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Investigating the carbonate preservation efficiency (CPE) of continental crust is crucial to understand the global carbon cycle, which requires constraints on initial carbonate abundances (ICAs) of crustal rocks. To link Mg isotopes to ICAs, we present elemental and Mg isotopic data for Himalayan carbonate‐bearing and carbonate‐free metasedimentary rocks. Given no evident melt extraction or external‐fluid infiltration, ICAs of these samples can be independently estimated by elemental data. Despite different carbonate species in the protoliths, all the samples show congruent relationship between their δ26Mg and ICAs, owing to the elevated carbonate δ26Mg and Mg/Ca in protoliths of calcite‐rich samples resulting from diagenetic processes. When collated with literature data, we suggest the observed correlation here can be applied to most carbonate‐bearing (meta‐)sedimentary rocks. Based on a steady state box‐model, we constrained the modern net carbonate accretion flux (9.50−5.56+9.50${9.50}_{-5.56}^{+9.50}$Tmol/year) and the average time‐integrated CPE (∼80−43+20${80}_{-43}^{+20}$%) for continental crust. Investigating the fate of carbonate preserved in continental crust is fundamental for understanding its role playing in the global carbon cycle, but is hindered by the lack of knowledge about the initial carbonate abundance of metasedimentary rocks prior to modification (e.g., anatexis). By analyzing Himalayan metasedimentary rocks, here we show a congruent relationship between their δ26Mg and initial carbonate abundances, irrespective of their protolith carbonate species, and suggest it is applicable to most carbonate‐bearing (meta‐)sedimentary rocks. Based on this relationship, the carbonate preservation in continental crust was simulated using a steady state box‐model. The results indicate a very high carbonate accretion influx to the continental crust, seven times higher than the C degassing flux in the mid‐ocean ridge. Considering explosive degassing of the accreted carbonates during episodic tectonomagmatic events, the continental crust could have been an important driving force for regulating the atmospheric CO2during Earth's history. A strong correlation between bulk δ26Mg of carbonate‐bearing metasedimentary rocks and their initial carbonate abundances was establishedDespite different carbonate species in protoliths, both the calcite‐rich and dolomite‐rich samples show nearly identical correlationsSteady state box‐model on the carbonate preservation efficiency of continent indicates it is a notable force to regulate atmospheric CO2 A strong correlation between bulk δ26Mg of carbonate‐bearing metasedimentary rocks and their initial carbonate abundances was established Despite different carbonate species in protoliths, both the calcite‐rich and dolomite‐rich samples show nearly identical correlations Steady state box‐model on the carbonate preservation efficiency of continent indicates it is a notable force to regulate atmospheric CO2

A combined study of detailed petrographic observation, mineral chemistry analysis and phase equilibrium modeling indicates that the high-temperature eclogites from the Dabie orogen, central China, experienced two episodes of anatexis: the first is phengite dehydration melting during the exhumation of deeply subducted slices, and the second is heating melting related to the post-orogenic collapse. Petrographic evidence and clues of the anatectic events include biotite + plagioclase + garnet ± amphibole intergrowth in matrix and biotite + plagioclase intergrowth within amphibole porphyroblast. Pressure–temperature (P–T) pseudosection and modal variation diagram indicate that the biotite + plagioclase + garnet ± amphibole in matrix was formed by the reactions phengite + clinopyroxene + quartz = melt + sanidine + garnet + plagioclase and later melt + sanidine + garnet = biotite + plagioclase, while the biotite + plagioclase intergrowths within poikiloblastic amphibole were formed by the reaction amphibole + muscovite + epidote = biotite + plagioclase + melt. In addition, the combination of petrological observations and P–T estimates suggests that the first melting event occurred at the late Triassic, while the second is related to the early Cretaceous mountain-root removal and subsequent asthenospheric upwelling and heat input. As the P–T paths of high-temperature/ultrahigh-pressure rocks have high probabilities to cross-cut phengite-melting curves, phengite melting during decompression may be a common process in these rocks. Moreover, the coexistence of multiple episodes of anatexis in a single tectonic slice suggests caution when identifying and dating partial melting in high-temperature/(ultra)high-pressure rocks.

The Brossasco-Isasca Unit (BIU) of the southern Dora-Maira Massif (DMM), Western Alps, is one of the most studied ultra-high pressure (UHP) units in the world. However, the interpretation of UHP metamorphism in the BIU is still a highly debated and challenging issue. The structural and tectonometamorphic setting of the southern DMM is described in the literature as a tectonic “sandwich”, with the UHP unit in the middle, bounded by two high-pressure (HP) eclogitic units in the footwall (the San Chiaffredo Unit, SCU) and hanging wall (the Rocca Solei Unit, RSU), respectively. These three units are in turn sandwiched between two blueschist-facies units (the Pinerolo Unit, PU, at the bottom, and the Dronero-Sampeyre Unit, DSU, at the top). In contrast to the well-constrained P–T evolution of the BIU, peak P–T conditions for its bounding HP units are poorly constrained, most studies dating back to over 20 years ago and mostly relying on conventional thermobarometric methods. This study aims to update our knowledge about the P–T evolution experienced by the whole tectonometamorphic package of the southern DMM. For the first time, peak P–T conditions and prograde evolution for the five units (PU, SCU, BIU, RSU, DSU) forming the southern DMM tectonic “sandwich” are estimated using the same, internally consistent and therefore comparable, modern thermobarometric approaches. The study focuses on metapelites (i.e., garnet-bearing phengitic micaschists) and combines multi-equilibrium thermobarometry (Average PT) with the P–T pseudosection approach. Our results demonstrate that most of the southern DMM nappe stack (i.e., SCU, RSU and also the PU, that was originally considered as a blueschist-facies unit) experienced eclogite-facies metamorphism under similar peak P–T conditions (500–520 °C, 20–24 kbar), and followed the same prograde path, suggesting similar burial mechanisms. The UHP BIU followed an early prograde evolution similar to that of the other eclogitic units of the southern DMM tectonic “sandwich”. The attainment of UHP peak conditions occurred through an earlier steep, almost isothermal increase in pressure and a later increase in temperature. The DSU is the only unit of the southern DMM nappe stack that did not experience eclogite-facies metamorphism (peak metamorphism at blueschist-facies conditions: 450–470 °C, 17–18 kbar) and it is separated from the eclogitic units by a shear zone (the Valmala Shear Zone), whose interpretation requires further studies. These new data represent the inescapable starting point for any conceptual model aiming for a deeper understanding of the subduction/exhumation processes of UHP continental units.

As a tribute to Bruno Lombardo, our senior co-worker who sadly passed away in 2014, the geologic setting and metamorphic evolution of the first eclogites discovered in the Eastern Himalaya are summarised in the context of U-Pb analytical results from the original granulitized eclogite samples. These eclogites occur at the top of the Main Central Thrust (MCT) zone in the Ama Drime Range (Arun - Phung Chu Valley), southeastern Tibet. The eclogites record five superposed metamorphic stages: a first eclogite-facies stage ( P > 1.5 GPa, T > 580°C), two granulite-facies stages at decreasing pressure ( P = 0.8 – 1.0 GPa, T > 750°C; P ~ 0.4 GPa, T ~ 750°C), an amphibolitefacies stage (T = 700°C, P = 0.4 – 0.6 GPa) and a late retrogression stage (T < 690°C, P <0.4 GPa). The timing of eclogite-facies metamorphism in the Ama Drime Range is difficult to constrain because of the widespread high-temperature granulite-facies overprint and the poor preservation of high-pressure assemblages. Zircons separated and analysed by the U-Pb SIMS (secondary ionization mass spectrometry) method display complex and varied systematics. Low-U rim domains with low Th/U ratios (0.02 – 0.03) give young206 Pb/238 U ages of 13 – 14 Ma, which are consistent with age estimates of other workers for the timing of granulite-facies overprint, but do not constrain the age of eclogitization. Proterozoic ages (1.8 Ga) are attributed to inherited components. Discordant analyses define an array toward an upper intercept value of ca . 1000 Ma. Five grains that give ages of ca. 90 Ma may record contaminants incorporated during processing, or may represent the age of the protolith and thus may be the first indication in the Himalayan basement nappes for the widespread Cretaceous Rajmahal Trap basalt volcanism of NE India.

This contribution presents a geological mapping of the Quaternary succession of the Bulè and low Alpetto tributary valleys (Po Valley, Piedmont), together with preliminary data on its petrography. The investigated area, covering an altitude range between 3015 and 1485 m a.s.l., is located in the Monviso Massif Ophiolite Complex, at the contact with marble and calcschist of the Dora-Maira Massif cover. The Quaternary sequence mainly consists of glacial, outwash, debris and landslide deposits, with local torrential/avalanche and colluvial bodies. The investigation on the petrographic composition of these sediments has allowed to recognize the occasional occurrence of jadeitite boulders, likely exploited during the Neolithic period for axe head production. Four possible jadeitite blocks have been found; one of them is described here for the first time, sizing up to 1 m3. In detail, one of the finds is a Neolithic site of jade polishing in the high Bulè Valley that was referred to the middle of V millennium BC (Allisio, La Preistoria e l’alta Valle Po, Lar Editore, Perosa Argentina, 2012). The jadeitite blocks have been found in the debris (site A), landslide (site B), glacial (site C) and outwash (site d) sediments. The petrographic characterization of the new jadeitite samples might potentially shed light on the still controversial genesis of these peculiar rocks.

One of the peculiarities of Turin (NW Italy) lies on the presence of monumental arcades which mainly consist of stone material. These arcades, characterized by more than 12 km of interconnected paths, represent one of the widest city promenadesof Europe and are an architectural, aesthetic and socio-economic example unique in the world. This paper, analysing the urban axis of Via Roma (Rome Street), aims to study the material used in arcade construction. The main stones occurring in Via Roma have been identified and described from a petrographic and mineralogical point of view in order to find out the corresponding geological units and original quarry sites.The minero-petrographic study is accompanied by an architectural survey that was performed applying different methods, as well as the geometric mapping and the perspective rectification of span-types, of block terminations and of other architectonical elements, in plan and in elevation, of the arcades.This allows us to emphasize the merging of cultural and scientific interest for the stone materials used in the historical architecture of a town closely interconnected to the surrounding Western Alps orogenic chain.

The overall metamorphic reaction in an impure calcite marble from the ultra-high pressure (UHP) zone at Changpu, eastern Dabie Shan, China, is partitioned into local reaction domains defined by isolated grains of omphacite, garnet, epidote, quartz and ilmenite within the calcite matrix. These reaction domains witness different stages of the metamorphic evolution. Chemically homogeneous omphacite in some instances has partial or complete coronas of quartz which are interpreted as a prograde phenomenon, i.e. caused by replacement of a plagioclase precursor by omphacite with increasing pressure. Rare relics of talc in epidote indicate that the first breakdown reaction during exhumation was talc + omphacite + garnet = amphibole + epidote ± quartz (still within the eclogite facies). During this early retrogression stage, omphacite was partially or completely rimmed by chemically heterogeneous amphibole. Most garnets developed a thick corona of epidote plus some quartz and calcite during this early retrogression or are completely pseudomorphed. Where garnet occurs near or next to omphacite, this corona consists partly or entirely of amphibole.The second retrograde stage is commonly characterized by the breakdown of omphacite + quartz to albite + diopside + amphibole, often in the form of symplectite. Localized reactions become evident as omphacite at its site can decompose to three distinct types of symplectite (albite + diopside, albite + amphibole, albite + diopside + amphibole). Matrix quartz grains develop rims of individual diopside crystals at distances up to 2–3 millimetres from symplectites; additional outer rims of albite develop adjacent to garnet or corona epidote. Matrix quartz remote from omphacite sites is locally transformed into albite, where an Al-bearing mineral (mostly epidote) is found nearby. Epidote in contact with quartz develops albite coronas as it decomposes into albite + minute grains of Cerich epidote (allanite). Primary epidote and corona epidote remote from any quartz develop a complex compositional zoning with enrichment of mainly Ce3+ at the outer rim, indicating exposure to a fluid undersaturated in alumina. Finally, large ilmenite grains in the matrix develop thick overgrowths of titanite, but in several instances an intermediate step of rutile formation is preserved. All these features indicate a direct correlation of textures with transport distances of dissolved chemical species in a non-pervasively infiltrating pore fluid. Simplified open-system reactions were derived for each site at each metamorphic stage and show that Al must have been quite mobile during prograde quartz corona formation, whereas both Si and Al were relatively immobile during the two retrograde stages – the relative sequence of mobility being Na+ > Mg2+, Ca2+, Fe2+ >> Si4+ > Al3+, Ti4+. Mass-balance between sites is consistent and no significant gain or loss of cations is indicated.

Slices of continental crust pertinent to the lower Austroalpine domain of the western Alps, crop out within the ophiolitic Piemonte Zone. Among them, the Châtillon slice was studied in detail. The slice consists of orthogneiss with subordinate metabasics and very minor paraschist. The garnet-phengite-epidote-albite orthogneiss is characterised by polyphase garnet porphyroclasts. Metabasics consist of prasinite lenses and eclogite relics. Phengite-clinozoisite eclogite is characterised by small garnet idioblasts with prograde zoning; jadeite content in omphacite increases towards the rim; Si content in phengite decreases towards the rim. Garnet-glaucophane-phengiteparagonite micaschist is characterised by polymetamorphic garnet porphyroclasts, and small Alpine garnet idioblasts. A pre-Alpine amphibolite-facies metamorphism is inferred for the polymetamorphic rocks of the Châtillon slice. Paragneiss and micaschist probably derive from pre-Alpine “kinzigites”; the orthogneiss protolith was a late-Variscan porphyritic granitoid. Thermobarometry in the eclogite constrains the metamorphic peak at T ≤ 560 °C and P = 16 kbar. The HP minerals were partly retrogressed to greenschist-facies assemblages during the late Alpine tectono-metamorphic recrystallisation. The inferred Alpine P-T conditions are consistent with those for other Penninic and Austro-Alpine nappes of the northwestern internal Alps. The Châtillon slice is very similar to the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone and to the other eclogite-facies Austroalpine slices of the Dent Blanche Nappe, but it could also represent a portion of the Sesia-Lanzo Zone basement, which experienced a somewhat different subduction depth. The tectonic position of the Châtillon slice within the Piemonte Zone is essential to reconstruct the geometric relationships in the Austroalpine-Piemonte nappe stack of the northwestern internal Alps.

Abstract We report the first occurrence of a talc-kyanite assemblage typical of whiteschists in felsic ultrahigh-pressure metamorphic rocks of Dabie Shan, China. The whiteschist assemblage occurs in a leucocratic layer, less than half a meter thick, which crosscuts a coesite-bearing eclogite. Both its geologic setting and geochemistry suggest that the protolith was a felsic dyke, which suffered loss of some elements, in particular alkalies. The whiteschist consists of quartz, minor epidote, kyanite, talc and omphacite. Common accessories are rutile, apatite and zircon. Epidote is zoned, and the core includes quartz pseudomorphs after coesite and contains up to 5 wt% REE. Kyanite includes fresh omphacite with the highest jadeite content (XJd = 0.60). The host eclogite consists of omphacite (XJd = 0.44 to 0.37 from core to rim), garnet, quartz, kyanite, epidote, phengite (Si = 3.47 a.p.f.u.) and amphibole, and accessory rutile, apatite, zircon and ilmenite. Fresh coesite occurs in epidote, and its polycrystalline quartz pseudomorphs are included in both omphacite and kyanite. The whiteschist and the host eclogite share the same tectonometamorphic evolution, and show three metamorphic stages: (I) coesite-eclogite-facies stage at P = 2.6-2.7 GPa and T = 710 ± 20°C; (II) early decompression stage at P = 1.5-2.0 GPa and T = 650 ± 30°C; (III) late adiabatic decompression to P = 0.7 GPa and T = 670 ± 40°C.

The main felsic lithologies experiencing ultrahigh-pressure metamorphism (UHPM) are described, with a focus on rock association, nature, and composition of the peak minerals, as well as nature of the protolith. Felsic rocks usually preserve poor evidence of UHPM, being pervasively deformed and retrogressed during exhumation. Locally, however, unstrained lithologies are preserved that show UHPM minerals and even microstructures inherited from the protolith. Among the number of localities where UHPM has been reported, the following have been considered in detail— the southern Dora-Maira massif and Lago di Cignana in the Western Alps, the Dabie Shan-Sulu region in northeastern China, the Kokchetav massif in northern Kazakhstan, and the Western Gneiss Region of the Scandinavian Caledonides.At Lago di Cignana, metasediments of oceanic affinity are carbonate-bearing garnet-phengite-quartz schist and oxidized manganiferous quartz schists, all preserving rare coesite relics. In the Dora-Maira massif, the Brossasco-Isasca Unit includes coesite-bearing phengite-jadeite-almandine-quartz micaschist, pyrope whiteschist, jadeite-kyanite quartzite, sodic whiteschist, and jadeite-almandine orthogneiss. In the Dabie Shan-Sulu region, felsic rocks with clear evidence of UHPM include jadeite quartzite, talc-kyanite quartzite, and other kyanite-bearing quartzites. Moreover, other lithotypes like aegirine orthogneiss, paragneiss, and micaschist also may have been subjected to UHPM. In the Kokchetav massif, microdiamond-bearing rocks include garnet-biotite gneisses and schists, garnet-pyroxene and pyroxene-carbonate-garnet rocks, and possibly also whiteschists. Other localities where UHPM felsic rocks have been reported include the northern Tien Shan, northern Mali, the Maksyutov Complex in the southern Urals, and the Western Gneiss Region of the southwestern Caledonides.In felsic rocks, the most “common” UHP minerals are coesite, clinopyroxenes, garnets, epidotes, kyanite, phengite, talc, aragonite, dolomite, and accessory rutile, titanite, apatite, tourmaline, and zircon. Exceptionally, rare or unique phases also may be found as inclusions armored in more competent minerals. In the highest-P areas, microdiamond occurs, its growth being strongly controlled by the composition of the fluid phase. Mineral chemistry is sensitive to both bulk-rock chemical composition and P-T conditions: garnets are solid solutions of the pyralspite series, the Mn-rich members being high in calderite; pyroxenes are jadeite-diopside solid solutions; epidotes include allanite, zoisite, clinozoisite-epidote solid solutions, and piemontite; and micas are highly celadonitic phengites. In rocks with microdiamond, extreme UHPM conditions are further suggested by the unusual composition of some minerals, such as garnet high in Si, Y, and Yb, or Na-pyroxenes rich in K.Clear evidence of partial melting during UHMP is missing, although peak conditions are close to or above the minimum granite wet solidus. This may be due to either the aH2O < 1, or to melt extraction.

AbstractMetamorphism of the Askore Amphibolite, metabasaltic and metasedimentary medium-grade hornblendebearing schists at the northernmost portion of the Ladakh Terrane and of the Shyok Suture Zone, mainly a low-grade volcano-sedimentary series, has been studied in the area between the Chogo Lungma glacier and the Indus river halfway between Skardu and Rondu.In the Askore Amphibolite the peak assemblage in the amphibolite facies defines the regional metamorphic foliation, and is overprinted by a later static recrystallization at comparable P-T conditions. In spite of similar peak temperatures (630–650°C), geobarometry based on amphibole composition reveals a marked difference between garnet – epidote – andesine amphibolites exposed just above the Main Mantle Thrust at the head of Turmik valley, which equilibrated at high pressures (about 10 kbar) in late Miocene (Tortonian), and biotite – epidote – oligoclase amphibolites outcropping at the mouth of Turmik valley, which equilibrated at pressures of c. 6 kbar before late Eocene (Priabonian).The Dasu Ultramafite and other smaller lens-shaped bodies of low- to medium-grade metaperidotite separate the Ladakh Terrane from the Shyok Suture Zone. They are antigorite serpentinites, often with talc and magnesite, in which relict cumulitic structures are locally recognisable. The ultramafites may represent remnants of oceanic lithosphere separating the Ladakh-Kohistan island arc from the Asian plate, or they may be deep crustal rocks stripped from the basement of the arc.The mostly greenschist-facies Shyok Suture Zone shows the lithology of a calc-alkaline volcano-sedimentary series. It is supposed to be a remnant of a back arc basin of early Cretaceous age, separating the arc from the southern margin of Asia. Chloritoid, kyanite and biotite have been found in individual thrust sheets occurring at different structural levels and totally subordinate in volume to very low- and low-grade rocks. Such sharp differences in mineral paragenesis, together with field evidence of local shear, suggest a complex internal structure for the Shyok Suture Zone. From the head of Chogo Lungma glacier to the Basha valley, close to the contact with the Karakorum Metamorphic Complex, the rocks of the Shyok Suture Zone record a late Miocene metamorphic event at medium pressures and temperatures. Thermobarometric and geochronological evidence suggests that this event can be related to the exhumation and thrusting of the Karakorum metamorphic core over the Shyok Suture Zone.

The Zhu‐Jia‐Chong quartz eclogites occur within two mica–epidote–garnet–plagioclase gneiss as lenses up to 200 m thick and more than 1 km long. The eclogite is medium‐grained and consists of omphacite, garnet, clinozoisite, minor kyanite and quartz. No evidence for the former presence of coesite was found. Locally, poikiloblastic amphibole occurs, which is zoned from glaucophane in the core to Mg–cummingtonite in the rim. In some localities, clinozoisite occurs as polycrystalline aggregates, interpreted as pseudomorphs after former porphyroblasts of lawsonite. The eclogite is cut by irregular metamorphic veins up to 20 cm thick and several meters long, consisting of kyanite ± zoisite ± omphacite ± rutile ± apatite ± clinozoisite in a quartz matrix. Locally, vein minerals attain pegmatite‐like grain size. Geothermobarometric calculations and mineral compatibilities of peak and retrograde minerals of both eclogite and veins indicate a clockwise P–T path, characterized by initial eclogitic conditions prograding from the lawsonite to the epidote stability field. The veins are considered to have formed before peak metamorphic conditions by prograde dehydration of lawsonite to kyanite + zoisite/clinozoisite + quartz + fluid; the fluid released from the reaction favored the growth of coarse‐grained vein minerals. The eclogitic peak, estimated at ∼ 24 kbar and 700 °C, is followed by a two‐stage retrograde evolution: the earlier stage, characterized by a quasi‐adiabatic decompression from eclogite‐ to the amphibolite‐facies conditions, implies a rapid exhumation from depths of ∼ 90 km to mid‐crustal levels; the later stage, characterized by slower exhumation rates, led to the final exposure of eclogites and country rocks. The role of lawsonite as storage of fluid in subducting plates is therefore discussed.

New structural and metamorphic data from the Chogo Lungma glacier area and Basha valley enlighten the polyphased tectonometamorphic evolution of the Metamorphic Complex of Karakorum. A pre- to early D1 event is defined by a metamorphic cleavage in the core of some zoned garnet and staurolite porphyroblasts. The main tectonometamorphic event (D1) is characterized by N110°E south-vergent isoclinal folds associated with a metamorphic axial-plane cleavage formed in a predominantly flattening regime of deformation. It corresponds to a Grt-Bt-Mu-Ky peak metamorphism (M1: 620–730°C and 7.5–11 kbar). The D2 tectonometamorphic event corresponds to the development of several conical domes elongated striking N110°E to N140°E. The S2 cleavage is seldom individualized as it usually corresponds to a reworking of S1. In some areas, late evolution of D2 doming deformation is marked by a crenulation. Petrologically, D2 is characterized by shearing and boudinage of M1 minerals and by development of sillimanite. It corresponds to a decompressional evolution (7.5 down to 4 kbar) with only slight changes in temperature. A late event corresponds to large undulations and open folds which affect the D2 crenulation. We propose here a model of vertical plus dextral extrusion of the middle crust to explain the dome structures in Karakorum. The steep pattern of the metamorphic P-Tpath implies rapid exhumation rates and suggests that the different phases could be part of a continuum of deformation. Preliminary results by 40Ar39Armethod give young cooling ages (<10 Ma) for the D2 event (Villa et al., 1996, this issue). Thus, most of the observed tectonometamorphic evolution in central Karakorum could be fairly young.