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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)6.51on 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 Ho11Ge10type, 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 Mn5Si3and Ho11Ge10. In eight of these systems the Mn5Si3analog melts congruently, whereas the Ho11Ge10analog never does. Instead, the Ho11Ge10analog melts peritectically, generally to an Mn5Si3analog 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 unit-cell parameter aof antigorite (usually expressed as the polysome mvalue) 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 mvalues 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 mvalue. 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 mvalue of 20–21, whereas high-Al antigorite formed during isothermal decompression displays a lower mvalue of 19. Combination of our results with the published data of mvalues from metamorphic antigorite that experienced various conditions allowed construction of a P-T-mdiagram that can be used in future studies to better constrain formation conditions of serpentinites. The decrease of mvalues 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 mvalues 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.

As reported in our prior work, we have recovered microdiamonds and other unusual minerals, including pseudomorph stishovite, moissanite, qingsongite, native elements, metallic alloys, and some crustal minerals (i.e., zircon, quartz, amphibole, and rutile) from ophiolitic peridotites and chromitites. These ophiolite-hosted microdiamonds display different features than kimberlitic, metamorphic, and meteoritic diamonds in terms of isotopic values and mineral inclusions. The characteristic of their light carbon isotopic composition implies that the material source of ophiolite-hosted diamonds is surface-derived organic matter. Coesite inclusions coexisting with kyanite rimming an FeTi alloy from the Luobusa ophiolite show a polycrystalline nature and a prismatic habit, indicating their origin as a replacement of stishovite. The occurrence in kyanite and coesite with inclusions of qingsongite, a cubic boron nitride mineral, and a high-pressure polymorph of rutile (TiO2II) point to formation pressures of 10–15 GPa at temperatures ∼1300 °C, consistent with depths greater than 380 km, near the mantle transition zone (MTZ). Minerals such as moissanite, native elements, and metallic alloys in chromite grains indicate a highly reduced environment for ophiolitic peridotites and chromitites. Widespread occurrence of diamonds in ophiolitic peridotites and chromitites suggests that the oceanic mantle may be a more significant carbon reservoir than previously thought. These ophiolite-hosted diamonds have proved that surface carbon can be subducted into the deep mantle, and have provided us with a new window for probing deep carbon cycling.

Geophysical investigations and laboratory experiments provide strong evidence for subduction of ancient oceanic crust, and geological and mineralogical observations suggest that subducted oceanic crust is recycled into the upper mantle. This model is supported by some direct petrologic and mineralogical evidence, principally the recovery of super-deep diamonds from kimberlites and the presence of crustal materials in ophiolitic chromitites and peridotites, but many details are still unclear. Here we report the discovery of ophiolite-hosted diamonds in the podiform chromitites of the Skenderbeu massif of the Mirdita ophiolite in the western part of Neo-Tethys. The diamonds are characterized by exceedingly light C isotopes (δ13CPDB~ −25‰), which we interpret as evidence for subduction of organic carbon from Earth’s surface. They are also characterized by an exceptionally large range in δ15Nair(−12.9‰ to +25.5‰), accompanied by a low N aggregation state. Materials sparsely included in diamonds include amorphous material, Ni-Mn-Co alloy, nanocrystals (20 × 2 nm) of calcium silicate with an orthorhombic perovskite structure (Ca-Pv), and fluids. The fluids coexisting with the alloy and Ca-Pv provide clear evidence that the diamonds are natural rather than synthetic. We suggest that the Skenderbeu diamonds nucleated and grew from a C-saturated, NiMnCo-rich melt derived from a subducted slab of ocean crust and lithosphere in the deep mantle, at least in the diamond stability field, perhaps near the top of the mantle transition zone. The subsequent rapid upward transport in channeled networks related to slab rollback during subduction initiation may explain the formation and preservation of Skenderbeu diamonds. The discovery of diamonds from the Mirdita ophiolite not only provides new evidence of diamonds in these settings but also provides a valuable opportunity to understand deep cycling of subducted oceanic crust and mantle composition.

The Lhasa terrane, forming one of the main tectonic components of the Himalayan–Tibetan orogen, has received a lot of attention as it records multiple episodes of plate spreading, subduction and collision within the realm of the Paleo-Tethys Ocean. A review of the mineralogical, petrological, geochemical and geochronological data of eclogites, associated blueschists and garnet-bearing mica schists from the Sumdo high- and ultrahigh-pressure metamorphic (HP/UHP) complex in the central/southern part of the Lhasa terrane, is present here so that the origin and tectono-metamorphic evolution of this important suture can be deduced. By re-evaluating the original published P–Tconditions for the metamorphic rocks of the Sumdo Complex, we consider that the Sumdo Complex has experienced low temperature HP/UHP metamorphic conditions, characteristic of fast subduction (and exhumation) in a typical oceanic subduction zone setting. The original wide spread in the maximal peak P–Tconditions could be reduced in size due to thus far unknown inconsistencies in the usage of applied geothermobarometric techniques. The remaining spread in the maximal P–Tconditions (c.200°C/10 kbar) of the HP/UHP regions can be explained by a mechanism that the rocks from individual tectonic slices were subducted to different depths and followed by juxtaposition on their way back to the surface. A re-consideration of the isotopic ages of eclogites from the Sumdo Complex demonstrates that the opening of the Paleo-Tethys Ocean, located in between the two major components of the Lhasa terrane, was initiated prior to c.280 Ma and the eclogite facies metamorphism is likely to be of late Permian (c.260 Ma) to early Triassic age (245–225 Ma), recording different ages of subduction from individual slices of the oceanic crust. The closure of the Paleo-Tethys Ocean resulted, no earlier than 210 Ma, in the final collision between the northern and southern Lhasa blocks. This final collision event may have been triggered by the initial subduction of the Bangong–Nujiang Tethys Ocean in the north.

Magnetic properties of a suite of variably serpentinized peridotites from the Dongbo ophiolite, SW Tibet (China), have been investigated to determine the magnetic signatures of suture zones. The degree of serpentinization (S) for these peridotites is mainly in the range of S< 60%. Petrography, mineral chemistry, and thermomagnetic analyses reveal that magnetite occurring in the interior of various serpentine veins is the predominant magnetic phase. Magnetic hysteresis and first‐order reversal curve diagrams suggest that the magnetite is mixture of interacting single‐domain, and pseudo‐single‐domain and/or multidomain particles. Superparamagnetic magnetite occurs in the S= 40–60% serpentinized but weakly magnetic dunites. Overall, the magnetite content and magnetic susceptibility increase consistently from ~0 to 20% of serpentinization and then decrease from S= 30 to 60%. Peak magnetite abundance is at S~25% (density, ~3.1 ± 0.05 g cm–3) and may suggest localized enrichment of fluids. The low magnetite abundance in dunite results from low‐temperature serpentinization. Finally, strong intensities of magnetization (1–10 Am–1) reside in the S> 60% and S= 20–30% serpentinized peridotites, indicating that peridotites with such degrees of serpentinization contribute to the aeromagnetic anomalies within the Yarlung‐Zangbo suture zone in south Tibet. Magnetic properties of variably serpentinized peridotites from the Dongbo ophiolite (SW Tibet) are investigatedMagnetic susceptibility and magnetite abundance show peaks at approximately 25% of the degree of serpentinizationPeridotites in 20‐30% and >60% of serpentinization are major sources of magnetic anomalies in suture zones

The Pozanti-Karsanti ophiolite situated in the eastern Tauride belt, southern Turkey, is a well-preserved oceanic lithosphere remnant comprising, in ascending order, mantle peridotite, ultramafic and mafic cumulates, isotropic gabbros, sheeted dikes, and basaltic pillow lavas. Two types of chromitites are observed in the Pozanti-Karsanti ophiolite. One type of chromitites occurs in the cumulate dunites around the Moho, and the other type of chromitites is hosted by the mantle harzburgites below the Moho. The second type of chromitites has massive, nodular, and disseminated textures. We have conducted the mineral separation work on the podiform chromitites hosted by harzburgites. So far, more than 100 grains of microdiamond and moissanite (SiC) have been recovered from the podiform chromitite. The diamonds and moissanite are accompanied by large amounts of rutile. Besides zircon, monazite and sulfide are also very common phases within the separated minerals. The discovery of diamond, moissanite, and the other unusual minerals from podiform chromitite of the Pozanti-Karsanti ophiolite provides new evidences for the common occurrences of these unusual minerals in ophiolitic peridotites and chromitites. This discovery also suggests that deep mantle processes and materials have been involved in the formation of podiform chromitite.

Baotoudong syenite pluton is located to the east of Baotou City, Inner Mongolia, the westernmost part of the Triassic alkaline magmatic belt along the northern margin of the North China Craton (NCC). Zircon U-Pb age, petrological, mineralogical and geochemical data of the pluton were obtained in this paper, to constrain its origin and mantle source characteristics. The pluton is composed of nepheline-clinopyroxene syenite and alkali-feldspar syenite, with zircon U-Pb age of 214.7±1.1 Ma. Diopside (cores)-aegirine-augite (rims), biotite, orthoclase and nepheline are the major minerals. The Baotoudong syenites have high contents of rare earth elements (REE), and are characterized by enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE; e.g., Rb, Ba, Sr), depletion in heavy rare earth elements (HREE) and high field strength elements (HFSE). They show enriched Sr-Nd isotopic compositions with initial 87Sr/86Sr ranging from 0.7061 to 0.7067 and εNd(t) values from–9.0 to–11.2. Mineralogy, petrology and geochemical studies show that the parental magma of the syenites is SiO2-undersaturated potassic-ultrapotassic, and is characterized by high contents of CaO, Fe2O3, K2O, Na2O and fluid compositions (H2O), and by high temperature and high oxygen fugacity. The syenites were originated from a phlogopite-rich, enriched lithospheric mantle source in garnet-stable area (>80 km). The occurrence of the Baotoudong syenites, together with many other ultrapotassic, alkaline complexes of similar ages on the northern margin of the NCC in Late Triassic implies that the lithospheric mantle beneath the northern margin of the NCC was previously metasomatized by melts/fluids from the subducted, altered paleo-Mongolian oceanic crust, and the northern margin of the craton has entered into an extensively extensional regime as a destructive continental margin in Late Triassic.

Qingsongite (IMA 2013-30) is the natural analog of cubic boron nitride (c-BN), which is widely used as an abrasive under the name “Borazon.” The mineral is named for Qingsong Fang (1939-2010), who found the first diamond in the Luobusa chromitite. Qingsongite occurs in a rock fragment less than 1 mm across extracted from chromitite in deposit 31, Luobusa ophiolite, Yarlung Zangbu suture, southern Tibet at 29°13.86N and 92°11.41E. Five electron microprobe analyses gave B 48.54 ± 0.65 wt% (range 47.90-49.2 wt%); N 51.46 ± 0.65 wt% (range 52.10-50.8 wt%), corresponding to B1.113N0.887 and B1.087N0.913, for maximum and minimum B contents, respectively (based on 2 atoms per formula unit); no other elements that could substitute for B or N were detected. Crystallographic data on qingsongite obtained using fast Fourier transforms gave cubic symmetry, a = 3.61 ± 0.045 Å. The density calculated for the mean composition B1.100N0.900is 3.46 g/cm3, i.e., qingsongite is nearly identical to synthetic c-BN. The synthetic analog has the sphalerite structure, space group F43m. Mohs hardness of the synthetic analog is between 9 and 10; its cleavage is {011}. Qingsongite forms isolated anhedral single crystals up to 1 mm in size in the marginal zone of the fragment; this zone consists of ~45 modal% coesite, ~15% kyanite, and ~40% amorphous material. Qingsongite is enclosed in kyanite, coesite, or in osbornite; other associated phases include native Fe; TiO2II, a high-pressure polymorph of rutile with the αPbO2structure; boron carbide of unknown stoichiometry; and amorphous carbon. Coesite forms prisms several tens of micrometers long, but is polycrystalline, and thus interpreted to be pseudomorphic after stishovite. Associated minerals constrain the estimated pressure to 10-15 GPa assuming temperature was about 1300 °C. Our proposed scenario for formation of qingsongite begins with a pelitic rock fragment that was subducted to mid-mantle depths where crustal B originally present in mica or clay combined with mantle N (δ15N = -10.4 ± 3‰ in osbornite) and subsequently exhumed by entrainment in chromitite. The presence of qingsongite has implications for understanding the recycling of crustal material back to the Earth’s mantle since boron, an essential constituent of qingsongite, is potentially an ideal tracer of material from Earth’s surface.

Abstract: Recently, an over 100 km long MORB-type eclogite belt of Permian was discovered in the Sumdo (松多) region of the Lhasa block, Tibet. A critical question thus is: what is the tectonic setting of the eclogite belt and is it related to an unrecognized suture in the region? Further investigations show that there are some mafic and ultramafic rocks spacially associated with the eclogite belt in the region. Three ultramafic massifs were recognized in the Sumdo region, and called the Luomaling (罗马岭), Gongbupala (贡布爬拉) and Qiazhasumdo (卡扎松多) massifs. All the massifs are fault-contacted with greenschist (Chasagang (岔萨岗) Formation) or muscovite-quartz schist (Mabuku (马布库) Formation), and individuals are about 100 m×50 m in size extending in EW as the regional structure. All the ultramafic rocks have been entirely serpentinized, and the Gongbupala massif has been selected for study in geochemistry. Eleven chemical analyses of the rocks from the Gongbupala massif show a narrow range in contents: SiO2 (35.97–40.63) wt.%, MgO (37.02–38.60) wt.%, TiO2 (0.01–0.08) wt.%, Al2O3 (0.80–1.64) wt.%, (Na2O+K2O) less than 0.1 wt.%, with high volatile contents (H2O+CO2) (11.24–14.91) wt.%. After recalculation without H2O+CO2, the mean values are SiO2 45.24 wt.%, MgO 43.54 wt.%, FeOT (7.45–9.97) wt.% (8.55 wt.% in average), (MgO+FeOT) 52.09 wt.%, Mg# (100×Mg/(Mg+Fe*), where Fe* represents total Fe)=89.42–90.08, (m+f)/Si ((atomicity Mg+atomicity Fe) /atomicity Si)=1.53–1.75 (1.59 in average), respectively. The mean M/F (atomicity Mg/atomicity Fe) ratio of the rocks is 9.05, which is classified as magnesium enriched-type of ultramafic rocks. The compositional features, depleted in K, Na, Ca, Al and Ti and enriched in Mg#, indicate the characteristics of peridotite originated from a depleted mantle. The rocks have low ΣREE with (1.60–2.68)×10−6 similar to those of the primitive mantle. The chondrite-normalized REE patterns of all samples show slightly enrichment in LREE, with (Ce/Yb)N 1.03–2.46, but a little depleted in HREE. Most samples show a slight negative anomaly in Eu, a feature in REE from a relic mantle and common features in highly serpentinized ultramafic rocks in the Yarlung-Zangbo (雅鲁藏布) ophiolite and the Bangong (班公)-Nujiang (怒江) ophiolite in Tibet. The primitive mantle-normalized spiderdiagram of trace elements for Gongbupala ultramafic rocks yields uniform distributed pattern. They are relatively enriched in Rb, Ba, La, P element (LHSE) and de pleted in Sm, Ti, Y, Yb element (HFSE), a feature of metasomatic mantle peridotite. The geochemical features of the rocks suggest that the protolith of Gongbupala serpentinite in Sumdo region is harzburgite, a typical depleted mantle rock, and may represent a dismembered ophiolite unit in the region.

Abstract: The A’nyemaqen (阿尼玛卿) ophiolite belt along the southern margin of the East Kunlun (昆仑) Mountains marks the suture formed by the closure of paleo-Tethys. The Dur’ngoi ophiolite in the eastern part of this belt consists of meta-peridotite, mafic-ultramafic cumulates, sheeted dikes and basaltic lavas. The meta-peridotites consist of dunite, harzburgite, lherzolite, feldspar-bearing lherzolite and garnet-bearing lherzolite and contain residual spinel with Cr# [100×Cr/(Cr+Al)] ranging from 30 to 57 and Mg# [100×Mg/(Mg+Fe2+)] ranging from 50 to 75, indicating an Al- and Mg-rich series. The meta-peridotites have a relatively narrow range of composition with Mg# of 89.2–92.6, Al2O3 contents of (1–4) wt.% and slightly depleted chondrite normalized REE patterns, indicating that they represent relict mantle material that has undergone intermediate to low degrees of partial melting. Garnets in the lherzolite are andradite, enriched in Ca and Fe and depleted in Mg and Al (And=95–97, Pyr=0.3–5, Gro=0–3), indicating a metamorphic origin. The cumulate rocks mainly consist of dunite, wehrlite, pyroxenite and gabbro. A well-layered gabbro-pyroxenite complex is defined by modal variations in plagioclase and pyroxene. Blocks of garnet-pyroxenite or rodingite are locally present in the meta-peridotites. Garnets in the cumulate rocks are grossular (Gro=69–90, And=9–19, Br=1–12), also metamorphic origin. The diabase dikes are moderately depleted in LREE [(La/Sm)N=0.5–0.8] and HREE resulting in slightly convex chondrite-normalized patterns with slightly positive Eu anomalies (δEu=1.1–1.3). The basaltic lavas have REE patterns similar to those of MORB with (La/Sm)N ratios of 0.5–1 and small negative Eu anomalies. They appear to have been derived from a depleted mantle source and to have undergone little or no differentiation during crystallization. SHRIMP U-Pb dating of zircons from the basalts yields 206Pb/238U ages of 276–319 Ma (average 308.0±4.9 Ma). The Dur’ngoi ophiolite is interpreted as a dismembered fragment of paleo-oceanic crust emplaced during closure of the paleo-Tethyan Ocean basin. Three other suites of oceanic lavas are recognized in the area: island arc volcanic (IAV) rocks, possible back arc basin (BAB) basalts and possible post-collisional volcanic (PCV) and plutonic rocks. The distribution of these rocks suggests north-directed subduction. Opening of the A’nyemaqen oceanic basin started at least as early as Late Carboniferous (308 Ma) and the basin probably closed during the Early Triassic. The IAV formed in Late Permian (260 Ma), the BAB in Early-Middle Triassic, and the PCV in Late Triassic. Several large scale, ductile, sinistral strike-slip fault zones, extending hundreds to thousands kilometers, formed along or north of the suture during the Early-Late Triassic, e.g., they are the south margin fault zone of East Kunlun (200–220 Ma), the Altyn Tagh fault (220–230 Ma), and the North Qaidam fault zone (240–250 Ma). These strike-slip faults were probably generated by oblique subduction and closure of the paleo-Tethyan Ocean basin, possibly during exhumation of the subducted plate or uplift of the overriding plate, coincident with post-collisional magmatism.

Abstract: Based on the deformation characteristics of the ductile shear zones in Sumdo (松多) Group, the quartz fabric by EBSD (electron backscatter diffraction), the data of muscovite 40Ar-39Ar geochronology (220–230 Ma) from ductile shear zones and the zircon SHRIMP U-Pb chronology (190 Ma) of granites in Sumdo region, Lhasa (拉萨) terrane is thought to have experienced an important Indosinian orogenic event at 220–230 Ma, which caused the closure of the paleo-Tethys Ocean along the tectonic zone of eclogite and the collision between northern part and southern part of the Lhasa terrane. The zircon SHRIMP U-Pb chronology of 190 Ma for biotite adamellite, with the distributing characteristics of the granite massif intruding in Sumdo Group, indicates that the biotite adamellite should be the late orogenic or post-orogenic granite resulting from the Indosinian orogenesis. The discovery of Indosinian orogenic belt in Lhasa terrane expansed the southern boundary of Indosinian orogenic belt in Qinghai (青海)-Tibet plateau to Lhasa terrane from Qiangtang (羌塘) terrane, which changed the understanding about the distribution of Indosinian orogenic belt in Qinghai-Tibet plateau and extended the “T” type Indosinian orogenic belt in China. The study is very important for the formation and distribution of paleo-Tethys Ocean in Tibet. The ancient terrane framework and evolution of Qinghai-Tibet plateau need further research.

Abstract: Silicon-bearing rutile has been found in chromitite from the Luobusa (罗布莎) ophiolite, Tibet. However, the extent of SiO2 solubility in rutile and the nature of its origin are still unclear. At high pressure, SiO2 takes a rutile structure with Si in 6-fold coordination. Thus, high pressures may enhance its solubility in rutile because of possible isovalent exchange in the octahedral site. In this study, we report new experimental results on SiO2 solubility in rutile up to 23 GPa and 2 000 °C. Starting materials were mixtures of powdered pure rutile and pure quartz, with compositions of (Ti0.5Si0.5)O2, (Ti0.93Si0.07)O2, and (Ti0.75Si0.25)O2. The mixtures were loaded into either platinum capsules (for a 10/5 assembly) or rhenium capsules (for an 8/3 assembly). The experiments were carried out using multi-anvil high-pressure apparatus with a rhenium resistance heater. Sample temperatures were measured with a W5%Re-W26%Re thermocouple and were controlled within ±1 °C of the set temperature. TiO2-rich and SiO2-rich phases were produced in all the quenched samples. Microprobe analyses of the phases show that the solubility of SiO2 in rutile increases with increasing pressure, from 1.5 wt.% SiO2 at 10 GPa to 3.8 wt.% SiO2 at 23 GPa at a temperature of 1 800 °C. The solubility also increases with increasing temperature from 0.5 wt.% SiO2 at 1 500° to 4.5 wt.% SiO2 at 2 000° at a pressure of 18 GPa. On the other hand, the solubility of TiO2 in coesite or stishovite is very limited, with an average of 0.6 wt.% TiO2 over the experimental P-T ranges. Temperature has a much larger effect on the solubility of SiO2 in rutile than pressure. At high pressure, the melting point of SiO2 is definitely higher than that of TiO2 and the eutectic point moves towards SiO2 in the TiO2-SiO2 system. Lower oxygen fugacity decreases the solubility of SiO2 in rutile, whereas water has little effect on the solubility. Our experimental data are extremely useful for determining the depth of origin of the SiO2-bearing rutile found in nature.

Abstract: A wide variety of unusual mantle has been reported from podiform chromitite orebodies Cr-31 and Cr-74 in the Luobusa (罗布莎) ophiolite, Tibet. A detailed investigation of chromitite orebody Cr-11, located in the Kangjinla (康金拉) district at the eastern end of the ophiolite, has revealed many of the same minerals, including diamond, moissanite, and some native elements, alloys, oxides, sulphides, silicates, carbonates, and tungstates. This orebody is particularly rich in diamonds, with over 1 000 grains recovered from about 1 100 kg sample of chromitite. More detailed studies and experiments are needed to understand the origin and significance of these unusual minerals because they have not been found in situ. It is a great breakthrough in mineralogical research that we have picked up more than 40 kinds of minerals from the Kangjinla chromite deposit in Luobusa. It is notable that a large amount of diamonds were firstly discovered from the Kangjinla chromite deposit as well as many other unusual minerals, such as moissanites, rutiles, native irons, and metal alloys. Especially, that diamond was found again in different chromitites in the same ophiolite belt provided new key evidence for discussing the origin of the diamond and the hosted chromitite and ophiolite. The mantle mineral group in Tibet has great significance in mineralogy and geodynamics.

An unusual group of mantle minerals including about 70-80 species has been recovered from podiform chromitites of the Luobusa ophiolite, Qusong County, Tibet, China. All of the minerals were hand picked from heavy mineral separates of the chromitite. The minerals include diamond, coesite, moissanite, wüstite, intermetallic compounds, Os-Ir alloys with diamond inclusions, Fe-silicides, and a new mineral, qusongite. Qusongite is associated with chromian chlorite, calcite, (W,Ti)C and (Ti,W) C alloys, and chromite. It occurs as angular grains generally 4-8 μm in diameter, but some are as large as 0.2 × 0.3 × 0.25 mm. The grains are opaque and steel-gray with a metallic luster and grayish-yellow reflection. The empirical formula (based on 2 atoms) is W1.006Cr0.02C0.992, and the simplified formula is WC. Qusongite has a hexagonal structure and belongs to space group P6m2, with a = 2.902(1) Å, c = 2.831(1) Å, c:a = 0.9775, V = 20.05 (1) Å3, Z = 1.

Yarlongite occurs in ophiolitic chromitite at the Luobusha mine (29°5′N 92°5′E, about 200 km ESE of Lhasa), Qusum County, Shannan Prefecture, Tibet Autonomous Region, People's Republic of China. Associated minerals are: diamond, moissanite, wüstite, iridium (“osmiridium”), osmium (“iridosmine”), periclase, chromite, native iron, native nickel, native chromium, forsterite, Cr‐rich diopside, intermetallic compounds Ni‐Fe‐Cr, Ni‐Cr, Cr‐C, etc. Yarlongite and its associated minerals were handpicked from a large heavy mineral sample of chromitite. The metallic carbides associated with yarlongite are cohenite, tongbaite, khamrabaevite and qusongite (IMA2007–034). Yarlongite occurs as irregular grains, with a size between 0.02 and 0.06 mm, steel‐grey colour, H Mohs: 5½–6. Tenacity: brittle. Cleavage: {0 0 1} perfect. Fracture: conchoidal. Chemical formula: (Cr4Fe4Ni)Σ9C4, or (Cr,Fe,Ni)Σ9C4, Crystal system: Hexagonal, Space Group: P63/mc, a= 18.839(2) Å, c= 4.4960 (9) Å, V= 745.7(2) Å3, Z= 6, Density (calc.) = 7.19 g/cm3(with simplified formula). Yarlongite has been approved as a new mineral by the CNMNC (IMA2007–035). Holotype material is deposited at the Geological Museum of China (No. M11650).

Analytical electron microscopic observations have been carried out on a garnet peridotite from the Maobei area, Sulu ultrahigh-pressure terrane. The results showed that olivine in this garnet peridotite (5.3-6.6 GPa; 853-957 °C), contains precipitates of chromian magnetite and chromian-titanian hematite at dislocations and (001) faults. Specific crystallographic relationships were determined between these precipitates and the olivine host, viz. [101]Mt//[001]Ol, [110]Mt//[01̄1]Ol, and [01̄1]Mt//[011]Ol; and [0001]Hm//[100]Oland [101̄0]Hm//[001]Ol. These oriented oxides are not associated with silicate/silica phases and therefore cannot be accounted for by the mechanism of olivine oxidation. It is postulated that these magnetite and hematite precipitates most likely have resulted from dehydrogenation-oxidation of nominally anhydrous mantle olivine during rock exhumation. In view of the contrasting diffusion rates of H and Fe in the olivine lattice, it is suggested that the formation process might actually take place in steps. Hydrogen diffusion with concomitant quantitative oxidation of Fe2+to Fe3+in olivine occurred early during initial rock exhumation and was followed by slow Fe diffusion forming magnetite/hematite at stacking faults and dislocations within the olivine lattice. Two requirements are essential under such a scenario: an ample amount of H content of the olivine, and an appropriate exhumation rate, probably in the range of 6-11 mm/year, of the host rock. It is also noted that such dehydrogenation-oxidation processes may hamper a correct estimate of the actual P-T conditions and mantle oxidation state based on mineral chemistries present in mantle eclogite/peridotite. The present study demonstrates that oriented mineral inclusions may not necessarily form through exsolution processes sensu stricto, but may form through a series of more complicated reaction mechanisms.

The Kangxiwar ductile strike-slip shear zone marks the southern boundary of the West Kunlun terrane, a large, nearly E-W trending metamorphic terrane in the western Qinghai Tibet Plateau region. This ductile shear zone is ∼7 km wide, and consists of mylonitized khondalites. Protoliths of the khondalites were alumina-rich pelitic sedimentary and subordinate volcanic rocks. The pelitic khondalites have pronounced positive Th anomalies and subdued positive Ce and Zr anomalies, whereas the metavolcanic rocks have positive Nb and Zr anomalies. Both types of khondalite are LREE enriched, and show weak HREE depletions and moderate negative Eu anomalies. P-T conditions for the formation of the khondalites are estimated to be 6.8 kbar and 700°C. The khondalites formed in the Caledonian orogeny (428-445 Ma) and underwent strong shear deformation during the Indosinian (250-210 Ma). SHRIMP dating of detrital zircons in the khondalites suggests that they were derived from an older metamorphic basement, probably older than 644 Ma. The Kangxiwar khondalites are similar in their protoliths, trace- element and rare-earth element geochemistry, P-T conditions, and age of formation to those of the South Altyn Tagh khondalite series. This lateral correlation suggests that the West Kunlun and Altyn Tagh terranes were once contiguous, and provides evidence for the existence of a Caledonian orogenic belt in this region.

Field observations and petrological data suggest a new high-pressure/low temperature (HP/LT) metamorphic terrane in the northern Altyn Tagh. Newly recognized eclogites, blueschists, and metapelites constitute part of a coherent HP/LT metamorphic terrane that occurs as a tectonic slab in an ophiolitic melange. Geobarometry and geothemometry show that peak metamorphic conditions of the eclogites were T = 430-540°C and P = 20-23 kbar. Petrographic data, P-T estimates, and amphibole compositions indicate distinctly different metamorphic histories for two outcrops about 1.2 km apart. For blueschists from outcrop I, the retrograde P-T path is a relatively straight line directed towards the greenschist facies, suggesting simultaneous cooling and decompression. In contrast, interlayered blueschist and eclogite in outcrop II underwent isothermal decompression from eclogite/blueschist facies to epidote-amphibole facies conditions during exhumation. This implies that rocks from these two outcrops experienced different tectono-thermal histories. Preliminary data regarding geological setting, lithologies, and P-T estimates reveal significant similarities between the HP/LT metamorphic terranes in the north Altyn Tagh and the north Qilian Mountains, and suggest that they may be parts of the same early Paleozoic HP/LT belt, subsequently displaced by the Altyn Tagh fault.

A ~4000 km long ultrahigh-pressure metamorphic (UHPM) belt in northern China has been documented on the basis of the discovery of coesite-bearing rocks in the Altun-North Qaidam terrane in the western Central Orogenic Belt (COB), and diamond-bearing rocks in Qinling in the central and Dabie-Sulu terrane in the east. New SIMS and SHRIMP U-Pb dates of zircons from coesitebearing UHPM rocks indicate two UHPM events: one in the early Paleozoic and the other in the Triassic. Coesite-bearing zircons from a North Qaidam gneiss yielded UHP metamorphic ages of 452 ± 13.8 Ma and retrograde ages of 419 ± 6.7 Ma. A diamond-bearing gneiss from Qinling gave a lower intercept age of 502 ± 45 Ma, and an upper intercept age of 1545 ± 100 Ma, whereas a Qinling eclogite sample gave a lower intercept age of 493 ± 170 Ma and an upper intercept age of 1381 ± 82 Ma. The lower and upper intercept ages of the Qinling samples are interpreted as UHPM and protolith ages of the rocks, respectively. Coesite-bearing zircons from a Qinglongshan eclogite in the south Sulu belt yielded early Paleozoic UHPM ages of 441 ± 9 Ma, 449 ± 9 Ma, and 442 ± 9 Ma, whereas the core of a zircon containing plagioclase and apatite inclusions gave a protolith age of 761 ± 13 Ma. These age data suggest that the early Paleozoic UHPM rocks extend from west to east for about 4000 km across the COB, whereas the Triassic UHPM belt extends across the Dabie-Sulu region for about 1000 km.Based on available geochronological and geochemical data, we suggest the following tectonic model for evolution of the COB. At about 1000 Ma, the area was amalgamated to form the Rodinian continent, which contained ophiolitic fragments of oceanic affinity. This part of Rodinia was then rifted at about 800-750 Ma to form an oceanic basin with a variety of MORB and intruded by granitic plutons. Closure of this ocean basin produced Neoproterozoic ophiolites and granitic gneisses. The UHPM rocks, along with subduction-related island-arc volcanics and granites of early Paleozoic age suggest a second cycle of rifting and subduction along the COB, whereas the Triassic UHPM rocks record a final subduction and collision event between the North China and South China blocks.

Variably serpentinized peridotites from the Zedang ophiolite in southern Tibet were magnetically and petrologically examined to understand the serpentinization process and evaluate the origin of magnetic anomalies in ultramafic‐hosted tectonic settings. Magnetite occurs in the serpentine and brucite veins and is identified as the dominant magnetic carrier by thermomagnetic and petrological analyses. The magnetic susceptibility increases rapidly from <0.001 to ~0.02 SI for the <50% serpentinized samples followed by nearly constant values of 0.02–0.03 SI above 50% serpentinization. This transition corresponds with the formation of Fe‐poor serpentine mesh (2–3 wt% FeO) and magnetite in the early stages and the replacement of mesh center olivine by Fe‐rich serpentine (4–5 wt% FeO) without magnetite in the late stages. Brucite veins occur in the 50–70% serpentinized samples and indicate serpentinization temperatures from ~250 to <100°C. The serpentinization may initiate at an oceanic spreading ridge center under high temperatures (>250–300°C) to produce magnetite and subsequently continue at lower temperatures (<200–250°C) in near‐seafloor settings and limit the magnetite formation, possibly associated with ophiolite emplacement. These serpentinized peridotites have higher magnetization intensities (average 2.26 Am−1) than dolerite dykes and basaltic volcanics (mostly <1 A m−1) in the area and should be the major source of aeromagnetic highs in the south Tibetan ophiolite belt. Variably serpentinized peridotites from the Zedang ophiolite in southern Tibet were magnetically and mineralogically examinedWe observed that magnetite formation is limited above ~50% serpentinization in the Zedang ophioliteThe serpentinization may start at >250–300°C at an oceanic spreading ridge center and continue at <200–250°C in near‐seafloor settings