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Subduction zones are important for understanding of the global carbon cycle from the surface to deep part of the mantle. The processes involved the metamorphism of carbonate-bearing rocks largely control the fate of carbon and contribute to local carbon isotopic heterogeneities of the mantle. In this study, we present petrological and geochemical results for marbles and carbonated eclogites in the Southwestern Tianshan UHP belt, NW China. Marbles are interlayered with coesite-bearing pelitic schists, and have Sr-Nd isotopic values (eNd (T=320Ma) = −3.7 to −8.9, 87Sr/86Sr (i) = 0.7084–0.7089), typical of marine carbonates. The marbles have dispersed low δ18OVSMOW values (ranging from 14 to 29‰) and unaffected carbon isotope (δ13CVPDB = −0.2–3.6‰), possibly due to infiltration of external H2O-rich fluids. Recycling of these marbles into mantle may play a key role in the carbon budget and contributed to the mantle carbon isotope heterogeneity. The carbonated eclogites have high Sr isotopic compositions (87Sr/86Sr (i) = 0.7077–0.7082) and positive eNd (T = 320 Ma) values (from 7.6 to 8.2), indicative of strong seafloor alteration of their protolith. The carbonates in the carbonated eclogites are mainly dolomite (Fe# = 12–43, Fe# = Fe2+/(Fe2+ + Mg)) that were added into oceanic basalts during seafloor alteration and experienced calcite – dolomite – magnesite transformation during the subduction metamorphic process. The uniformly low δ18O values (∼11.44‰) of carbonates in the carbontaed eclogites can be explained by closed-system equilibrium between carbonate and silicate minerals. The low δ13C values (from −3.3 to −7.7‰) of the carbonated eclogites most likely reflect contribution from organic carbon. Recycling of these carbonated eclogites with C isotope similar to typical mantle reservoirs into mantle may have little effect on the mantle carbon isotope heterogeneity.

In order to study the behavior of high field strength elements (HFSE) during retrograde overprint of ultrahigh-pressure (UHP) eclogites, analysis of Nb and Ta concentrations was carried out on bulk rock, rutile (in both veins and host rocks) and titanite in the host eclogite. The studied samples were collected from the UHP metamorphic belt of southwestern Tianshan, China. Petrographic observation and phase equilibria modeling show that the host eclogites have experienced UHP metamorphism and the rutile-bearing veins are thought to be originated from an internal fluid source, probably by lawsonite dehydration during exhumation. The presence of vein rutile indicates HFSE could be mobilized from host eclogites to veins, which is probably facilitated by complexation with dissolved Na–Al silicates and fluorine-rich fluids. Changes in fluid composition (e.g., F−1, X(CO2)) may trigger the precipitation of rutile. Rutile/fluid partitioning may be the key to fractionating Nb and Ta, with preference for Ta in the fluid, resulting in Nb/Ta ratio of rutile in the veins lower than that in the host eclogite. Besides, the transformation of rutile into titanite also might be an effective mechanism for fractionating Nb from Ta, resulting in the intra-grain Nb–Ta zonations in vein rutile. The Nb–Ta mobility and fractionation can happen during exhumation of the UHP eclogite, which should be very important for understanding the behavior of HFSE in subduction zone metamorphism.

The magnitude of crustal contamination in intra-continental OIB-like basalts is commonly difficult to assess in terms of major, trace-element and Pb–Sr–Nd isotopes. However, the Re–Os isotope system is a very sensitive tracer of the crustal component during formation of basaltic rocks. Here we report new Sr–Nd–Pb–Os geochemical and isotopic data from the Cretaceous Duofutun Volcanic Rocks (DVRs) of West Qinling, central China. The DVRs exhibit low Os abundances (4.46–42.02 ppt) with Re abundances ranging from 45.58 to 812.98 ppt, variably high Re/Os (3–126), and high initial 187Os/188Os ratios (0.2112–0.6784). We propose that the degree of partial melting is responsible for the first-order control on the variation of Os concentrations. Even the least radiogenic sample still possesses a higher initial 187Os/188Os ratio than the primitive upper mantle, implying a non-negligible contribution from high Re/Os and 187Os/188Os reservoir(s). Both the crust and pyroxenite-rich mantle are reliable candidates, but the latter cannot be reconciled with the Os mass balance between pyroxenite and ambient peridotite. Hence, we contend that these rocks were contaminated by crustal materials en route to the surface. Assimilation/fractional crystallization modeling manifests the observed isotopic variations can be produced by minor assimilation of the Proterozoic mafic lower crust. In addition, the intra-continental OIB-like basalts show a relatively wider range of 187Os/188Os ratios in comparison with MORB and OIB, confirming the continental crust can have a higher concentration of radioactive Os, which is induced by time-integrated growth.

The intermediate–mafic–ultramafic rocks in the Jianzha Complex (JZC) at the northern margin of the West Qinling Orogenic Belt have been interpreted to be a part of an ophiolite suite. In this study, we present new geochronological, petrological, geochemical and Sr–Nd–Hf isotopic data and provide a different interpretation. The JZC is composed of dunite, wehrlite, olivine clinopyroxenite, olivine gabbro, gabbro, and pyroxene diorite. The suite shows characteristics of Alaskan-type complexes, including (1) the low CaO concentrations in olivine; (2) evidence of crystal accumulation; (3) high calcic composition of clinopyroxene; and (4) negative correlation between FeOtot and Cr2O3 of spinels. Hornblende and phlogopite are ubiquitous in the wehrlites, but minor orthopyroxene is also present. Hornblende and biotite are abundant late crystallized phases in the gabbros and diorites. The two pyroxene-bearing diorite samples from JZC yield zircon U–Pb ages of 245.7 ± 1.3 Ma and 241.8 ± 1.3 Ma. The mafic and ultramafic rocks display slightly enriched LREE patterns. The wehrlites display moderate to weak negative Eu anomalies (0.74–0.94), whereas the olivine gabbros and gabbros have pronounced positive Eu anomalies. Diorites show slight LREE enrichment, with (La/Yb)N ratios ranging from 4.42 to 7.79, and moderate to weak negative Eu anomalies (Eu/Eu∗ = 0.64–0.86). The mafic and ultramafic rocks from this suite are characterized by negative Nb–Ta–Zr anomalies as well as positive Pb anomalies. Diorites show pronounced negative Ba, Nb–Ta and Ti spikes, and typical Th–U, K and Pb peaks. Combined with petrographic observations and chemical variations, we suggest that the magmatism was dominantly controlled by fractional crystallization and crystal accumulation, with limited crustal contamination. The arc-affinity signature and weekly negative to moderately positive eNd(t) values (−2.3 to 1.2) suggest that these rocks may have been generated by partial melting of the juvenile sub-continental lithospheric mantle that was metasomatized previously by slab-derived fluids. The lithologies in the JZC are related in space and time and originated from a common parental magma. Geochemical modeling suggests that their primitive parental magma had a basaltic composition. The ultramafic rocks were generated through olivine accumulation, and variable degrees of fractional crystallization with minor crustal contamination produced the diorites. The data presented here suggest that the subduction in West Qinling did not cease before the early stage of the Middle Triassic (∼242 Ma), a back-arc developed in the northern part of West Qinling during this period, and the JZC formed within the incipient back-arc.

How to identify ultrahigh-pressure (UH P ) metamorphism in the absence of coesite is a key problem to gain the correct P–T history for an orogenic belt. In this study, garnet composition combined with the pseudosection approach was used to identify ultrahigh-pressure metamorphism and to determine P–T paths for eclogites and metapelites from Chinese southwestern Tianshan. Porphyroblastic garnets from both eclogites and metapelites develop pronounced chemical core-rim textures: the relatively homogeneous core with low pyrope [Prp; Mg/(Ca + Mn + Mg + Fe 2+ ) × 100] and grossular [Grs; Ca/(Ca + Mn + Mg + Fe 2+ ) × 100] content is overgrown by a thin rim with sharply increased Prp and Grs. Phase equilibria modeling indicates that the ultrahigh-pressure rocks have undergone a clockwise P–T path characterized by heating during early exhumation with peak P–T at 31–33 kbar and 490–520 °C. The P–T pseudosections for eclogites show that isopleths of Prp and Grs strongly depend on temperature and pressure, respectively, especially in the stability fields of glaucophane–lawsonite-bearing eclogite facies assemblages. This indicates that garnet composition provides robust thermobarometric constraints. Consequently, we propose a Prp–Grs diagram which is subdivided into a high-pressure region and an ultrahigh-pressure region by the quartz–coesite–transition curve. Those garnet compositions which fall into the ultrahigh-pressure region are regarded to have experienced ultrahigh-pressure metamorphism. This approach is expected to be a useful tool to qualitatively identify ultrahigh-pressure metamorphism for glaucophane–lawsonite-bearing eclogites and its particular strength is the quick examination of large datasets comprising samples with similar bulk composition. Using this method, garnet compositions of eclogites and mafic blueschists from Chinese southwestern Tianshan and lawsonite eclogites worldwide are plotted in the Prp–Grs diagram and several possible ultrahigh-pressure eclogite occurrences are newly identified.

This paper deals with the petrology and U–Pb dating of coesite-bearing garnet–phengite schist from the Kebuerte Valley, Chinese western Tianshan. It mainly consists of porphyroblastic garnet, phengite, quartz and chlorite with minor amounts of paragonite, albite, zoisite and chloritoid. The well preserved coesite inclusions (∼100 μm) in garnet are encircled by a narrow rim of quartz. They were identified by optical microscopy and confirmed by Raman spectroscopy. Using the computer program THERMOCALC, the peak metamorphic conditions of 29 kbar and 565 °C were obtained via garnet isopleth geothermobarometry. The predicted UHP peak mineral assemblage comprises garnet + jadeite + lawsonite + carpholite + coesite + phengite. The metapelite records prograde quartz–eclogite-facies metamorphism, UHP coesite–eclogite-facies peak metamorphism, and a late greenschist-facies overprint. Phase equilibrium modeling predicts that garnet mainly grew in the mineral assemblages garnet + jadeite + lawsonite + chloritoid + glaucophane + quartz + phengite and garnet + jadeite + lawsonite + carpholite + glaucophane + quartz + phengite. SHRIMP U–Pb zircon dating of the coesite-bearing metapelite yielded the peak metamorphic age 320.4 ± 3.7 Ma. For the first time, age data of coesite-bearing UHP metapelite from the Chinese western Tianshan are presented in this paper. They are in accord with published ages obtained from eclogite from other localities in the Chinese western Tianshan and the Kyrgyz South Tianshan and therefore prove a widespread occurrence of UHP metamorphism.

The Xitieshan terrane, located in the central part of the North Qaidam ultrahigh pressure (UHP) metamorphic belt, China, is mainly composed of orthogneiss and paragneiss and a few intercalated eclogite layers and boudins. Based on their bulk-rock TiO 2 -contents, the eclogites can be subdivided into a high-Ti group (TiO 2 > 2%) and a low-Ti group (TiO 2 e Nd ( T ) and restricted I Sr values and therefore provide further evidence for the formation of the protoliths of the eclogites in an oceanic environment. On the other hand, the Sr–Nd isotopes of high-Ti eclogites show mainly positive but also some negative e Nd ( T ) values and relatively broadly distributed I Sr values, indicating minor crustal contamination of the ocean floor basalts. Considering available 750–877 Ma protolith ages preserved in zircon cores, it is inferred that some of the eclogites derived from Neoproterozoic protoliths were emplaced onto the crust far ahead of the Paleozoic deep subduction, while the other eclogites originate from a different oceanic crust, e.g., the Paleo-Qilian ocean, indicating multiple orogenies in the geological history of the Xitieshan terrane, China. Whole-rock and in-situ LA-ICP-MS mineral trace element analyses of eclogites revealed two stages of fluid behavior during retrogression that correspond to the two exhumation stages uncovered by phase equilibrium calculations. The mineral scale trace element distributions and trace element inheritance of newly formed amphibole from its precursors indicate that, at the peak metamorphic stage (M1) and at the earlier (eclogite facies) overprint (M2), the fluid was internally controlled by the rock itself. Within a mafic lens, the amount of water-soluble elements (e.g., Rb, Sr, Ba, U, Pb and LREE), observed in the whole-rock compositions as well as in amphiboles, increases from the core (phengite-bearing eclogite) to the rim (amphibolite) and implies an external fluid source for the amphibolite facies retrogression (M3) which should be helpful for the final exhumation of UHP eclogite.