processes, these sediments are metamorphosed at variable temperatures and pressures, and undergo decarbonation reactions. While the orogenic metamorphism of some of these lithologies (i.e. impure limestones and dolostones, marls sensu stricto and calcareous pelites) is relatively well understood, very little is known about the metamorphic evolution and decarbonation history of mixed carbonate–silicate rocks in which either dolomite or magnesite is the dominant carbonate component. Here we present the results of a petrologic study of representative samples of metasediments from Central Nepal, derived from Proterozoic dolomitic and magnesitic protoliths metamorphosed during the Himalayan orogeny. The main metamorphic assemblages developed in sediments originally containing different amounts of dolomite or magnesite are characterised in detail. Forward thermodynamic modelling applied to seven samples allows constraints to be placed on (i) the main decarbonation reactions, (ii) the P–T conditions under which these reactions took place, (iii) the composition of the fluids, and (iv) the amounts of CO2 released. We conclude that the CO2 productivity of dolomitic and magnesitic pelites and marls originally containing 15–40% carbonate is significant (>5.5 ± 1.0 CO2 wt% and up to 10.5 ± 1.5 CO2 wt%), whereas for carbonate contents above 60–70%, CO2 productivity is negligible unless aqueous fluids infiltrate from the outside and trigger decarbonation reactions. Since the dolomitic and magnesitic protoliths are significantly abundant in the sedimentary sequences involved in the still active Himalayan orogen, the decarbonation processes described here could contribute to the diffuse CO2 degassing currently observed at the surface. Furthermore, we propose for the first time that the peculiar magnesium-rich assemblages investigated in this study may derive from evaporitic protoliths, and that the whole Upper Lesser Himalayan Sequence may therefore represent the metamorphic product of a Proterozoic sequence consisting of alternating layers of carbonatic, evaporitic and pelitic sediments. [ABSTRACT FROM AUTHOR]

The coexistence of chloritoid and biotite in medium-pressure Barrovian terranes is quite uncommon, and the parameters controlling their equilibrium relations are still controversial. Various studies have already investigated the influence of pressure (P), temperature (T), bulk rock (X bulk) and fluid (X fluid) compositions on the stability of this assemblage. Here we apply forward thermodynamic modelling on amphibolite-facies metapelites from the upper portion of the Lesser Himalayan Sequence (eastern Nepal Himalaya) to test which parameters mostly influence the stability of the chloritoid + biotite assemblage. P – T isochemical phase diagrams calculated in the MnNKCFMASHTO system fail in reproducing the coexistence of chloritoid and biotite, predicting biotite appearance at higher temperatures than chloritoid breakdown. Neither the fluid composition (i.e. reduced H 2 O activity due to the presence of CO 2) nor a more oxidated state of the system favours their coexistence, while slightly H 2 O-undersaturated conditions expand the biotite stability field toward lower temperatures, allowing the development of the chloritoid + biotite assemblage. Kinetic factors could have further contributed to the stability of this assemblage: thermal overstepping of the chloritoid-consuming and staurolite-producing reaction, induced by the difficulty in the staurolite nucleation and/or by the sluggishness of chloritoid dissolution, could have enhanced the metastable persistence of chloritoid at temperatures compatible with the presence of biotite. Being the kinetics efficiency intrinsically linked to the degree of fluid availability, the two factors (i.e. H 2 O-undersaturated conditions and kinetics of the chloritoid-consuming reaction) were likely complementary rather than mutually exclusive. [ABSTRACT FROM AUTHOR]

Recent studies have demonstrated that kinetic factors can significantly influence garnet nucleation, delaying its appearance with respect to the equilibrium predictions. Overstepping of garnet nucleation occurs in both contact and regional metamorphic settings, with extremely variable degrees: The factors controlling such highly variable degrees of overstepping are still unclear. This study focuses on garnet nucleation and growth in aluminous metapelites from the Barrovian inverted metamorphic pile of the upper portion of the Lesser Himalayan Sequence (Upper‐LHS; central Nepal), with the aim of (i) investigating if (and how) the bulk‐rock composition can influence overstepping of garnet nucleation and (ii) which are the implications of garnet nucleation overstepping for the P–T evolution of a Barrovian metamorphic sequence. Detailed petrographic, microstructural and compositional data are presented for six phyllitic schists, containing porphyroblasts of garnet, staurolite and/or kyanite. Their P–T evolution is constrained through thermodynamic forward modelling (i.e., isochemical phase diagrams combined with isopleth thermobarometry), assuming that equilibrium was attained at every stage of their metamorphic evolution. Comparison between the P–T conditions inferred for the growth of garnet core, the assemblages predicted to be stable at these P–T conditions and the modelled garnet‐in reaction boundary suggests that the studied samples have experienced different degrees of apparent thermal (ΔT) and/or baric (ΔP) overstepping of garnet nucleation. We suggest that the bulk‐rock MnO and CaO amounts might have influenced the apparent ΔT and ΔP overstepping of the garnet‐in reaction: more specifically, the higher the bulk‐rock MnO content, the more pronounced the apparent ΔT overstepping, whereas the lower the bulk‐rock CaO content, the greater the ΔP overstepping. However, rather than an effective delay of garnet appearance with respect to equilibrium predictions, the apparent ΔT overstepping of garnet nucleation could reflect the attainment of the critical 0.5% threshold of garnet abundance, below which garnet is not readily detected in thin section. Kinetic factors seem much less critical in controlling the growth of the garnet rim at peak P–T conditions, confirming that peak metamorphic conditions constrained through equilibrium approaches based on the composition of garnet rim and of the matrix assemblage can be considered as reliable. Overall, the P–T paths of the studied samples are characterized by prograde heating coupled with tectonic overload (peak‐P conditions of 9.5–10.5 kbar, 580–590°C), followed by heating during exhumation (peak‐T conditions of 8.2–8.9 kbar, 610–630°C), supporting those thermo‐mechanical models that predict a period of slowdown (or quiescence) of the Main Central Thrust activity. [ABSTRACT FROM AUTHOR]

Partial melting of deep continental crust may occur during either prograde heating or decompression. Although the effect of temperature on crustal melting has been widely investigated, few experimental studies have addressed the question of the influence of pressure on crustal anatexis. To understand the influence of decreasing pressure on partial melting processes, the thermodynamic approach of isochemical phase diagrams has been applied to garnet–K-feldspar–kyanite–sillimanite anatectic gneisses (Barun Gneiss) from the Higher Himalayan Crystallines (HHC) of eastern Nepal. The main melt-producing reactions, the amount of melt produced during heating vs decompression, and the effects of melt loss on the mineral assemblages and compositions have been investigated along four ideal P–T trajectories, dominated by either heating or decompression. Based on these results, the observed microstructures and mineral compositions of the Barun Gneiss have been interpreted in terms of melt-producing vs melt-consuming reactions (e.g. growth of peritectic garnet with preserved ‘nanogranite’ inclusions vs microstructures related to back-reactions between solids and melt), and used to derive the metamorphic evolution of the studied samples. The P–T pseudosection modelling predicts that at least 15–20 vol. % of melt was produced at peak P–T conditions through dehydration melting of both muscovite and biotite, and that melt production was mainly triggered by heating, with or without the combined effect of decompression. The preserved granulitic peak metamorphic assemblage, however, is consistent with a significant loss of most of this melt. The P–T evolution inferred for samples from different, strategically located, structural levels of the Barun Gneiss is consistent with the expectations of a ‘channel flow’ model, including: (1) the clockwise shape of the P–T paths; (2) the estimated P at peak T (new data: 10–8 kbar at 800°C; model: 13–7 kbar at 800°C); (3) the decreasing P structurally upward, which defines a ‘normal’ metamorphic sequence, in contrast to the inverted metamorphic sequence occurring in the lowermost Main Central Thrust Zone; (4) the nearly isothermal exhumation of the structurally lowest sample, reflecting the progressive exhumation of rocks that have been entrained in the deep, high-T region of the channel, versus the nearly isobaric heating of the structurally uppermost sample, reflecting the evolution of those rocks that flowed outwards with the underlying channel. [ABSTRACT FROM PUBLISHER]

Besides carbonates, carbonaceous material in meta-sediments represents an important carbon reservoir on Earth. In orogenic belts, carbon can be stored as graphite, a contribution to the global carbon cycle that remains insufficiently constrained. Here, we focus on the Himalayan belt where moderately crystalline graphite is found at the footwall of the Main Central Thrust. Using various analytical methods, we investigate metamorphic conditions of graphite at increasing spatial scales in the Upper Lesser Himalayan Sequence (upper-LHS) phyllites of Nepal. First, we develop and validate a new SEM-BSE technique to measure total organic carbon (TOC) content. Graphite shows mostly inter- and intra-granular flaky crystals associated with carbonates. Elemental carbon analyses yield large range of TOC content in phyllites with a mean of 3.7 ± 0.4 vol.%. Inferred from micro-Raman spectroscopy, peak metamorphic temperature (peak- T) ranges from 519 ± 9 to 590 ± 8 °C. We observe a significant coherence of metamorphic conditions of graphitic phyllites at metric, decametric and kilometric scales using extensive data obtained in the Upper Trisuli valley, Central Nepal (n = 77), and, at larger spatial scales, using data from Far-Western to Eastern Nepal (n = 67). Mean carbon isotopic composition (δ13C) of −26.3 ± 0.1 ‰ indicates biogenic source. Less negative δ13C values, also reported in other orogenic belts, suggest interaction with pre-existing sedimentary carbonates, probably related to metamorphic isotopic re-equilibration. First-order extrapolations at the upper-LHS scale suggest meta-sediment organic carbon may represent up to 20 % of the total amount of carbon stored in orogens. Central Nepal segment, with higher TOC content and peak- T , is also characterized by the largest current metamorphic carbon dioxide (CO 2) emissions of the chain. This hardly coincidental association, at the scale of the Himalayan belt, between organic carbon stored as graphite and mobile inorganic carbon released as CO 2 , represents an important clue to investigate further for global carbon budget estimations. • Carbon stored as flaky graphite revealed in Nepal in Lesser Himalayan phyllites. • A microscopy method to quantify organic carbon content is developed and validated. • Carbon content and peak temperature from Raman spectroscopy coherent at all scales. • Higher peak metamorphic temperature in Upper Lesser Himalaya of Central Nepal. • Significant organic carbon stored in orogens to consider in global carbon budget. [ABSTRACT FROM AUTHOR]