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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]

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]

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]

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]