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Wodegongjieite occurs in the Cr-11 chromitite orebody of the Luobusa ophiolite in the Kangjinla district, Tibet, China. It is found in two inclusions in corundum: (1) as a partial overgrowth (holotype) up to 1.5 μm thick around a spheroid 20 μm across of wenjiite (Ti10(Si,P,□)7), kangjinlaite (Ti11(Si,P)10), zhiqinite (TiSi2) and badengzhuite (TiP), and (2) as pools up to 0.25 μm wide filling interstices between wenjiite, jingsuiite (TiB2), osbornite–khamrabaevite (Ti[N,C]) and corundum. Energy dispersive analyses gave Al2O3 34.09, SiO2 49.11, K2O 2.56, CaO 11.71, SrO 2.53, total 100.0 wt.%, corresponding to K0.58Sr0.26Ca2.25Al7.20Si8.80O31.20, ideally KCa3(Al7Si9)O32, for Si + Al = 16 cations.Single-crystal studies were carried out with three-dimensional electron diffraction providing data for an ab initio structure solution in the hexagonal space group P6/mcc (#192) with a = 10.2(2) Å, c = 14.9(3) Å, V = 1340(50) Å3 and Z = 2. Density (calc.) = 2.694 g⋅cm–3. The refinement, which assumes complete Si–Al disorder, gives average T1–O and T2–O bond lengths both as 1.65 Å. It was not practical to use unconstrained refinement for the occupancies of the large cation sites 6f and 2a. The ab initio model shows clearly that the two cation sites have different sizes and coordination. Consequently, we imposed the condition (1) that all the K occupies the 2a site as the average K–O bond length of 3.07 Å is close to the average K–O bond lengths reported in kokchetavite and (2) that all the Ca occupies the 6f site as the average Ca–O bond length of 2.60 Å (2.36 Å and 2.84 Å for Ca–O1 and Ca–O3, respectively) is reasonable for Ca–O. Assuming that all K and all Ca are located at the 2a site and 6f site, respectively, Sr occupancies of these sites could be refined. Thermal parameters are positive and in a reasonable range. The structure is a sheet silicate isostructural with the K-feldspar polymorph kokchetavite, with two crystallographically distinct sites for K, but not with the topologically identical anorthite polymorph dmisteinbergite (CaAl2Si2O8) with only a single site for Ca. Substitution of K by Ca at the 6f site is associated with marked rotation of the Si,Al tetrahedra and a collapse of the structure to accommodate the smaller Ca ion.The spheroid of intermetallic phases is believed to have formed from the interaction of mantle-derived CH4 + H2 fluids with basaltic magmas at depths of ~30–100 km, resulting in precipitation of corundum that entrapped intermetallic melts. Associated immiscible silicate melt of granodioritic composition crystallised metastably to wodegongjieite instead of a mixture of anorthite and K-feldspar.

The new minerals wenjiite, Ti10(Si,P,☐)7 (IMA2019-107c) and kangjinlaite, Ti11(Si,P)10 (IMA2019-112b) occur with badengzhuite, zhiqinite, and a K-bearing dmisteinbergite-like mineral in a spheroid 20 μm across enclosed in corundum from the Cr-11 podiform chromitite orebody near the Kangjinla, Luobusa ophiolite, Tibet, China. In addition, wenjiite occurs with deltalumite, jingsuiite, osbornitekhambaraevite, and the K-bearing dmisteinbergite-like mineral in a lamellar intergrowth 100 μm long, also enclosed in corundum from the same locality. The new minerals were characterized by energy-dispersive spectroscopy and three-dimensional electron diffraction, which enabled us to obtain an ab initio structure solution and dynamical refinement from grains a few micrometers across hosted in a FIB lamella. Four analyses of wenjiite from the spheroid gave in wt% Si 21.67, P 6.24, Ti 66.39, V 1.37, Cr 2.20, Mn 0.97, and Fe 1.17 (normalized to 100), which corresponds to (Ti0.93Cr0.03Mn0.01Fe0.01V0.02)10 (Si0.79P0.21)6.51 on the basis of 10 cations excluding Si and P. The simplified formula is Ti10(Si,P)6.5, or more generally Ti10SixPy, where x > y and 6 ≤ (x + y) ≤ 7, i.e., Ti10(Si,P,☐)7. Wenjiite has hexagonal symmetry, space group: P63/mcm (no. 193), with a = 7.30(10) Å, c = 5.09(10) Å, V = 235(6) Å3, Z = 1, and is isostructural with xifengite, mavlyanovite, synthetic Ti5Si3, and synthetic Ti5P3.15. Four analyses of kangjinlaite gave in wt% Si 25.56, P 9.68, Ti 62.35, V 0.21, Cr 0.83, Mn 0.42, and Fe 0.95 (normalized to 100), which corresponds to (Ti10.65V0.03Cr0.13Mn0.06Fe0.14)Σ11.01(Si7.43P2.55)Σ9.99. The simplified formula is Ti11(Si,P)10. Kangjinlaite is tetragonal, with space group: I4/mmm (no. 139), a = 9.4(2) Å, c = 13.5(3) Å, V = 1210(50) Å3, Z = 4, and is isostructural with synthetic compounds of the Ho11Ge10 type, being the most compact of these phases. Despite there now being over 70 compounds containing 38 elements isostructural with Ho11Ge10, synthesis of an analog of kangjinlaite has not been previously reported in either the Ti-P or Ti-Si binary systems or in a multicomponent system. The previously deduced crystallization sequence with decreasing temperature of the four minerals in the spheroid is wenjiite → kangjinlaite → zhiqinite + badengzhuite. This sequence is consistent with relationships reported in 9 binary systems containing intermetallic compounds of Ge and Sn isostructural with Mn5Si3 and Ho11Ge10. In eight of these systems the Mn5Si3 analog melts congruently, whereas the Ho11Ge10 analog never does. Instead, the Ho11Ge10 analog melts peritectically, generally to an Mn5Si3 analog and less commonly to compounds with 5:4 stoichiometry. Final crystallization of the spheroid to zhiqinite + badengzhuite is expected to be well below the temperature of 1500 °C for the congruent melting of zhiqinite in the Ti-Si system, i.e., in the range of ~1100–1300 °C.

The structure and lithological sequences of ophiolites provide critical information for reconstructing their tectonic setting and thus, the history of the paleo-oceanic basins in which they formed. Our systematic mapping of three of the best-preserved massifs in the Xigaze ophiolite from the middle segment of the Yarlung Zangbo Suture Zone (YZSZ) in South Tibet, revealed a nearly intact structure of the ophiolitic sequence that has largely escaped emplacement and post-emplacement dismemberment. One of the most important results is the identification of a nearly E-W striking, serpentinized shear zone (SPSZ) preserved within the sequence. The SPSZ is rooted in the serpentinized front of the upper mantle and is mainly composed of meta-gabbroic and rodingite lenses floating in a serpentinite matrix. Extensive talc-serpentinite metasomatism, shearing and cataclastic deformation in the SPSZ reveal a complex interplay of fluid metasomatism and progressive ductile-brittle deformation in hundreds of meters of shear zone between the mantle and the crust. The structural and petrological features of the SPSZ are very similar to those of detachment faults within modern in situ oceanic core complexes (OCCs) in the Mid-Atlantic Ridge and the Southwest Indian Ridge. Zircon U Pb ages from the footwall of the fault, and the age of radiolarian chert overlying it suggest that the detachment faulting took place between ca. 125 to 123 Ma. Combined with the stratigraphic and geochronological relationships between the ophiolite and the overlying sediments, we suggest that the Xigaze ophiolite developed in the supra-subduction zone on the southern margin of the Lhasa terrane during rollback of the Neo-Tethyan oceanic slab. Recognition of detachment faulting in the Xigaze ophiolites, along with other features, suggest that the Xigaze ophiolite formed at a spreading ridge, whereas the other large ultramafic massifs in YZSZ, may have formed in an off-axis location on the slow-spreading ridge.

Early Cretaceous magmatism suggested to be related with the Kerguelen mantle plume has been reported in both eastern and western Tethyan Himalaya (TH). Coeval magmatism has been recently di...

Ophiolites, which represent segments of oceanic lithosphere obducted onto the continental crust, provide an important window into processes such as continental rifting, mantle melting, asthenospheric upwelling and cooling of oceanic lithosphere. Traditionally, research has focused on crustal sections of ophiolites. However, there is a growing recognition that mantle sections contain important information on ophiolite formation and crustal recycling. In this Review, we outline the importance of chromium spinel and associated mineral inclusions for recording the tectonic setting and past histories of ophiolitic mantle sections, with a focus on the insights they provide into deep crustal recycling. In particular, the presence of ultra-high-pressure mineral inclusions, such as microdiamonds, metal alloys, Mn silicates and coesites, in podiform chromitites and other ophiolitic mantle rocks offer evidence of deep and reduced formation conditions. The composition of ultra-high-pressure minerals, and especially the light carbon isotope composition of ophiolite-hosted diamonds, indicates a contribution of recycled crustal material to the mantle portions of ophiolites. The details of crustal material transport through the mantle remain a subject of debate and ongoing research. A global investigation of high-pressure minerals in ophiolitic peridotites and chromitites should be a target of future research to help clarify understanding of deep crustal recycling. Ophiolites, segments of oceanic lithosphere emplaced onto continental crust, are thought to form in the shallow mantle, but the discovery of ultra-high-pressure minerals in ophiolites has sparked debate over their formation mechanisms. This Review examines how these mantle components can record the tectonic history and deep mantle formation of ophiolites.

Ophiolites in the southern belt (SB) occur as much larger peridotite massifs compared with those of the northern belt (NB), sporadically overlain by a thin layer of isotropic gabbro in the ...

Textural and compositional characteristics of mineral inclusions in podiform chromitites flourish thoughtful information about the genesis of the Koycegiz ophiolite (SW Turkey). In the chromitite bodies, olivine, orthopyroxene, clinopyroxene, and sulfide inclusions are mostly sub-rounded or globular. Less abundant needleshaped silicate inclusions (10–50 μm-long) are aligned along crystallographic orientations in the host magnesiochromite grains. Raman spectroscopy and micro analytical data indicate that these lamellar inclusions have consistently a diopside composition. Uncommon, discrete micro-diamonds and moissanite (SiC) particles, observed in some magnesiochromite grains, may suggest formation under ultra-high pressure (UHP) superreduced (SuR) conditions. The Koycegiz chromitites are characterized by variable platinum-group element (PGE) abundances (80–441 ppb), with unfailing IPGE > PPGE contents. Fractionated chondrite-normalized PGE patterns of chromitites and associated peridotites and data scatter in the Pd/Ir versus Pt/Pt* space indicate chromitite formation by partial melting of the associated mantle rocks. The variable 187Os/188Os(i) (0.1278–0.1380) and γOs ( 0.38 to +7.54) values demonstrate extensive contamination of crustal components, likely in a supra-subduction zone (SSZ) setting. Combined with the occurrence of unusual ‘exotic’ mineral inclusions, geochemical and isotopic characteristics of the Koycegiz chromitites replicate superimposed deep mantle processes, recycling and subduction related phase transformations.

The unit-cell parameter a of antigorite (usually expressed as the polysome m value) has been determined as a function of temperature (T) and pressure (P) in the range of 600–650 °C, 25–45 kbar in weeklong piston-cylinder experiments. A well-characterized natural antigorite (with m = 16 and less abundant m = 15) was used as a starting material that coexisted with olivine, chlorite, Ti-humite, and aqueous fluid at run conditions. Transmission electron microscope (TEM) measurements on selected focused ion beam (FIB) wafers showed that antigorite m values after the experiments varied between 14 and 22. More than 40 punctual analyses for each run condition were acquired to determine the range and the primary m value. The most frequent antigorite m-value decreased systematically from 17–19 at 600 °C to 15–16 at 650 °C. The spacing of the m-isolines is getting narrower as the antigorite breakdown reaction is approached. The topology of the m-isolines is similar to that previously characterized for the simple MgO-SiO2-H2O (MSH) system. However, the isolines are shifted to about 50–100 °C higher temperatures due to the incorporation of Al into antigorite. Powder samples and FIB wafers of natural antigorite from the Tianshan UHP belt (China) with peak metamorphic conditions of ~35 kbar, ~520 °C were also investigated with TEM. Low Al-antigorite formed at peak metamorphic conditions displays a peak m value of 20–21, whereas high-Al antigorite formed during isothermal decompression displays a lower m value of 19. Combination of our results with the published data of m values from metamorphic antigorite that experienced various conditions allowed construction of a P-T-m diagram that can be used in future studies to better constrain formation conditions of serpentinites. The decrease of m values and the increase of Al in antigorite with increasing temperature result in small, continuous dehydration whereby the H2O content of antigorite changes from 12.4 to 12.1 wt%. Therefore, it is expected that a pore fluid is present during the prograde deformation of serpentinites. TEM observations showed that antigorite adjusted its Al content by segregation of chlorite at the nanoscale. Together with the observation that multiple m values are always present in a single sample, this result indicates that full equilibration of antigorite at the micrometer-scale is rare, with important implications for the interpretation of geochemical signatures obtained by in situ techniques.

The >2000 km Indus-Yarlung Tsangpo suture zone (IYSZ) is composed of the Neo-tethys oceanic remnants, flysch units and related continental rocks, which has been regarded as the boundary between the Eurasian and Indian terranes. Among the ophiolitic complexes, the Purang ophiolite is the biggest massif in the IYSZ, and many studies have been conducted on this ophiolite. However, previous studies have mainly focused on harzburgite, clinopyroxenite and dunite. Field observations show that mafic dykes were emplaced within the Purang ophiolite. However, petrogenetic evolutions of those mafic dykes are poorly understood. In this study, we present new LA-ICP-MS zircon U–Pb dating results, whole-rock geochemistry and Sr–Nd–Hf isotope analyses for microgabbro, gabbro and dolerite dykes from the Purang ophiolite of the southwestern IYSZ, respectively. Three samples yielded zircon U–Pb ages of 144.2 ± 2.1 Ma, 127.9 ± 2.3 Ma and 126.5 ± 0.42 Ma, suggesting two different phases of magmatic activities distinctly. Whole-rock geochemical results suggest that the gabbro samples show alkaline features marked by enrichments of light rare earth elements (LREE) and large-ion lithophile elements (LILE), as well as Nb–Ta elements, suggesting an oceanic island basalt-like (OIB-like) geochemical affinity. However, the dolerite and microgabbro samples demonstrate sub-alkaline characteristics with normal mid-oceanic ridge basalt-like (N-MORB-like) geochemical features. Three distinct mafic dykes show significant Rb element depletion. The geochemical data and Sr–Nd–Hf isotopic features suggest that the microgabbro and gabbro rocks were derived from a depleted mantle that had been metasomatized by partial melts of sediments and enriched slab-derived fluids. The dolerite was also originated from a depleted mantle marked by significantly depleted Sr–Nd–Hf compositions, which was not influenced by enriched slab-derived fluids and sediments contamination during subsequent evolution. The isotope and geochemical data and tectonic diagrams suggest a tectonic transition from a within-plate to a mid-oceanic ridge basalt-like (MORB-like) setting during the period from ca. 144 Ma to 127 Ma. Combined with regional background and this study, we propose that these mafic dykes were formed in an oceanic back-arc basin setting. Additionally, integrated with previous studies, we suggest that the geodynamic evolution of the southwestern and central parts of the Neo-Tethys oceanic basin is comparable in Early Cretaceous. Keywords: Isotope geochemistry, Mafic rocks, Purang ophiolite, Yarlung Tsangpo suture zone

In this study, we report systematically field observations, geochronology, whole-rock geochemistry and Sr-Nd-Hf isotopic dataset on the various types of magmatic rocks collected from the Zedang area. Chemically, the diabase and gabbro have a low-K calc-alkaline affinity, whereas the basalt and plagiogranite have medium to high-K calc-alkaline characteristics. In addition, the basalts are highly enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), but strongly depleted in high strength field elements (HFSE), indicating that their magma source probably was derived from a subduction- or arc-related setting. In contrast, both the gabbro and diabase mainly demonstrate an N-MORB-like affinity consistent with normal mid-oceanic ridge basalt (N-MORB) origin. The zircon U-Pb dating results suggest that the basalts were crystallized earlier at ca. 158–161 Ma (Oxfordian stage), but the gabbro was crystallized at ca. 131 Ma (Hauterivian stage of Early Cretaceous). The zircon U-Pb dating results correspond with the field observations that the veins of gabbro intruded basalt. Furthermore, the plagiogranite has a weighted mean age of ca. 160 Ma (MSWD=2.1) consistent with the basalt within the uncertainty. The basalt and the plagiogranite have significantly positive eHf(t) values (+5.8 to +15.6 and +8.6 to +16.1, respectively), suggesting that they were originated from partial melting of a depleted source. However, basalt and plagiogranite are characterized by the wide variations of eHf(t) values indicating minor amounts of exotic crustal material input during the later magma evolution. Additionally, the basalt shows duplex geochemical features of island-arc and mid-oceanic ridge basalt, corresponding to the supra-subduction zone-(SSZ) type affinity. To sum up, two distinct magmatic events identified in this study probably suggest an intra-oceanic arc system existing in the Zedang area during the Late Jurassic, but the intra-oceanic arc subduction extinguished in the Early Cretaceous as suggested by the N-MORB-like gabbro and diabase. Integrated with regional background and different rock types, as well as geochemical features, we conclude that intra-oceanic arc subduction setting developed during the Late Jurassic in the Zedang area, southern Tibet.

Most of the diamond deposits in China are in the North China Craton. In addition to gem diamonds in kimberlite, a large number of microdiamonds have also been discovered in alkaline dolerites. These microdiamonds show very different characteristics from those recovered in kimberlite. Here, we report the morphology, colour, nitrogen contents, and carbon isotopic compositions of the diamonds recovered from the alkalic dolerites in eastern China. The microdiamonds are mainly cube and rhombic dodecahedron with diameters of 0.2 to 0.6 mm. Infrared spectrum analysis shows that these microdiamonds are mostly type Ib with a small amount of type Ia. The Y centre is obvious in type Ib diamond. Modelling mantle residence times for the IaAB diamonds is about 550 Ma. Nitrogen contents of the diamonds range from 4.5–503 ppm, with a median value of 173 ppm. The total δ13C range of the microdiamonds varies between −18.6 and −21.1‰ and are similar to those of ophiolite diamond.

We report new U–Pb zircon age data, zircon in situ oxygen isotope, mineral chemistry, whole-rock geochemistry and Sr–Nd isotopic compositions from the Early Devonian ultrapotassic Gucheng pluton in the North China Craton, and discuss its petrogenesis. The Gucheng pluton is exposed in the northern part of the North China Craton and forms a composite intrusion, consisting of K-feldspar-bearing clinopyroxenite, clinopyroxene-bearing syenite and alkali-feldspar syenite. Mineral phases in these lithologies include clinopyroxene (Wo43–48En19–35Fs18–38), sanidine (An0Ab3–11Or89–97), and subordinate titanite, andradite and Na-feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in situ oxygen isotopes (δ18O = 5.2–6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the North China Craton. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo-Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. Sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean-type continental margin evolution along the northern edge of the North China Craton facing the Palaeo-Asian Ocean.

We investigated lherzolitic peridotites in the Cretaceous Purang ophiolite along the Yarlung Zhangbo suture zone (YZSZ) in SW Tibet to constrain their mantle–melt evolution history. Coarse-grained Purang lherzolites contain orthopyroxene (Opx) and olivine (Ol) porphyroclasts with embayments filled by small olivine (Ol) neoblasts. Both clinopyroxene (Cpx) and Opx display exsolution textures represented by lamellae structures. Opx exsolution (Opx1) in clinopyroxene (Cpx1) is made of enstatite, whose compositions (Al2O3 = 3.85–4.90 wt%, CaO =

Helium isotopic compositions are considered to be ideal tracers to identify whether mantle materials have been added to crustal rocks or fluids. In this paper, we present the helium isotopic compositions of the Songduo eclogites in the Lhasa terrane, Tibet. We found that garnet and omphacite in the eclogites have different helium retention characteristics. The 4He content of most omphacite grains are about 10-20 times of that of garnet, suggesting that omphacite has a higher ability to capture 4He than garnet. Similarly, there is about 10-20 times difference in 3He content between omphacite and garnet in the same eclogite samples. The 3He/4He ratios of garnet and omphacite in these rocks range from 0.27 to 0.60 Ra (relative to the modern air 3He/4He ratio, 1.4×10-6). These ratios are within the range of both mantle-and crust-derived helium, suggesting mixed sources. The Songduo eclogites have much higher 3 He/4He ratios than those observed in the Dabie eclogites of eastern China. Such high ratios are typically thought to be associated with deep mantle sources. We cautiously conclude that deep mantle materials might have been involved during the formation of the Songduo eclogites.

The Songshugou peridotite massif is located in the north of Shangdan suture zone, North Qinling orogenic belt of Central China. The massif is mainly composed of fine-grained mylonitic dunites, coarse-grained dunites, fine- and coarse-grained harzburgites, and minor clinopyroxenites. The coarsegrained dunites as well as parts of the harzburgites host small-scale chromitites. Chromite grains from various textural types of chromitites and dunites pervasively contain primary and secondary silicate inclusions. Primary inclusions are dominated by monophase olivine, with minor clinopyroxene and a few multiphase mineral assemblages consisting of olivine and clinopyroxene. Secondary inclusions, mainly Cr-chlorite and tremolite, show irregular crystal shapes. Besides, Cr2O3 contents (0.08 wt.%–0.71 wt.%) of primary olivine inclusions are remarkably higher than those of interstitial olivine (

Post-collisional potassic and ultrapotassic volcanic rocks are widely distributed across the Tibetan Plateau, and they are considered to be indicators of evolving mantle dynamics. A suite of potassic basalts younger than 55 Ma from the Saga area of western Tibet has been reported. The geochemical characteristics of these rocks distinguish themselves from other potassic-ultrapotassic volcanic rocks in Tibet, such as positive Nb, Ta, and Ti anomalies and strong enrichment in large ion lithophile elements (LILE), suggesting that phlogopite, rutile and/or sphene might have originated from the mantle source. These basalts are also characterized by a very wide range of 87Sr/86Sr ratios (0.709 043–0.711 915) and relatively high 143Nd/144Nd ratios (0.512 426–0.512 470, eNd= −4.60 to −3.87). We propose a petrogenetic model for the Saga potassic rocks in which the lithospheric mantle source was infiltrated by a volatile-rich (H2O, CO2) and low-degree silicate melt derived from the asthenosphere in the Middle to Late Proterozoic. After the initial Indo-Asian collision, Neo-Tethyan slab breakoff resulted in the partial melting of the previously metasomatized lithospheric mantle and the formation of the Saga potassic rocks. It is likely that the eruption of these volcanic rocks lasted at least 10 Ma.

The Sumdo eclogite-bearing (U)HP metamorphic belt extends over 100 km across the middle part of the Lhasa terrane in southern Tibet, which forms a Permian-Triassic oceanic subduction zone between the south and the north Lhasa sub-terranes, leading to the reinterpretation of the tectonic evolution of the Lhasa terrane in the Tibetan-Himalayan orogeny. Previous studies show that there are significant differences in temperature and pressure conditions of the eclogites in four areas, e.g., Sumdo, Xindaduo, Bailang and Jilang areas. Studying the peak metamorphic P-T conditions and path of eclogite in the Sumdo belt is of great significance to reveal the subduction and exhumation mechanism of Paleo-Tethys Ocean in the Lhasa terrane. In this contribution, eclogite in the Jilang area of the Sumdo belt is chosen as an example to study its metamorphic evolution. The mineral assemblage of the eclogite is garnet, omphacite, phengite, hornblende, epidote, quartz and minor biotite. Garnet has a "dirty" core with abundant inclusions such as epidote, amphibole, plagioclase and a "clear" rim with few inclusions of omphacite and phengite. From the core to the rim, pyrope content in garnet increases while grossular content decreases, showing typical growth zoning. The rim of garnet is wrapped by the pargasite+plagioclase corona, showing amphibolite facies overprint during retrogression. Three stages of metamorphism are inferred as (1) prograde stage, represented by the core of garnet and mineral inclusions therein; (2) peak stage, represented by the garnet rim, omphacite, lawsonite, phengite, and quartz; (3) retrograde stage characterized by decomposition of lawsonite to zoisite, followed by symplectite of omphacite and corona rimmed garnet. A P-T pseudosection contoured with isopleths of grossular and pyrope contents in garnet is used to constrain the near peak P-T condition at 2.85 GPa, 575 C. In general, the Jilang eclogite shows a clockwise P-T path with a near isothermal decompression process during exhumation. Combined with the age peaks of 583, 911, and 1 134 Ma from the detrital zircons of the country metaquartzite, a continental margin material involving exhumation process at shallow depth after the subduction channel exhumation is inferred for the Jilang eclogite and may further indicate that the subduction direction of the Sumdo eclogite belt is from north to south.

The preservation of ultra-high-pressure and super-reducing phases in the Neotethyan Luobusa ophiolite in Tibet suggests their deep origin near the mantle transition zone. Dunite and harzburgite core samples from the Luobusa Scientific Drilling Project show supra-subduction zone geochemical signatures and equilibration temperatures of c. 950–1080°C. Olivine shows A-, B-, C- and E-type fabrics, and combinations of A- and E-type or B- and E-type fabrics. Transmission electron microscopy observations show straight dislocations and the activation of multiple slip systems [100](010), [001](010), [001](100) and [100](001) in olivine. The mean water content in olivine, orthopyroxene (Opx) and clinopyroxene (Cpx) from 24 peridotite samples was 16 ± 5, 90 ± 21 and 492 ± 64 ppm, respectively, which is different from the water content of hydrated peridotites above the mantle wedge. The trace element compositions of Cpx exclude significant metasomatism after melt extraction. The high hydrogen partition coefficient between Cpx and Opx (DHCpx/Opx = 5.56 ± 0.96) implies equilibrium at high pressures and rapid exhumation. Based on deformation experiments, the B- and C-type fabrics could be formed in a subduction zone at depths >200 km, whereas the A- and E-type fabrics were produced in the shallow mantle. In a process triggered by slab rollback, the Luobusa peridotites may have been rapidly exhumed within a subduction channel and mixed with the lithospheric mantle of the forearc. Supplementary material: Major oxide contents in Opx, Cpx and spinel, trace element concentration in Cpx, micrographs and TEM images of peridotite are available at https://doi.org/10.6084/m9.figshare.c.4307828 Thematic collection: This article is part of the ‘Tethyan ophiolites and Tethyan seaways collection’ available at: https://www.lyellcollection.org/cc/tethyan-ophiolites-and-tethyan-seaways

The Zhangguangcai Range in the Xing’an Mongolian Orogenic Belt, NE China, contains Early Jurassic (c. 188 Ma) Dabaizigou (DBZG) porphyritic dolerite. Compared with other island-arc mafic rocks, the DBZG dolerite is characterized by high trace-element contents, relatively weak Nb and Ta enrichments, and no Zr, Hf or Ti depletions, similar to OIB-type rocks. Analysed rocks have (87Sr/86Sr)i ratios of 0.7033–0.7044, relatively uniform positive ɛNd(t) values of 2.3–3.2 and positive ɛHf(t) values of 8.5–17.1. Trace-element and isotopic modelling indicates that the DBZG mafic rocks were generated by partial melting of asthenospheric mantle under garnet- to spinel-facies conditions. The occurrence of OIB-like mafic intrusion suggests significant upwelling of the asthenosphere in response to lithospheric attenuation caused by continental rifting. These processes occurred in an incipient continental back-arc environment in the upper plate of a palaeo-Pacific slab subducting W–NW beneath East Asia.

Submicroscopic to micro-diamonds, silicates, wüstite, platinum group minerals (PGM), sulfides and ultra-high pressure (UHP) mineral inclusions have been recovered from podiform and banded chromitites of the Bulqiza ophiolite in Albania. The alignment of acicular silicate inclusions with crystallographic planes of magnesiochromite suggests an exsolution origin. Positive and negative-crystal faces of cubic silicate inclusions are consistent withs a high-temperature paragenesis. Micro-inclusions and nanoparticles (