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The application of whole-rock compositions to trace magma evolution or crystal-melt equilibrium may be called into question when foreign crystals are incorporated into host magmas. To address this challenge, establishing the origins (orthocrysts, antecrysts, xenocrysts, etc.) of minerals with variable textures in magmatic plumbing systems is necessary. In this paper, we describe complex oscillatory zoning patterns of clinopyroxenes (Cpx) from the Miocene Majuangou (MJG) alkali basalts from West Qinling, China. Our detailed petrographic, mineralogical, and geochemical analyses unravel the origins of various Cpx crystals with distinct textural patterns, thereby providing information about magma storage, recharging and/or mixing, and transportation, as well as the reactions between crystals and melts/fluids. Based on textural patterns, Cpx may be divided into four types: normal (Type-1 Cpx), simple oscillatory (Type-2 Cpx), complex oscillatory (Type-3 Cpx), and grains that lack zoning (Type-4 Cpx, suggested to be orthocrysts). Through the textural characterization of Cpx, the comparison between different types of Cpx, and the relationships between Cpx major compositions from different lithologies, we concluded that Type-1–3 Cpx cores are antecrysts or xenocrysts with diverse origins: primitive magma (Type-1 Cpx cores), magma mush (Type-2 Cpx cores), and crustal granulite (Type-3 Cpx cores). The zoning patterns and the compositions of these Cpx crystals indicate at least three batches of magmatic activity, i.e., the Batch-1 low-Mg# magma (Mg#: 47.4–53.3), the Batch-2 primitive magma (Mg#: 57.2–64.5), and the Batch-3 low-Mg# host alkali magma (Mg#: 47.2–54.6). Cpx-melt thermobarometry demonstrates that at least two crustal magma reservoirs existed in the magma plumbing system at depths of 30.1 and 40.9 km. The antecrystic/xenocrystic Cpx cores were captured by, continued to grow in, and subsequently reacted with ascending K-rich melt/fluid. The spongy textures in Cpx cores/mantles are attributed to this reaction, which may be expressed as: Melt 1 (primitive or evolved) + K-rich melts/fluids + Cpx (CaMgSi2O6) = K-feldspar (KAlSi3O8) + ilmenite (FeTiO3) + Melt 2 (derivative). The products of this reaction (K-feldspar and ilmenite) filled the sieves in the spongy zones of Type-1–3 Cpx. This detailed investigation of compositional and textural features of Cpx antecrysts/xenocrysts suggests that the interactions between various interconnected magma reservoirs are widespread beneath the magmatic plumbing system. Our study emphasizes the importance of the incorporation of foreign crystals and the Cpx-melt/fluid reaction in magmatic plumbing system, which can significantly modify the whole-rock compositions and lead to the formation of spongy textures without the need for fractures and cracks in minerals. [ABSTRACT FROM AUTHOR]

Methane (CH4) is a critical but overlooked component in the study of the deep carbon cycle. Abiotic CH4 produced by serpentinization of ultramafic rocks has received extensive attention, but its formation and flux in mafic rocks during subduction remain poorly understood. Here, we report massive CH4-rich fluid inclusions in well-zoned garnet from eclogites in Western Tianshan, China. Petrological characteristics and carbon–hydrogen isotopic compositions confirm the abiotic origin of this CH4. Reconstructed P–T– f O2–fluid trajectories and Deep Earth Water modeling imply that massive abiotic CH4 was generated during cold subduction at depths of 50–120 km, whereas CO2 was produced during exhumation. The massive production of abiotic CH4 in eclogites may result from multiple mechanisms during prograde high pressure-ultrahigh pressure metamorphism. Our flux calculation proposes that abiotic CH4 that has been formed in HP-UHP eclogites in cold subduction zones may represent one of the largest, yet overlooked, sources of abiotic CH4 on Earth. [ABSTRACT FROM AUTHOR]

High‐pressure (HP) granulites provide telling records of mineral reactions at upper mantle to lower crustal levels and key information on the fate of material in subduction systems. The latter especially applies when they abut eclogite and mantle dunite because such rock associations are crucial for understanding the incompletely known processes at the interface of converging plates. A continental arc, active c. 520–395 Ma ago, formed an enigmatic example of such a rock association in the Songshugou area, Qinling Orogen. To unravel the juxtaposition of the distinct rocks, this study combines petrography, phase equilibria modelling, conventional thermobarometry, and zircon U–Th–Pb–Ti–REE analysis. Two mafic HP granulites, which contain the mineral assemblages garnet–clinopyroxene–plagioclase–rutile–mesoperthite–quartz and garnet–clinopyroxene–plagioclase–rutile, experienced peak metamorphic conditions of ≤1.4 GPa, 860°C and ~1.3 GPa, ≥910°C, respectively. During decompression and cooling, at 489 ± 4 Ma, amphibole lamellae unmixed from a clinopyroxene solid solution and orthopyroxene in part replaced garnet. A felsic HP granulite shows equilibration of garnet, perthite, antiperthite, kyanite, quartz, and rutile at 810–860°C, ~1.2 GPa, sillimanite growth during decompression, and upper amphibolite facies cooling at 510 ± 4 Ma. Though the thermobarometric data are just within the methodological errors, the U/Pb zircon ages imply the HP granulites did not evolve coherently. The HP granulites either represent foundered lower arc crust or originated from subduction erosion because their geochemistry is indistinguishable from that of the hanging‐wall plate. Published and new pressure–temperature–time–deformation paths converge at ~710°C, ~0.9 GPa, and ≲470 Ma, implying exhumation tectonics juxtaposed the HP granulites with a mélange of eclogite and mantle dunite at lower crustal levels. This study highlights that lower arc crust can comprise material of diverse evolution. [ABSTRACT FROM AUTHOR]

Abstract: The Chinese Western Tianshan terrane is a well‐known ultrahigh‐pressure (UHP) metamorphic belt that mainly consists of subducted oceanic crustal materials. While two uplift stages have been recognized, the early exhumation history has not been fully constrained. We conducted U–Th–Pb and trace element microanalysis of titanite as well as zircon from UHP eclogites. Apart from several inclusions in garnet, three types of retrograde titanite have been identified based on their petrographic occurrences: (a) in matrix enclosing relict rutile; (b) as coarse‐grained coronas around garnet; and (c) in veins coexisting with epidote and/or apatite. The three types of titanite are homogeneous in chemical composition, low in Al, enriched in middle rare earth elements (REEs), and depleted in light REEs and heavy REEs, suggesting their growth during retrograde decompression. Sensitive high‐resolution ion microprobe (SHRIMP) and LA‐ICP‐MS U–Pb dating of titanite from four samples, regardless of their textural occurrences, gave consistent ages of c. 306 Ma. Phase equilibria modelling and Zr‐in‐titanite thermometry indicate that titanite replaced rutile at ~14 kbar and ~570°C, following a nearly isothermal decompression from the UHP peak conditions of 30 kbar and 550–570°C. SHRIMP zircon U–Pb dating yields two age groups: c. 320 Ma and c. 305 Ma. The former is generally interpreted to be the age of UHP stage. The latter, overlapping the titanite ages, represents the timing of titanite growth during exhumation. Thus, we estimate that the near‐isothermal exhumation from the UHP peak conditions to the epidote–amphibolite facies retrogression (14 kbar) lasted c. 15 Ma, corresponding to an average exhumation rate of 3.4 ± 2.5 mm/yr (2σ). This exhumation rate lies within the range of oceanic LT‐HP belts in the absence of continental subduction, suggesting a similar slab exhumation mechanism that operates in the UHP regime. The consistent retrograde metamorphic ages recorded by titanite from different localities, and the general agreement among P–T paths in the adjacent area, suggest that the UHP unit of Western Tianshan metamorphic belt was likely exhumed as a coherent unit. [ABSTRACT FROM AUTHOR]

Inclusions, reaction textures, and the compositional zoning of minerals are telling records of metamorphic processes and windows into Earth's interior. Evolving pressures, temperatures, water activities, and oxygen fugacities may cause the obscuration of these features—they become ambiguous and difficult to interpret. This contribution discusses examples of such modifications in eclogites and shows limitations for quantitative thermobarometry.The eclogites detailed herein occur as lenses and layers in mafic mylonite. They stem from the north-eastern section of the Qinling Belt, China, and are instances of a limited number of dismembered bodies of (ultrahigh-pressure) eclogites, which formed in a subduction–accretion system ~520–485 Ma ago.The best-preserved eclogites mainly consist of garnet and extensive clinopyroxene(III)–plagioclase symplectites, which mantle relics of clinopyroxene(I+II). Garnets show prograde zoning and include clinopyroxene(I+III), plagioclase with concentric zoning, K feldspar in contact with epidote, amphibole surrounded by compositional halos in garnet, prehnite, pumpellyite, epidote, rutile, ilmenite, and titanite. In the matrix, clinopyroxene(I) is in contact with nearly Na-free but oriented orthopyroxene rods-hosting clinopyroxene(II); both are mantled by the symplectites. The symplectites are separated from garnet by amphibole coronas.In eclogites with poor garnet and clinopyroxene preservation, amphibole-rich cleavage domains wrap around amphibole porphyroclasts and garnets with plagioclase–amphibole–clinozoisite coronas. The amphibole porphyroclasts have whitish actinolite cores, which sometimes contain nearly Na-free clinopyroxene, and brown pargasite–hornblende rims; at the core-rim interface, oriented rods of quartz and plagioclase precipitated.The mafic mylonite shows amphibole-rich cleavage domains and stretched ribbons of plagioclase with minor amphibole and clinozoisite.The principle results are: (1) Host–inclusion reactions in garnet, perhaps facilitated by diffusional component exchange with the matrix, led to concentric zoning in plagioclase inclusions and compositional halos in garnet around amphibole; all these inclusions cannot be used for conventional thermobarometry. The modification of inclusions happened even at rather low temperatures given the presence of prehnite and pumpellyite. Early mineral assemblages may be discussed with the help of phase diagrams but they can hardly be deduced solely from textures. Phase equilibria modelling revealed 1.93–2.54 GPa, 462–542°C for the growth of garnets with the highest Mn and lowest Mg content in the cores, assuming their zoning was negligibly modified after growth. (2) The highly stretched ribbons in mafic mylonite are the end-point of a continuous decomposition of garnet: During exhumation, garnet first was mantled by coronas at 0.7–1.2 GPa, 660–710°C. During an isothermal uplift to 0.5–0.8 GPa, these coronas evolved into stretched ribbons. (3) Clinopyroxene(I), stable at the pressure peak, evolved into an orthopyroxene-bearing solid solution at the temperature peak and was decomposed to clinopyroxene(III)-plagioclase symplectites during retrogression. (4) The zoning of the amphibole porphyroclasts reflects the limited availability of components in the centre of a corona but not continuous growth: Amphibole coronas first mantled the symplectites and the symplectites then were transformed into the whitish cores. (5) K feldspar–epidote inclusions in garnet are relics of melt; together with the exsolutions of orthopyroxene in clinopyroxene(II) they confirm significant heating during uplift. [ABSTRACT FROM AUTHOR]

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 FeO tot and Cr 2 O 3 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 ε Nd ( 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. [ABSTRACT FROM AUTHOR]

Abstract: 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 TiO2-contents, the eclogites can be subdivided into a high-Ti group (TiO2 >2%) and a low-Ti group (TiO2 <2%). Whole-rock major and trace element analyses revealed that the protoliths of the low-Ti eclogites are normal-type mid-ocean ridge basalts (N-MORB), whereas those of the high-Ti eclogites are either enriched-type mid-ocean ridge basalts (E-MORB) or near ridge seamount basalts, respectively. The Sr–Nd isotopes of eclogites of both groups are similar to those of MORB. Those of the low-Ti eclogites are characterized by positive ε 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 ε Nd(T) values and relatively broadly distributed I Sr values, indicating minor crustal contamination of the ocean floor basalts. Considering available 750–877Ma 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. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

The southwestern Tianshan orogenic belt hosts one of the Earth’s rare occurrences of ultrahigh-pressure (UHP) metamorphic rocks derived from subducted oceanic crust. To address its poorly constrained and controversial P-T-t evolution, for this study, three representative samples, two paragonite-epidote eclogites (H505-26 and H76-10) and one glaucophane-phengite eclogite (H711-1) are selected, and Sm-Nd geochronology and detailed phase equilibria modeling are combined. Garnet porphyroblasts from the three studied eclogites have well-preserved growth zoning with rim-ward increasing pyrope and decreasing spessartine contents. Phase equilibria modeling reveals that the investigated garnets grew in the lawsonite eclogite facies; in eclogite H505-26 during heating and burial, and in eclogites H76-10 and H711-1 during heating and decompression. Temperatures of 460–590°C derived for the three eclogites are below the closure temperature of the Sm-Nd system, and, thus, the garnet-omphacite-whole rock Sm-Nd isochron ages have the potential to reflect garnet growth, i.e., eclogite facies metamorphism. The three eclogites yield consistent Sm-Nd isochron ages of 309±4.6Ma (H505-26), 306±15Ma (H76-10) and 305±11Ma (H711-1), reflecting HP eclogite facies metamorphism. Based on the new geochronology, detailed investigation of phase equilibria and published data, a clockwise P-T-t path with post-P max heating – decompression, subsequent isothermal decompression and final cooling – decompression is predicted for the eclogites from Chinese southwestern Tianshan. We propose, subduction of the South Tianshan paleo-ocean began before 346Ma, the eclogite facies prograde metamorphism of the subducting oceanic crust occurred at 346–333Ma, peak eclogite facies metamorphism at 320–305Ma, and buoyancy-driven exhumation of the oceanic crust at 296–226Ma. [ABSTRACT FROM AUTHOR]