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Serpentinites in subduction zones are important fluid reservoirs for crust-mantle interaction and arc magma formation. The serpentinite-derived fluids can carry fluid-mobile elements such as Ba and potentially affect the geochemical composition of the metamorphic rocks in the subduction zones. However, the Ba isotope behavior during interaction between serpentinite-derived fluids and metamorphic rocks is still poorly understood. The whiteschists of the Dora-Maira Massif in the Western Alps have experienced fluid infiltration by serpentinite-derived fluids during deep subduction. Here we report the Ba isotope data of the whiteschist and its protolith metagranite to investigate the Ba isotope behavior during fluid-rock interaction. The metagranites, which have experienced ultrahigh-pressure metamorphism, have a small variation of δ138Ba (−0.25 to 0.26‰), while the whiteschists display a large Ba isotope variation from −0.99‰ to 0.48‰. Such a large variation in whiteschists cannot be explained by chemical weathering, protolith heterogeneity, or kinetic effects. Instead, considering the negative correlations of δ138Ba with Ba and other fluid-mobile elements (e.g., Sr, Cs, Zn, and Cu) and the geochemical composition features of serpentinite-derived fluids, the large Ba isotope variation in whiteschists is inferred to be induced by the interaction between serpentinite-derived fluid and metagranite during intense fluid infiltration. This is consistent with the observation that the whiteschists with low Ba contents tend to exhibit heavier Mg-Fe isotope compositions but lighter Li isotope compositions, which indicates that the related fluids were derived from the dehydration of mantle wedge serpentinite. The fluid infiltrating process is well explained by a numerical model with high fluid-rock ratios of 4 to 12. Our modeling further suggests that serpentinite-derived fluid has a distinctly higher δ138Ba value than the depleted upper mantle. During fluid-rock interaction, heavy Ba isotopes prefer to be infiltrated into fluids with an apparent fractionation factor (Δ138Ba rock-fluid) of mostly −0.5‰ to −0.1‰ but can be down to −0.8‰ to −0.7‰ required for a few samples. A two-stage dehydration and metasomatism model at different depths can well explain the formation of Dora-Maira whiteschists. Whereas the Ba isotope composition of mantle wedge serpentinite is mainly dictated by crust-derived fluids from the subducting slab, an interaction between the serpentinite-derived fluids and the metagranite protolith resulted in a large Ba isotope variation of whiteschists. In case of a general involvement of serpentinites in subduction zones, serpentinite-derived fluids would significantly affect the Ba isotope compositions of subarc mantle peridotites and also arc lavas. [ABSTRACT FROM AUTHOR]

Serpentinite plays a crucial role in element mobility during seawater hydrothermal alteration of mid-ocean ridge peridotite on the seafloor and fluid metasomatism of peridotite in the subduction zone. However, element mobility and Mg isotope fractionation during serpentinization of peridotites are still uncertain. In order to address this issue, we present a geochemical study of serpentinites and their associated peridotites in the Xigaze ophiolite from southern Tibet. The serpentinites have lower MgO contents and Mg/Si ratios than the peridotites. The low Mg/Si ratios in serpentinites, along with the Isocon calculation results, indicate the Mg loss during serpentinization. In terms of trace elements, the serpentinites have higher contents of Ba, U, Sr and Pb than the peridotites, though both types of ultramafic rocks exhibit similar REE distribution patterns and HFSE contents. The serpentine-magnetite O isotope thermometry yields a relatively low serpentinization temperatures of 200–300 °C. The trace element composition of the serpentinites is distinct from that of forearc serpentinites but consistent with that of mid-ocean ridge ones. The peridotites show mantle-like δ26Mg values of –0.27 to –0.23‰ with an average of –0.25 ± 0.03‰ (2SD, n = 6). In contrast, the serpentinites have systematically higher δ26Mg values of –0.21 to –0.09‰ with an average of –0.16 ± 0.09‰ (2SD, n = 8). The Mg isotope composition of serpentinites shows no correlations with either the chemical weathering proxy or the talc modal content, indicating that the Mg isotope fractionation between serpentinite and peridotite is not caused by chemical weathering of the serpentinites. Instead, it probably resulted from the serpentinization of peridotite on the seafloor. Thus, there is the loss of not only element Mg but also isotopically light Mg during the serpentinization of peridotites. The isotopically heavy Mg serpentinites can be carried to oceanic subduction zones, giving rise to higher δ26Mg mantle sources for various basalts. The isotopically light Mg can be released into the seawater, regulating the Mg isotope composition of seawater in geological history. Taking into account the sources and sinks of Mg in subduction zone fluids, the formation of isotopically heavy Mg serpentinites during seafloor serpentinization can be probably an important process in changing the Mg isotope composition of mantle sources. [ABSTRACT FROM AUTHOR]

Subduction of low-density continental crust to subarc depths is generally associated with preceding subduction of the oceanic slab. However, the original oceanic signature is often overprinted by subsequent fluid metasomatism from the subducting continental crust. As a result, it is generally difficult to trace the geochemical processes of previous oceanic subduction in collisional orogens. This issue can be potentially resolved by applying B isotopes to metamorphic rocks from continental subduction zones. Here a combined study of whole-rock geochemistry and in situ tourmaline B isotopes was carried out for coesite-bearing whiteschist and phengite schist, as well as country rock metagranitoids from the Dora-Maira Massif in the Western Alps. While all these metamorphic rocks have a similar protolith of Permian granites, whiteschist and phengite schist experienced Mg-rich fluid metasomatism during the continental subduction, evidenced by their much higher MgO contents than the metagranitoids. Tourmaline in the metagranitoid (Tur-G) is schorlitic (X Mg = 15–55) with low δ11B values from −13 to −6‰, and is consistent with a magmatic origin. In contrast, tourmaline in metasomatic rocks (Tur-S) is mainly dravitic with the highest X Mg worldwide (X Mg = 90–98) and high δ11B values of −5 to +1‰. Integrated with whole-rock geochemistry and previous studies, Tur-S is interpreted to grow during the infiltration of external fluids that were highly enriched in MgO and relatively enriched in 11B. According to the B isotope compositions of precursor tourmalines, it is estimated that the external fluids had significantly higher δ11B values than +2.4‰. Based on B isotope compositions of both Tur-S and metasomatic fluids, our quantitative modeling suggests that the metasomatic fluids most likely originated from the mantle wedge serpentinite that was formed during the preceding oceanic subduction stage. Therefore, tourmaline B isotopes in the ultrahigh pressure metamorphic continental crust can be used to trace preceding fluid metasomatism at the interface between oceanic slab and mantle wedge. The mantle wedge serpentinite plays an important role in modifying the geochemical composition of deeply subducted supracrustal rocks and probably also the mantle sources of arc magmas. [ABSTRACT FROM AUTHOR]

As a highly fluid mobile element, barium (Ba) and its isotopes are excellent tracers for fluid associated processes in subduction zones. However, the behavior of Ba isotopes during high-pressure (HP) fluid–rock interaction in subduction zones is poorly constrained. To shed light on this issue, we focus on typical HP fluid−metasomatized rocks from the Sopron area of the Eastern Alps. These fluid−metasomatized rocks comprise Mg-rich leucophyllite and transitional gneiss that occurs between leucophyllite and metagranite protolith. These rocks provide excellent targets for the exploration of Ba isotope variations during fluid−rock interactions. From unaltered metagranite to metasomatized interiors, Ba contents steadily decrease, but δ138/134Ba values increase. In the first stage of fluid−related metasomatism, the transitional gneiss shows significantly higher δ138/134Ba values but lower Ba contents compared to its metagranite protolith, which can be attributed to the dissolution of K-feldspar characterized by high Ba contents but low δ138/134Ba values. In the second stage, leucophyllite was produced, displaying remarkably high δ138/134Ba values ranging from 0.20 to 0.47‰ and extremely low Ba contents down to 29.5 μg/g. These features are interpreted to result from an infiltration of serpentinite-derived fluids with high δ138/134Ba values and relatively low Ba contents, which caused an increase in the δ138/134Ba values of leucophyllite and a removal of Ba during fluid−rock interaction. Additionally, the contribution of fluids from mélange at the slab-mantle interface composed of dragged-down forearc serpentinite, carbonate-rich sediments and altered oceanic crust can result in the heterogeneous Ba isotope compositions of mantle wedges and arc lavas. This study provides new insights into Ba isotope behavior during HP fluid−rock interactions at forearc depths, which has a great bearing on understanding Ba isotope variations in mantle wedges and arc magmas. • Negative correlation occurs between δ138/134Ba and Ba contents of fluid-metasomatized rocks. • Dissolution of K-feldspar causes high δ138/134Ba of the transitional gneiss. • Serpentinite derived fluid-rock interaction explains high δ138/134Ba of leucophyllite. [ABSTRACT FROM AUTHOR]

• The slab-unfolding links plates and slabs without assumed vertical slab sinking. • The Lesser Antilles slab was laterally transported up to ∼900 km after subduction. • We reconstruct up to 18 mm/yr lateral slab transport since 50 Ma. • A new pre-subduction ridge-transform system of the Proto-Caribbean is presented. The link between surface tectonic plates and mantle slabs is fundamental for paleo-tectonic reconstructions and for our understanding of mantle dynamics. Many seismic tomography-based studies have assumed vertical slab sinking and projected mantle features to the surface to reconstruct paleo-trench locations or explain tectonic features. Here, we used a slab-unfolding approach that does not require assumptions about sinking paths or rates to re-interpret the seismic structure of the Lesser Antilles slab underneath the Caribbean. A recent study invoked mainly vertical slab sinking and a highly folded and deformed slab to explain seismic Caribbean mantle structures. However, our results show that the upper-mantle Lesser Antilles slab structure can be better explained by limited intra-slab deformation and up to ∼900 km lateral slab transport towards the northwest after subduction. Our results indicate that such lateral slab transport can occur even with probable weaknesses in the slab that originate from a subducted fossil ridge-transform system. We ascribe the lateral slab transport in the mantle to a kinematic connection with the North American plate, which has migrated northwestward since the Eocene. [ABSTRACT FROM AUTHOR]

• The Tianli Schist in South China records blueschist-facies metamorphism. • Monazite U–Pb dating shows the blueschist-facies metamorphism occurred at ∼ 930 Ma. • Modern-style plate tectonics might reinitiate in the Early Neoproterozoic. When and how plate tectonics operated throughout Earth's history remain a matter of debate. The absence of low-temperature/high-pressure metamorphic rocks during Earth's middle age (∼1.8–0.8 Ga) leads to some arguing for a temporary shutdown or change of modern-style plate tectonics throughout the Mesoproterozoic and Early Neoproterozoic. Here we report the first discovery of ∼930 Ma blueschist-facies metamorphic rocks (Tianli Schist) from the Jiangnan Orogen, South China. Peak pressure–temperature conditions of ∼425–475 ℃ and ∼0.8–1.1 GPa recorded by these units represent an average geothermal gradient of ∼14 ℃/km, characteristic of cold subduction zones. Available geologic and geochronologic constraints indicate that the Tianli Schist protolith was deposited in the southeast margin of the Yangtze Block and subsequently experienced blueschist-facies metamorphism during ocean closure at ∼930 Ma. Combined with the global metamorphic record, our results suggest that modern-style plate tectonics did not operate continuously since initiation in the Early Paleoproterozoic, but was inhibited in the Mesoproterozoic and restarted again at the end of Earth's middle age. [ABSTRACT FROM AUTHOR]

• Coesite firstly found in the granitic gneiss of the Dora Maira Massif provides direct evidence for the UHP metamorphism of country rocks. • Jadeite quartzite has a similar protolith of Permain granite to its host whiteschist. • The distinct Fe-O isotopes of jadeite quartzite are the result of complex fluid-rock interaction in the continental subduction zone. • Fe-O isotopes are powerful tracers for the fluid source and composition in the subduction zone. Fluid is a key agent for the mass transfer between the subducting slab and the mantle wedge, which greatly affects the evolution of the crust-mantle system at convergent plate boundaries. A geochemical study was carried out for coesite-bearing jadeite quartzite and its country rock granitic gneiss from the Dora Maira Massif in the Western Alps. The results provide new insights into the composition of metamorphic fluids and fluid-rock interaction processes in the continental subduction zone. Coesite inclusions are found for the first time in metamorphic zircons of the granitic gneiss. The ultrahigh-pressure (UHP) metamorphism was dated to occur at 34.7 ± 0.3 Ma, confirming that the country rock experienced the UHP metamorphism synchronously with the whiteschist and jadeite quartzite. The jadeite quartzite occurs as layers and boudins within the coesite-bearing whiteschist, and shows similar whole-rock REE distribution patterns to both granitic gneiss and whiteschist. In addition, relict domains of magmatic zircon in the three types of UHP metamorphic rocks exhibit similar U-Pb ages and δ18O values, indicating that they have the same protolith of granites. However, metamorphic zircons in the jadeite quartzite show significantly lower δ18O values of 6.1–7.3‰ than the relict magmatic domains of 9.4–10.6‰. Furthermore, the jadeite quartzite has whole-rock δ18O values of 7.9–8.7‰, significantly lower than those of 9.5–10.9‰ for the granitic gneiss but higher than 6.5–7.9‰ for the whiteschist. These differences suggest that the jadeite quartzite was probably formed through metasomatism of metagranite by external fluids with relatively low δ18O values. The jadeite quartzite shows high δ56Fe values of 0.69–0.87‰, considerably higher than those of 0.18–0.38‰ for the granitic gneiss, but falling within the range of 0.32–1.03‰ for the whiteschist from the same outcrop. The Fe isotope modeling results suggest that the metasomatic fluids responsible for the formation of jadeite quartzite were possibly derived from the host whiteschist under UHP metamorphic conditions. Such fluids are enriched in Si, Al and Mg, and have high δ56Fe but low δ18O values. Therefore, the jadeite quartzite records the action of metamorphic supercritical fluids at the subarc depth. If the metasomatized rocks in the continental subduction zone would dehydrate to generate fluids with variable geochemical compositions due to multistage fluid-rock interactions, the fluids could further infiltrate the surrounding rock and possibly the mantle wedge to result in further geochemical transfer from the subducting slab to the mantle wedge. [ABSTRACT FROM AUTHOR]

The mobility of redox sensitive elements such as iron, carbon and sulfur in subduction zone fluids is important for redox transfer between the subducting slab and mantle wedge. However, the speciation of Fe during its mobility in subduction zone fluids at forearc depths is still unclear. Here, we studied the Fe isotope compositions of high-pressure (HP) fluid-metasomatized rocks (Mg-rich leucophyllite and gneiss, which is mineralogically transitional between leucophyllite and metagranite) from the Eastern Alps and found evidence for two distinct types of Fe species during Fe mobility in subduction zone fluids. The protolith of both types of rocks is metagranite, which shows δ56Fe values of 0.12–0.25‰. Compared to the metagranite, the transitional gneiss shows slightly higher Fe 2 O 3t contents, lower Fe3+/∑Fe ratios and significantly lower δ56Fe values of −0.03–0.16‰, whereas the leucophyllite exhibits much lower Fe 2 O 3t contents, comparable or slightly lower Fe3+/∑Fe ratios and generally higher δ56Fe values of up to 0.55‰. These observations indicate that the granitic protolith has experienced two types of fluid metasomatism. Combined with previous results, we infer the mobility of Fe in subduction zone fluids at forearc depths via two kinds of Fe species, Fe2+-(CO 3), and Fe2+-(HS) or Fe2+-Cl. The Fe mobility was dominant in the form of the Fe2+-(CO 3) complex in a carbonate-rich fluid during the formation of transitional gneiss, consistent with reported radiogenic Sr and isotopically light Mg isotope compositions, whereas Fe loss in the form of Fe2+-(HS) or Fe2+-Cl complexes via dissolution of biotite and phlogopite can account for the formation of leucophyllites. According to the tectonic evolution of the Eastern Alps, both types of fluids possibly originated from the dehydration of the sediment-serpentinite mélange produced by the subducting oceanic slab of the Alpine Penninic unit. The diversity of ferrous iron species in the subduction zone fluids at forearc depths probably reflects the variation in the redox properties of fluids derived from carbonate-rich sediment and serpentinite under such conditions. Our results provide an excellent example of tracing Fe mobility with distinct Fe species in subduction zone fluids. [ABSTRACT FROM AUTHOR]

Fluids play a crucial role in element mobility and mass transfer at the slab-mantle interface in subduction zones. However, tracing the source and chemical composition of subduction zone fluids still remains challenging. High-pressure (HP) metamorphic leucophyllites, mainly composed of quartz, muscovite/phengite and Mg-chlorite, occur in several localities in the Eastern Alps and experienced significant Mg-metasomatism at forearc depths during the Eoalpine orogeny. They thus provide a good opportunity to explore the origin of Mg-rich fluids in the continental subduction zone. The leucophyllites are rich in Mg and poor in Fe, Na, and Ca and occur as lenses and layers in the country rocks of metagranites. Both leucophyllites and metagranites show similar whole-rock REE distribution patterns, and magmatic zircons from them exhibit similar U-Pb ages of ∼271 Ma, suggesting that both types of rock have the same protolith of early Permian granite. One metagranite that preserves magmatic composition shows a low δ26Mg value of −0.64‰, suggesting a contribution of carbonate-rich sediment when granitic melts were produced. A profile of samples from metagranite through transitional gneiss to leucophyllite in Hungary shows two trends in δ26Mg values. In the Vashegy sections, δ26Mg values firstly decrease from −0.64‰ to −1.30‰ during gneiss formation, and then increase to 0.29‰ during leucophyllite formation. In a second locality, the TV tower section, the δ26Mg values firstly decrease from −0.29‰ to −0.89‰ during gneiss formation, and then remain constant during leucophyllite formation. Two leucophyllites in Austria exhibit higher δ26Mg values of 0.05 to 0.09‰ compared to their country rocks of about −0.20‰. Based on the Mg isotope systematics and their relationships to whole-rock geochemical compositions, we propose two types of fluid in the subduction zone: (1) a low δ26Mg (<−1.3‰) fluid produced by dissolution of mainly Mg-calcite at HP conditions; (2) a high δ26Mg (>0.3‰) fluid derived from dehydration of talc-rich serpentinite. This is for the first time to find subduction zone fluids with distinct δ26Mg values at forearc depths. Both fluids possibly originated from dehydration of the sediment-serpentinite mélange at the slab-mantle interface. The fluids were heterogeneous in δ26Mg values and they metasomatized slices of continental crust along shear zones. The presence of fluids with δ26Mg values as low as −1.3‰ suggests that the dissolution of Mg-rich carbonates can be significant at forearc depths. Our results provide an excellent example of tracing different sources of subduction zone fluids by coupling Mg isotopes with other petrological and geochemical variables. They may also have great implications for the generation of magmas with variable Mg isotope compositions at convergent plate boundaries. [ABSTRACT FROM AUTHOR]

Fluids released by subducting slabs are important to the geochemical evolution of Earth's crust-mantle system. However, it is still not well constrained for the mobility of redox sensitive elements (like Fe, C or S) in subduction zone fluids, and its effect on the redox property of the overlying mantle wedge and its derived arc magmas. Iron isotope fractionation is sensitive to the redox state and speciation of Fe in fluids, which can potentially provide clues on the oxygen fugacity (f O 2) of slab-derived fluids. Whiteschists at the Dora-Maira Massif in the Western Alps have experienced ultrahigh-pressure metamorphism at subarc depths. They are rich in both SiO 2 (mostly >65 wt.%) and MgO (4–10 wt.%), and mainly composed of pyrope, quartz/coesite, talc, phengite and kyanite. They were demonstrated to have a granite protolith, but metasomatized by serpentinite-derived Mg-rich fluids at the slab-mantle interface in a subduction channel. Thus these rocks provide an excellent target to explore the fluid-rock interaction by Fe isotopes. The whiteschists show extremely high δ56Fe values of +0.32 to +1.22‰, being much higher than those of +0.10 to +0.38‰ for the surrounding metagranites. The Fe isotope composition of whiteschists significantly deviates from the igneous differentiation trend defined by rocks with basaltic to rhyolitic compositions. The heavy Fe isotope compositions must be produced by a metasomatic process. The much lower Fe 2 O 3 and FeO t contents and Fe3+/∑Fe ratios but higher δ56Fe values for the whiteschists relative to their protolith suggest the reduction of Fe3+ to Fe2+ and the loss of isotopically light Fe, probably in the form of Fe(II)-Cl and/or Fe(II)-(HS) complexes, through saline fluids and/or HS- bearing fluids. The reduction of Fe3+ to Fe2+ and the possible presence of HS- suggests the occurrence of relatively reduced fluids. Such reduced fluids were probably derived from serpentinite dehydration at the slab-mantle interface in the subduction channel. Therefore, the Fe isotope results demonstrate that the f O 2 at the slab-mantle interface can be locally highly heterogeneous, leading to various oxidation states in the mantle wedge and arc magmas. [ABSTRACT FROM AUTHOR]

Partial melting of crustal rocks may take place during continental collision, giving rise to anatectic melts. Their composition and evolution are major concerns in the chemical geodynamics of continental subduction zones. They are often recorded by multiphase solid (MS) inclusions enclosed by peritectic minerals in ultrahigh-pressure (UHP) metamorphic terranes inside collisional orogens. These MS inclusions generally show negative crystal shapes with the wedge-shaped off-shoot structure in garnet, omphacite and kyanite. They contain a number of silicate, carbonate and sulfate minerals such as K-feldspar, plagioclase, quartz, epidote, calcite and barite, with occasional occurrences of magnetite, zircon and pyrite. An integrated study of petrology, mineralogy and geochemistry indicates that the MS inclusions are the primary crystallization product of former silicate and carbonate melts. The silicate melts were derived from dehydration melting of hydrous minerals such as phengite and paragonite in UHP metamorphic rocks, and the carbonate melts were produced by partial melting of subducted carbonate minerals. Some MS inclusions show remarkably high Na contents, suggesting their derivation from dehydration melting of paragonite. In contrast, K-bearing MS inclusions are produced by dehydration melting of K-bearing hydrous minerals like phengite. Many studies have been devoted to the mineralogical and geochemical compositions of MS inclusions in UHP metamorphic rocks, with the aim to determine the time and mechanism of crustal anataxis during collisional orogeny. Various analytical methods were used to characterize the morphology, texture, mineral chemistry and trace element composition of MS inclusions. The results provide insights into the physicochemical properties of anatectic melts in continental subduction zones. The partial melting of deeply subducted crustal rocks would lead to their significant differentiation in lithochemistry and geochemistry. This process has great bearing on the tectonothermal evolution of continental subduction zones and the exhumation mechanism of deeply subducted crustal slices. [ABSTRACT FROM AUTHOR]

Fluids are important for mass transfer at the slab–mantle interface in subduction zones. However, it is usually difficult to trace fluids from specific sources in a subducting slab, especially those derived from dehydration of serpentinite. Coesite-bearing whiteschist at Dora-Maira in the Western Alps is characterized by strong Mg enrichment relative to the country rocks, which requires infiltration of Mg-rich fluids into the supracrustal rock. In order to constrain the origin of such Mg-rich fluids, we have performed an integrated study of whole-rock Mg and O isotopes, zircon U–Pb ages and O isotopes for the whiteschist and related rocks. Zircons in the whiteschist show two groups of U–Pb ages at ∼262 Ma and ∼34 Ma, respectively, for relict and newly grown domains. The Permian U–Pb ages of relict magmatic domains are consistent with the protolith age of host metagranite, suggesting that their common protolith is the Permian granite. The Tertiary U–Pb ages occur in coesite-bearing metamorphic domains, consistent with the known age for ultrahigh-pressure metamorphism. The metamorphic domains have δ 18 O values of 5.8 – 6.8 ‰ , whereas the relict magmatic domains have high δ 18 O values of ∼ 10 ‰ . Such high δ 18 O values are also characteristic of the metagranite, indicating that the whiteschist protolith underwent metasomatism by metamorphic fluids with low δ 18 O value of ∼ 2 – 4 ‰ . The whiteschist mostly has whole-rock δ 26 Mg values of −0.07 to 0.72‰, considerably higher than country-rock δ 26 Mg values of −0.54 to − 0.11 ‰ . Thus, the metamorphic fluids are not only rich in Mg but also heavy in Mg isotopes. They were probably derived from the breakdown of Mg-rich hydrous minerals such as talc and antigorite in serpentinite at the slab–mantle interface in the subduction channel. Therefore, the dehydration of mantle wedge serpentinite during the subduction and exhumation of continental crust can provide the Mg-rich fluids responsible for the metasomatism of crustal rocks at subarc depths. [ABSTRACT FROM AUTHOR]

Marble-hosted eclogite is volumetrically minor in collisional orogens, but its geochemistry has great bearing on the origin of deeply subducted crustal rocks and the fluid mobility of subduction zones. This paper presents a combined study of whole-rock major-trace elements and Sr Nd isotopes, mineral O isotopes, carbonate C and O isotopes, and zircon U Pb ages and Lu Hf isotopes for marble-hosted ultrahigh-pressure metamorphic eclogites from Rongcheng and Sanqingge in the Sulu orogen. The results provide insights into the protolith nature of eclogites and the fluid mobility of subduction zones. Zircon U Pb dating yields consistent middle Triassic ages for the two occurrences of eclogites, indicating new growth of metamorphic zircon during continental collision. The Sanqingge eclogite shows LREE-enriched patterns and negative ε Nd (t) of − 16.6 to − 14.3 for whole-rock and negative ε Hf (t) of − 27.1 to − 15.2 for metamorphic zircon. A few relict zircon domains show middle Neoproterozoic U Pb ages and negative ε Hf (t) of − 35.2 to − 15.5. Thus, the Sangqingge eclogite was metamorphosed from a mafic rock that was derived from partial melting of an anciently enriched mantle source. In contrast, the Rongcheng eclogite exhibits flat or even LREE-depleted patterns with negative ε Nd (t) values of − 12.2 to − 1.0 for whole-rock but positive ε Hf (t) values of 5.4 to 10.4 for zircon. The occurrence of interstitial and highly cuspate plagioclase along grain boundaries indicates the presence of partial melting in the eclogite. Thus, its positive zircon ε Hf (t) values are ascribed to the eclogite protolith of juvenile origin, whereas the LREE depletion is due to extraction of LREE-rich anatectic melt from the eclogite during the Triassic continental collision. As such, the Rongcheng eclogite was metamorphosed from a mafic rock that was derived from partial melting of a less enriched mantle source. All the eclogites from both areas show variably high δ 18 O values of 9.4‰ to 19.5‰. Oxygen isotope fractionations between mineral pairs mostly yield eclogite-facies temperature of 600 to 800 °C, suggesting that the high δ 18 O signature was inherited from their protoliths before the Triassic subduction. In combination with the field relation between the eclogite and marble, it is inferred that the eclogite protolith is probably basaltic tuff and its high δ 18 O value would be acquired together with the marble protolith during their deposition from the surface water. Therefore, there would be the limited isotopic exchange between marble and eclogite during continental collision. [ABSTRACT FROM AUTHOR]

• Delineation of the Neoproterozoic Kang-Dian Orogenic Belt was done. • Metamorphic monazite records metamorphism occurred between 885 and 778 Ma. • Metamorphic P-T conditions belong to amphibolite facies and intermediate P/T facies series. There have been hot debates about the tectonic affinity of the western margin of Yangtze Block (the N-S striking Kang-Dian region) in the Neoproterozoic, with former discussions mainly based on petrogenesis of diverse igneous rocks. This study provides new insights about the possible tectonic affinity of the Kang-Dian region in the Neoproterozoic from metamorphic perspective. The metamorphic ages of the Kang-Dian region have been determined by LA-ICP-MS U-Pb dating of monazite to be 885 ± 3 Ma, 778 ± 4 Ma and 840 ± 3 Ma, respectively, for three representative metapelitic samples collected from the Yuanmou, Yanbian and Miyi areas. Such long age duration of the metamorphic event (over 100 Ma) of the Kang-Dian region is obviously conflict to a super-plume scenario, but more likely to be the results of a long-lived orogenic process. Furthermore, the ubiquitous gneissosity and schistosity of the Neoproterozoic intrusions and metamorphic rocks of the Kang-Dian region also indicate an extensive compressional environment with prevailing deviatoric stress, i.e., most possibly being a subduction zone. The metamorphic P-T conditions of kyanite-bearing metapelite are estimated to be 7.5–9.0 kbar / 640–670 ℃, which belong to intermediate facies series and should be metamorphosed at middle to lower crustal level, this further confirms the collision process. Combined with previous data, we suggest that the Kang-Dian region was a Neoproterozoic orogenic belt at least during ∼ 880–780 Ma, and at that time the Yangtze Block was an isolated terrane or at the margin of the Rodinia supercontinent. [ABSTRACT FROM AUTHOR]

Subduction initiation is one of the key processes in the operation of plate tectonics. Our understanding of this process is largely derived from geochemical studies of basaltic rocks in ophiolites at continental margins. Because of the petrological inheritance in forearc basalts, it is important to acquire geochemical signatures for fluid metasomatism of the nascent mantle wedge at forearc depths. This may be provided by the study of peridotite in ophiolites, which is least affected by tectonic emplacement of the ophiolites onto continental margins. For this purpose, we have carried out a comprehensive study of whole-rock and mineral geochemical compositions for harzburgites from the Xigaze ophiolites in the Yarlung Zangbo suture zone, southern Tibet. The results provide insights into the forearc metasomatism during subduction initiation before the closure of Neo-Tethyan Ocean for the India-Asia continental collision in the Cenozoic. The harzburgites show high spinel Cr# values and strong whole-rock depletion in CaO, Al 2 O 3 and REE, suggesting high degrees of melt extraction (~10–20%) during seafloor spreading to form the Neo-Tethyan mid-ocean ridge basalts. They exhibit positive LILE anomalies in trace element distribution patterns, indicating metasomatism by subduction zone fluids. This is supported by the occurrence of tremolite and dolomite in some of the harzburgites. Based on the presence or absence of carbonate minerals, two types of metasomatic fluids are distinguished. One is the aqueous solutions, leading to the formation of tremolite and the enrichment of LILE in clinopyroxene. The other is the carbonate-bearing fluids, resulting in the formation of dolomites, tremolite and low Mg# (79–82) olivine. The metasomatism also results in the high contents of structural hydroxyl in pyroxenes and elevated δ18O values for the harzburgites. It is inferred that the fluid metasomatism was induced by incipient subduction of the juvenile Neo-Tethyan oceanic slab to forearc depths shortly after conversion of the Neo-Tethyan mid-ocean ridge into the trench. During this period of subduction initiation, the fluids were produced by heating dehydration of the sinking ridge crust to the forearc depths. It is these fluids that would metasomatize the overlying nascent mantle wedge, leading to superimposition of the geochemically enriched signature on the lithochemically depleted harzburgites in the ophiolites. Therefore, the forearc metasomatism of peridotite in ophiolite can be used as a geochemical proxy for subduction initiation. Furthermore, the Xigaze ophiolites were produced by the ridge-trench conversion for subduction initiation in the Mesozoic and they were emplaced onto the southern margin of the Asian continent in the Cenozoic. • The harzburgite experienced melt extraction at the mid-ocean ridge; • The harzburgite was metasomatized by two types of subduction zone fluids; • The fluid metasomatism is closely associated with subduction initiation; • The Xigaze ophiolites were generated through the ridge-trench conversion. [ABSTRACT FROM AUTHOR]

Deep-Earth cycling of nitrogen (N) along the subduction pathway, remains relatively poorly understood, particularly that related to deep subduction of continental crust. The ultrahigh-pressure (UHP) Dora-Maira Massif whiteschists in the Western Alps and their lower- P-T equivalent leucophyllites in the Eastern Alps show a decrease in N concentration and an increase in δ15N relative to their presumed country rock protoliths. On average, the whiteschists contain 20.7 ± 10.5 ppm N (mean ± 1σ) with δ15N air = +2.7 ± 2.5‰ (mean ± 1σ) and the leucophyllites contain 59.8 ± 12.3 ppm N with δ15N air = +4.3 ± 2.0‰, whereas the metagranitic country rocks from Dora-Maira contain 41.3 ± 12.5 ppm N with δ15N air = −1.9 ± 3.2‰ and the country rocks from the Eastern Alps contain 91.1 ± 42.5 ppm N with δ15N air = +1.7 ± 2.1‰. The loss of N and isotope shift in the whiteschists and leucophyllites from these two localities was accompanied by loss of LILE (Rb, Ba), Sr, CaO, Na 2 O, and FeO total. The other stable isotope systems applied to study both suites (Mg, Fe, O, Li and Ba) show no obvious correlation with variations in δ15N. Infiltration by and interaction with a serpentinite-derived fluid depleted in N and LILE could have led to leaching of N from the country rock protoliths and production of the +4.6‰ (Dora-Maira Massif) and +2.6‰ (Eastern Alps) isotope shifts adequately explained using a Rayleigh distillation model. The N exchange and leaching process is best explained as involving N 2 -NH 4 or NH 3 -NH 4 at about 550–600 °C, the latter the approximate temperatures at which the fluid infiltration is thought to have occurred. These results provide new insights regarding N behavior and isotopic fractionation during fluid-rock interactions occurring in UHP rocks and point to the importance of H 2 O-rich fluids from serpentinite dehydration as agents of metasomatism along deep subduction interfaces. Furthermore, the data for the metagranites indicate the potential of this lithology to retain significant amounts of N to at least 100 km and beyond sub-arc depths. • Alps whiteschists/leucophyllites contain less N with higher δ15N than protoliths. • Lowered N and elevated δ15N resulted from leaching during fluid-rock interaction. • The metasomatic fluid was most likely derived from serpentinite dehydration. • Metasomatism of this type could mobilize NH 3 /N 2 fluids into the upper mantle. • Metagranites can retain significant amounts of N during continental subduction. [ABSTRACT FROM AUTHOR]

Melt extraction and subduction zone fluid infiltration can produce light Fe isotope compositions of arc lavas relative to the MORBs. However, the Fe isotope signature of subduction zone fluid is poorly constrained, which has hampered our understanding of Fe cycling in the subduction zone. Jadeitite veins in forearc serpentinite provide a unique record for the characteristics of subduction zone fluids. Here we report Fe isotope data for Myanmar jadeitites and associated rocks to constrain the Fe isotope signature of subduction zone fluids. Two types of jadeitites from Myanmar display distinct δ56Fe much lower than typical igneous rocks. White jadeitites, directly precipitated from fluids, have variable δ56Fe from −0.60‰ to −0.04‰, while green jadeitites, formed by fluid-chromite interaction, have δ56Fe (−0.44‰ to −0.19‰) similar to the associated amphibole-rich blackwalls (−0.47‰ to −0.23‰). The light Fe isotope signature of the jadeitites is not induced by weathering, kinetic fractionation, dissolution of Fe-rich carbonate, or mineral dissolution-reprecipitation process. Instead, it most likely reflects the Fe isotope variation of serpentinite-derived fluids. Correlations of δ56Fe with REEs, Sb, and U in white jadeitites suggest that the jadeitite-forming fluid is a mixture of serpentinite-derived and sediment-derived fluids. In contrast, correlations of δ56Fe with Si, Al, and Cr in green jadeitites record the interaction of low-δ56Fe fluids with chromite. Considering the abundance of the serpentinite-derived fluid, it can effectively influence the Fe isotope compositions of the ambient mantle and arc lavas. • Myanmar jadeitites record signature of subduction zone fluids. • Both white and green jadeitites have variably low-δ56Fe. • Low δ56Fe of jadeitites reflect serpentinite-derived fluid signature. • Subduction zone fluid can influence the δ56Fe of ambient mantle and arc lavas. [ABSTRACT FROM AUTHOR]

Metamorphic fluids produced by dehydration of subducting crust transport mass and energy at the slab-mantle interface in subduction channels. It is commonly assumed that fluid flow is significant in oceanic subduction channels but insignificant in continental subduction channels. This assumption is challenged by a combined study of whole-rock geochemistry, Mg and O isotopes, zircon U-Pb ages and trace elements in coesite-bearing jadeite quartzites from the Dabie orogen, China. Although the target samples were collected from different outcrops in an area of ∼50 km2, zircon U-Pb dating yields similar discordia lines with not only consistent upper intercept ages of 1.9–2.0 Ga but also consistent lower intercept ages of 224–235 Ma. This indicates the same Paleoproterozoic protolith and the same Triassic metamorphism for these jadeite quartzites. The O isotope analysis of mineral separates and whole-rock yields variable δ18O values from 6.3‰ to 9.4‰, indicating involvement of supracrustal components. Except for one outlier at −0.43‰, all the other rocks give variable δ26Mg values from −0.16‰ to 0.61‰, much higher than normal mantle values. The whole-rock Mg isotopes show significant positive correlations not only with MgO contents but also with Rb/La, Rb/Gd and Rb/Nb ratios, but a negative correlation with Na 2 O contents. These observations indicate that the middle Paleoproterozoic protolith of jadeite quartzites was weathered to produce a kind of sedimentary rocks in a passive continental margin and then underwent significant metasomatism by metamorphic fluids with high δ26Mg values during the continental subduction in the Triassic. The metamorphic fluids were produced by the breakdown of biotite in the metasedimentary rocks during their subduction to subarc depths for ultrahigh-pressure metamorphism. They would have acquired their geochemical compositions not only from the biotite breakdown but also through leaching reaction with the TTG provenance. In view of the spatial occurrences of the target samples, the metamorphic fluids would have flowed inside the continental subduction channel on a large scale. This is the first report of the large-scale fluid flow in the continental subduction zone and therefore demonstrates that fluid flow can be significant in continental subduction channels. [ABSTRACT FROM AUTHOR]

Barium isotopes are a potential tracer of fluid-mediated recycling of subducted crustal materials into the mantle wedge. However, the behavior of Ba isotopes during metamorphic breakdown of Ba-rich hydrous minerals is not well understood. High-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks and associated veins in convergent orogens can provide important insight into Ba transport and Ba isotope fractionation during metasomatic fluid activities. This study reports Ba isotope data for whole rocks and mineral separates of a HP-UHP eclogite-quartz vein system from Bixiling in the Dabie orogen. The samples include epidote-phengite-bearing quartz veins and two types of eclogites. The Type I eclogites are the host of the quartz veins and were not affected by the metasomatic fluids, whereas the Type II eclogites are hosted in the veins and were strongly modified by the fluids. The results show that the Type I eclogites have relatively homogeneous δ138/134Ba ranging from −0.12 to −0.04‰. In contrast, the quartz veins have highly variable δ138/134Ba ranging from 0.16 to 0.51‰. The Type II eclogites have δ138/134Ba from 0.32 to 0.40‰, similar to those from 0.25 to 0.35‰ of the quartz separates within the veins, which may represent the average Ba isotopic composition of the fluid inclusions trapped in the quartz during vein formation. These results demonstrate that the vein-forming fluid has much higher δ138/134Ba than the host eclogites, indicating significant Ba isotope fractionation during dehydration of eclogites. Mineralogical evidence suggests that the fluid was probably derived from the breakdown of phengite at the HP eclogite-facies to amphibolite-facies stage during slab exhumation. The higher δ138/134Ba of the phengite separates (from −0.16 to −0.05‰) than those of the coexisting biotite separates (from −0.34 to −0.22‰) in the Type I eclogites indicate that the fluid released from the breakdown of phengite to biotite would be enriched in heavy Ba isotopes. This explains well the observed heavier Ba isotopic compositions of the quartz veins relative to the host eclogites. In summary, our results reveal that the breakdown of phengite during continental slab exhumation could produce Ba-rich fluids with remarkably higher δ138/134Ba than that of the host rocks, and thus might potentially influence the Ba isotopic systematics at convergent plate margins. [ABSTRACT FROM AUTHOR]

Impure marbles from ultra-high pressure (UHP) metamorphic belts bear significant information on the metamorphic evolution and carbon cycling during continental subduction and exhumation. In this study, detailed petrological data are presented and a P–T–X(CO 2 ) path is constructed for the impure marbles from the Dabie UHP terrane. Coesite relicts are discovered as inclusions within dolomite from the selected samples, which have a peak assemblage of dolomite, aragonite, garnet, omphacite, phengite, coesite, allanite and rutile. Estimated with the compositions of peak minerals, a P–T condition of 4.05–4.45 GPa at 740–820 °C is obtained by conventional geothermobarometry. The modeled fluid compositions have a low X(CO 2 ) (0.01–0.02) at the peak conditions, while the X(CO 2 ) firstly increased during isothermal exhumation and then decreased at later retrogression. The discovery of coesite within dolomite underscores the role of the “pressure vessel” models and highlights the significance of fluid unavailability in preserving coesite in UHP rocks. Neither petrological evidence nor independent peak P–T estimations support the breakdown of dolomite in the studied marbles, which contests recent suggestions. Analysis on the phase relations in the CaO–MgO–SiO 2 –H 2 O–CO 2 system shows that the bulk rock compositions have a large control on the stable UHP carbonate associations in carbonate-bearing rocks. The low X(CO 2 ) in the peak fluids indicates a weak decarbonation of the impure marbles under sub-arcs. In the last, a large fraction of CO 2 is shown to be sequestrated during regional retrogression of clinopyroxene marbles, which has a profound influence and must be considered for the global carbon cycling. [ABSTRACT FROM AUTHOR]

Abstract: Seven stages of mineral assemblage were recognized in the Zhimafang orogenic garnet lherzolite from the Sulu ultrahigh-pressure (UHP) metamorphic terrane of eastern China, which suggest that it experienced progressive mantle wedge convection during subduction of previous oceanic and subsequent continental slabs. M 1 is recorded by inclusions of high-Mg olivine, high-Mg–Al–Cr–Ca orthopyroxene and high-Mg clinopyroxene in the high Ca–Cr cores of the garnet porphyroclasts, suggesting that the Zhimafang peridotite was probably a garnet lherzolite originated from a deep and hot mantle wedge above the previously subducted oceanic slab. M 2 is represented by inclusions of high-Mg–Al chromite, high-Mg olivine and high-Mg orthopyroxene in the low-Cr mantles of the garnet porphyroclasts and core of the matrix garnet, suggesting that the rock was convected to shallow and hot mantle wedge, and was transformed to refractory spinel harzburgite or spinel dunite due to high degree of decompressional partial melting. M 3 is manifested by the origin of metasomatic clinopyroxene and orthopyroxene porphyroblasts with inclusions of high-Fe chromite, tremolitic or edenitic amphibole, phlogopite, barite and pyrite, indicating that the rock was convected to shallow, cold and wet corner of the mantle wedge, and was transformed to a fertile amphibole-bearing spinel lherzolite during continental subduction. The lherzolites was subsequently transformed to high-pressure (HP) amphibole-bearing garnet lherzolite (M 4) and UHP garnet lherzolite (M 5), indicating its convection to deep mantle wedge. Finally, the UHP garnet–lherzolite exhumed to the Earth’s surface, together with the UHP terrane, and subsequently retrograded to amphibole-bearing garnet lherzolite (M 6) and chlorite-amphibole-bearing spinel lherzolite (M 7). Detailed analyses of reaction textures and mineral compositions revealed several stages of metasomatism related to oceanic and continental subduction and exhumation. [Copyright &y& Elsevier]

Whole rock lithium (Li) and Li isotopic compositions of coesite-bearing whiteschists and their country rock metagranites from the Dora-Maira Massif in the Western Alps were investigated to understand their behavior during fluid-rock interaction in mantle depths of continental subduction zones. The whiteschists reveal relatively low δ7Li values (−6.4‰ to +2.4‰, with an average value of −2.1‰) in comparison to the country rock metagranites (δ7Li = −1.2‰ to +3.6‰, with an average value of +1.4‰). The δ7Li values of whiteschists do not show any correlations with major elements (e.g. Mg, Fe), Li, or isotopes (i.e., δ26Mg and δ18O values) and cannot be explained by kinetic fractionation. Hence, the low δ7Li values of whiteschists result from interactions with metasomatic fluids containing isotopically light Li that occurred in deep portions of a continental subduction zone. Fluid-rock interaction modeling suggests that the metasomatic fluids contain low δ7Li values (down to −4‰) and moderate Li concentrations (50 to 150 ppm), which were probably derived from the dehydration of serpentinites in the subduction channel. The presence of such low δ7Li fluids in deep subduction zones could cause Li isotopic heterogeneity in the earth's interior. • The whiteschists have relatively low δ7Li values (−6.4‰ to +2.4‰). • Isotopically light Li compositions resulted from metasomatism with fluids. • The metasomatic fluids were probably derived from the dehydration of serpentinites. • Such low δ7Li fluids could cause Li isotopic heterogeneity in the Earth's mantle. [ABSTRACT FROM AUTHOR]

Tourmaline, one of the most important hosts of boron (B) in crustal rocks, has rarely been found in natural high-pressure (HP) and ultra-HP (UHP) eclogites. Here, we report the first finding of tourmaline-bearing UHP eclogites and veins in the Dabie terrane, China. These distinctive samples occur exclusively in impure UHP marbles. Investigations on the eclogite-vein-marble system provide important insights into the origin, flow, and metasomatic effects of eclogite-facies, 11B-rich fluids in continental subduction zones. Petrologic and geochemical evidence indicates that tourmaline in the eclogites (Tur-E) formed by metasomatism due to the infiltration of a B-rich fluid under conditions of ca. 2.2–2.6 GPa and 610–660 °C. The HP veins, containing euhedral tourmaline (Tur-V) and occurring at the eclogite-marble contacts or in the interiors of eclogite lenses, represent the crystallized products of the infiltrating fluid. Systematic compositional variations along the profile from the interiors of the eclogite lenses to their margins and mass-balance calculations indicate large inputs of B, carbon, large ion lithophile elements, and light rare earth elements to the eclogites during fluid infiltration. All types of tourmaline, including the Tur-E and Tur-V as well as minor amounts of tourmaline in the marbles (Tur-M), have similar compositions (dravitic) with X Mg values [= Mg/(Mg + Fe)] of 0.7–0.8. In situ analyses using laser ablation multicollector inductively coupled plasma mass spectrometry show that all of tourmaline has high δ11B values (ranging from +6 to +15‰), which suggest a 11B-rich isotope signature for the infiltrating fluid. The tourmaline B isotope data, together with the detailed field observations and whole-rock Sr-Nd isotopes, reveal that the infiltrating fluid was derived from the impure marbles. Influx of such fluid was highly channelized and was mainly achieved along lithologic boundaries between UHP marbles and eclogites or along fractures in the eclogites. This study indicates that impure marbles are an important reservoir of isotopically heavy B in deeply subducted continental slabs. This point may be of particular importance because most B reservoirs in subducted continental crust typically have light B isotopic compositions. Our results highlight that subducted metacarbonate rocks could liberate eclogite-facies, B-rich, high-δ11B fluids and thus might exert important effects on the cycle of B and its isotopes in subduction zones. [ABSTRACT FROM AUTHOR]

Serpentinites entering subduction zones show selective enrichment in trace elements and are the most important carriers of B in this setting. The evolving B isotopic composition (δ11B) of serpentinite is used in studying water-mediated interactions between mantle and crust in subduction zones, but we lack a full understanding of the processes controlling B isotope fractionation during serpentinization that would allow this tracer to be better utilized. Boron isotope fractionation between lizardite/forsterite/diopside and aqueous fluids is investigated using quantum mechanics (density functional theory, DFT) and the first-principles molecular dynamics simulation (FPMD) here. The 11B enrichment sequence in minerals follows the order of forsterite > clinopyroxene (diopside) > white mica > phlogopite > lizardite. On the basis of boron configurations in aqueous fluid acquired from FPMD simulations based on DFT, the reduced isotopic partition function ratio (β-factor) of aqueous B(OH) 3 and B(OH) 4 − are 1000lnβ B(OH)3 = −20.1 + 37.07(1000/T) + 8.2(1000/T)2 and 1000lnβ B(OH)4- = −13.94 + 24.43(1000/T) + 9.12(1000/T)2 at the P-T range of 0–500 MPa and 298–1000 K. This theoretical approach gives an equilibrium B isotope fractionation of Δ11B lizardite-fluid = 15.89–22.88(1000/T) - 0.19(1000/T)2 between lizardite and neutral serpentinization fluid in the temperature range from 298 to 673 K. By contrast, the boron isotope fractionation between minerals formed as the slab descends (e.g., forsterite, diopside) and fluid released from subducted crustal lithologies is limited, indicating that secondary olivine minerals may inherit the δ11B signatures of the dehydrated serpentinites. This study provides new insights into how B isotopes can constrain boron sources and pathways in subduction zones, and trace phases recycled into the deep mantle and their later recycling in ocean island basalts. [ABSTRACT FROM AUTHOR]

Both slab-derived high-pressure (HP) or ultrahigh-pressure (UHP) aqueous fluids and hydrous melts are important agents for material cycling in subduction zones. Although numerous studies have investigated the genesis of both fluid phases, the temporal–spatial relationship and the chemical difference between aqueous fluids and hydrous melts during subduction-zone metamorphism remain mysterious. In this study, we present detailed petrological and geochemical studies on the UHP eclogites and enclosed epidote-quartz veins in the Dabie terrane (eastern China) to understand the compositions and behaviors of fluids/melts in subduction zones. The eclogites consist mainly of garnet, omphacite, and hydrous minerals (epidote, phengite, and paragonite). Two stages of epidote, including epidote inclusions in omphacite (Ep-in) and porphyroblastic epidote (Ep-p), are recognized in the eclogites. The obviously higher contents of strontium (Sr) and light rare earth elements (LREEs) in Ep-p than in Ep-in demonstrate the previous coexistence of lawsonite and Ep-in. The combination of petrographic analyses, epidote Sr isotopes, and phase equilibria modeling indicates lawsonite in the eclogite breakdown at ~2.2–2.7 GPa and 620–750 °C during the initial exhumation, accompanied by the significant release of H 2 O-rich fluids. This process led to the growth of Ep-p in eclogite and the precipitation of vein omphacite, quartz, and epidote (Ep-v1). On the other hand, many lines of evidence for partial melting in the eclogites are found, including (1) multiphase solid inclusions (MSIs) of Ky + Kfs ± other phases, which represent the in situ products of phengite partial melting; (2) MSIs of Kfs ± Pl ± Qz, which represent the crystallized products of hydrous melts; and (3) veinlets of Kfs ± Di that crosscut the omphacite. Importantly, the MSIs in texture (1) are exclusively hosted in the Ep-p, which formed by lawsonite breakdown. This distinctive phenomenon indicates that the partial melting of phengite was most likely triggered by the injection of H 2 O from the lawsonite breakdown (i.e., water-fluxed melting). Phase equilibria modeling suggests that the partial melting of phengite occurred at ~1.5–2.0 GPa and 700–750 °C during the further exhumation stage of the eclogites. The occurrence of Ep (Ep-v2) + Pl + Kfs in the veins indicates that melts derived from the eclogites infiltrated the vein system. The compositions of the fluids and melts are quantitatively constrained using the epidote compositions (Ep-v1 and Ep-v2, respectively) and epidote–fluid/melt partition coefficients. The results indicate that the fluids have trace element patterns similar to those of the melts with high contents of large ion lithophile elements (LILEs), Th, U, Pb, and LREEs. Thus, aqueous fluids have the ability to transfer significant amounts of mobile elements from the slab like the hydrous melts, if the appropriate P–T conditions and chemical characteristics of wallrocks can be satisified. Our study reveals multistage metamorphic-fluid/melt evolution in the UHP eclogites, involving metamorphic dehydration and subsequent partial melting during the exhumation of subducted continental crust. We provide petrological evidence linking fluid liberation to the occurrence of partial melting in eclogites. Moreover, this study indicates that the development of vein systems in HP–UHP rocks may provide high-permeability channels that allow the flow and escape of melts formed during later-stage process. We propose that the occurrence of early-stage dehydration events in deeply subducted slabs facilitates the generation of hydrous melts and their transport in subduction zones. • Evidence of both metamorphic dehydration and partial melting are observed in the eclogite–vein system. • The fluids induced by lawsonite breakdown may trigger partial melting of phengite. • Formation of veins in HP–UHP rocks facilitates the transport of subsequent melts in subduction zones. [ABSTRACT FROM AUTHOR]