SHRIMP dating of the Bangong Lake SSZ-type ophiolite: Constraints on the closure time of ocean in the Bangong Lake-Nujiang River,northwestern Tibet

The Bangong Lake ophiolite is located in the westernmost part of the Bangong Lake-Nujiang River suture zone. It is a tectonic mélange consisting of numerous individual blocks of peridotite, pillowed and massive lavas and mafic dykes with SSZ-type ophiolitic geochemical affinity formed at the end of a Wilson circle. The SHRIMP U-Pb ages of the co-magmatic zircon domains from one gabbroic dyke (Sample 01Y-155) range from 162.5±8.6 Ma to 177.1±1.4 Ma with an average of 167.0±1.4 Ma (n = 12, MSWD = 1.2), suggesting that the subduction of the Bangong Lake Neo-Tethyan Ocean started before the Middle Jurassic. It is inferred that the tectonic transform from spreading to subduction of the Neo-Tethyan Ocean began before the Middle Jurassic in the Bangong Lake area.

Melting geodynamics reveals a subduction origin for the Purang ophiolite,Tibet,China

The debate regarding whether the Yarlung-Zangbo ophiolite(YZO)on the south of the Qinghai-Tibet Plateau,formed in a mid-ocean ridge(MOR)or a supra-subduction zone(SSZ)setting has remained unresolved.Here we present petrological,mineralogical,and geochemical data associated with modeling melting geodynamics of the mantle peridotites from the Purang ophiolite in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)to explore its tectonic environment.The Purang lherzolites are characterized by the protogranular texture and have abyssal-peridotite-like mineral compositions,including low Cr^(#)(20-30)and TiO_(2) contents(<0.1wt%)in spinel,high Al_(2)O_(3)(2.9wt%-4.4wt%)and CaO(1.9wt%-3.7wt%)contents in orthopyroxene and LREE-depletion in clinopyroxene.Compositions of these lherzolites can be modeled by~11%dynamic melting of the DMM source with a small fraction of melt(~0.5%)entrapped within the source,a similar melting process to typical abyssal peridotites.The Purang harzburgites are characterized by the porphyroclastic texture and exhibit highly refractory mineral compositions such as high spinel Cr^(#)(40-68),low orthopyroxene Al_(2)O_(3)(<2.2wt%)and CaO(<1.1wt%)contents.Clinopyroxenes in these harzburgites are enriched in Sr(up to 6.0 ppm)and LREE[(Ce)N=0.02-0.4],but depleted in Ti(200 ppm,on average)and HREE[(Yb)N<2].Importantly,the more depleted samples tend to have higher clinopyroxene Sr and LREE contents.These observations indicate an open-system hydrous melting with a continuous influx of slab fluid at a subduction zone.The modeled results show that these harzburgites could be formed by 19%-23%hydrous melting with the supply rate of slab fluid at 0.1%-1%.The lower clinopyroxene V/Sc ratios in harzburgites than those in lherzolites suggest a high oxidation stage of the melting system of harzburgites,which is consistent with a hydrous melting environment for these harzburgites.It is therefore concluded that the Purang ophiolite has experienced a transformation of tectonic setting from MOR to SSZ.

Multi-stage formation of the Feragen ophiolite,Norway:Implication from petrology and geochemistry of peridotites and chromitites and its potential for prospecting

The ultramafic massif of Feragen,which belongs to the eastern ophiolitic belt of Norway,has abundant amounts of chromite ores.Recent studies have revealed a complex melt evolution in a supra-subduction zone(SSZ)environment.This study presents new whole-rock major element,trace element,and platinum-group element chemistry to evaluate their petrogenesis and tectonic evolution.Harzburgites have high CaO,Al_(2)O_(3),TiO_(2),MgO,and REE contents corresponding to abyssal peridotites,whereas dunites have low CaO,Al_(2)O_(3),TiO_(2),MgO,and REE contents corresponding to SSZ peridotites.The Cr^(#)and TiO_(2) of chromian spinels in the harzburgites suggest as much as about 15%–20%melting and the dunites are more depleted with>40%melting.The harzburgites and the dunites and high-Cr chromitites represent,respectively,the products of low-degree partial melting in a back-arc setting,and the products of melt-rock interaction in a SSZ environment.The calculated fO_(2) values for dunites and high-Cr chromitites(-0.17–+0.23 and+2.78–+5.65,respectively and generally above the FMQ buffer)are also consistent with the interaction between back-arc ophiolites with oxidized boninitic melts in a SSZ setting.

Early Paleozoic Ocean in the North Qaidam: Constraints from Kaipinggou Ophiolite

The North Qaidam orogenic belt(NQOB) lies at the northeast margin of Tibet Plateau, between the Qilian massif to the north and Qaidam massif to the south. It consists of predominantly gneisses and marbles with minor eclogite and garnet peridotite, and is known as an Early Paleozoic continental deep subduction ultrahigh pressure(UHP) metamorphic belt in northwest China. Recently some studies suggest that the NQOB experienced oceanic deep subduction prior to the continental deep subduction. However, the records of ocean events are rarely preserved because of the complex multi-stage transformation.Therefore, whether the Early Paleozoic ocean exists and the formation, nature and evolution of the Early Paleozoic ocean in North Qaidam are still in controversies. In this contribution, we report a new discovered SSZ-type ophiolite suite in Kaipinggou area, the western segment of the NQOB. The ophiolite is mainly composed of peridotite, gabbro,basalt, boninite, andesitic porphyrite and plagiogranite. In which,the basalt and homogeneous gabbro are both enriched in LILE and depleted in HFSE, and display flat or LREE-depleted REE patterns, similar to the MORB-like geochemical characteristics of fore-arc basalts;the boninite exhibits characteristics of forearc boninites with high Si and Mg, but low Ti and Ca contents;the andesitic porphyrite is enriched in LILE but depleted in HFSE, similar to island arc magmatic rocks. Rock assemblage and their geochemical characteristics suggest that this SSZ-type ophiolite was formed in fore-arc setting of intra-oceanic arc. The plagiogranite is a low-potassium tholeiite metaluminous granite with low trace elements contents, slight enrichment of LREE,depletion of HREE and positive Eu anomaly;combining with εHf(t) values(13.8–17.1) of zircons, the plagiogranite could be originated from partial melting of gabbros. LA–ICP–MS zircon U–Pb dating yielded the crystallization ages of 535±2 Ma, 513±3 Ma and 510±3 Ma for two cumulate gabbros and one homogeneous gabbro, respectively, and a formation age of 492±8 Ma for the plagiogranite. All these data suggest the existence of an Early Paleozoic ocean in the North Qaidam, and the initial subduction of the oceanic slab occurred earlier than 535 Ma.

Formation Age and Tectonic Setting of the Muli Arc-Ophiolite Complex in the South Qilian Belt, NW China

The Qilian orogenic belt is the northernmost orogen of the Tethyan domain and connects the Altaids to the north. It contains an assembly of Precambrian micro-continental fragments, early Paleozoic island arcs, accretionary complexes, ophiolites, forearc and backarc basins, and high-pressure(HP) metamorphic rocks, indicating a long history of accretionary processes. Spatially, this orogen is adjacent to the Tarim, Qaidam, and North China blocks, which also extends into accretionary orogenic belts to the east and SW such as the Qinling and Kunlun belts. Abundant ophiolites in this orogen record the closure of an early Tethyan Ocean and amalgamations between micro-continents of North China, Qaidam, and Tarim. Thus, the ages and tectonic settings of these ophiolites within this belt provide important information regarding evolution of the Proto-Tethys Ocean and assembly of micro-continental blocks, which aids understanding of the spatial and temporal relationship of this orogen within the Tethyan realm. Dismembered ophiolites sporadically crop out along the northern margin of the South Qilian belt, and, from east to west, are locally referred to as the Lajishan, Gangcha, Muli, and Dadaoerji ophiolites. Much attention had been paid to these ophiolites, and several competing models for the tectonic evolution of this belt have been suggested. Considerable disagreement remains in respect of the temporal and spatial framework of the Qilian Orogen and details such as timing of subduction(s) and associated polarities, early collision events, and final closure of oceanic basins. In particular, the formation age and tectonic setting of Muli arc-ophiolite complex remains unknown, which limits understanding of the tectonics of the South Qilian belt and the history of the Proto-Tethys Ocean. The Muli arc-ophiolite complex is distributed over 20 km^2 west of the township of Muli in the western segment of the South Qilian Belt and consists of serpentinite, dunite, cumulate gabbro, basalt, plagiogranite, and chert. Field mapping results demonstrate that these units have been largely destroyed by faulting and generally occur as blocks/slices. They are tectonically interlayered with Upper Ordovician – Lower Silurian siliciclastic turbidite. Arc-ophiolite rocks are intruded by 470–450 Ma subduction-related granitoid plutons and are unconformably overlain by shallow marine to non-marine sediments of Permian-Jurassic age. Basalts show typical subduction-related calc-alkaline geochemical affinity, representing portions of an island arc. Geochemical results for plagiogranites and spinels from serpentinite demonstrate that the Muli arc-ophiolite complex represents a super-subduction zone(SSZ)-type ophiolite. U-Pb zircon data indicate formation associated with southward subduction of the Proto-Tethyan Ocean during a short interval between 539–522 Ma. Voluminous Late Ordovician-Early Silurian deep-water marine siliciclastic and volcaniclastic turbidites and volcanic arc rocks are exposed to the south of the Muli arc-ophiolite complex, whereas fluvial coarse-grained sandstones and conglomerates unconformably overlie the Cambrian-Middle Ordovician ophiolite-arc systems in the eastern South Qilian Belt. These indicate that closure of the Proto-Tethys Ocean was diachronous during the early Paleozoic.

SHRIMP Zircon U-Pb Dating of Gabbro and Granite from the Huashan Ophiolite,Qinling Orogenic Belt,China: Neoproterozoic Suture on the Northern Margin of the Yangtze Craton

The recently identified Huashan ophiolitic mélange was considered as the eastern part of the Mianlüe suture in the Qinling orogenic belt. SHRIMP zircon U-Pb geochronology on gabbro from the Huashan ophiolite and granite intruding basic volcanic rocks indicates crystallization ages of 947±14 Ma and 876±17 Ma respectively. These ages do not support a recently proposed Hercynian Huashan Ocean, but rather favor that a Neoproterozoic suture assemblage （ophiolite） is incorporated into the younger （Phanerozoic） Qinling orogenic belt.

Geochemical Features of Ophiolite in Mianxian Lueyang Suture Zone，Qinling Orogenic Belt

The ultrahasic rocks in the Mianxian-Lueyang ophiolite belt are mainly harzburgite and dunite, and exhibit two types of REE distribution pattern: (1) LREE depletion, with remarkable Eu enrichment; (2) slight enrichment of LREE, with Eu deficiency. Both gabbros (accumulative gabbros) and diabasic dike swarms show a LREE enrichment but a slight Eu anomaly. The ratios of w (Ti) /w (V), w (Th) /w (Ta). w (Th) /w (Yb) and w (Ta) /w (Yb) indicate that the mid-ocean ridge basalts in this region originated from depleted mantle of asthenosphere, implying an association of MORB-type ophiolite and an ancient ocean basin between Qinling and Yangtze plates during Middle Paleozoic-Early Mesozoic era. The island arc volcanic rocks can be divided into tholeiitic and cafe-alkaline associations, which originated from the upper mantle wedge above slab by partial melting.

Sensitive High Resolution Ion Micro-Probe U-Pb Zircon Geochronology and Geochemistry of Mafic Rocks from the Pulan-Xiangquanhe Ophiolite,Tibet:Constraints on the Evolution of the Neo-tethys

The Pulan-Xiangquanhe ophiolite in the western Yarlung Tsangpo suture zone of Tibet is investigated for its geochemistry,geochronology,and tectonic implications in detail.Sensitive high resolution ion micro-probe zircon U-Pb dating reveals that diabases in the ophiolite from the three locations of Xugugab,Mapam Yum Co and La＇nga Co are dated at 122.3±2.5 Ma,118.8±1.8 Ma and 120.5±1.9 Ma,respectively.These early Cretaceous mafic rocks have Na_2O＋K_2O,rare earth element patterns,trace elemental spider diagram and other geochemical fingerprints of typical mature back-arc basin affinity.Therefore,the Pulan-Xiangquanhe ophiolite formed under a mature back-arc basin environment,which was a product of this intra-oceanic subduction system.A suprasubduction system could have existed in the southern margin of Eurasia,which involved both intra-oceanic and continent-ward subductions.Extension dominated the southern margin of the Eurasian continent during the early Cretaceous.

Tectonic Evolution of the Meso-Tethys in the Western Segment of Bangonghu-Nujiang Suture Zone: Insights from Geochemistry and Geochronology of the Lagkor Tso Ophiolite

The subduction of the Bangonghu-Nujiang Meso-Tethys and the collision between the Lhasa and Qiangtang blocks were important events in the growth of the Tibetan crust. However, the timing of collision initiation and closure timing, as well as nature and structure of the Bangonghu ocean basin, are still poorly constrained. The Lagkor Tso ophiolite, located in the south of Gerze County, Tibet, is one of the most completed ophiolites preserved in the southern side of the Bangonghu- Nujiang suture zone. This study discussed the tectonic evolution of the Bangonghu-Nujiang suture zone as revealed by the Lagkor Tso ophiolite investigated by field investigations, petrology, geochemistry, geochronology and tectonic analysis methods. We present new LA-ICP-MS zircon U-Pb and 39Ar/4~Ar ages for the Lagkor Tso ophiolite, in addition to geochemical and platinum-group element （PGE） data presented for the Lagkor Tso ophiolite in Tibet. It is suggested that the ancient Lagkor Tso oceanic basin split in Middle Jurassic （161.2 ± 2.7 Ma - 165.4 ± 3.5 Ma）, and experienced a second tectonic emplacement during the Early Cretaceous （137.90 ± 6.39 Ma）. The Lagkor Tso ophiolite likely developed in an independent suture zone. The Bangonghu-Nujiang ocean subducted southwards, and the dehydration of the subducting oceanic crust materials caused partial melting of the continental mantle wedge, which formed the second-order expanding center of the obduction dish. This led to inter-arc expansion, followed by the formation of inter-arc and back-arc basins with island arc features, which are represented by ophiolites around the Shiquanhe-Lagkor Tso -Yongzhu region. The tectonic environment presently can be considered to be similar to that of the current Western Pacific, in which a large number of island arc-ocean basin systems are developed.

Diamonds and Other Exotic Minerals Recovered from Peridotites of the Dangqiong Ophiolite, Western Yarlung-Zangbo Suture Zone, Tibet

Various combinations of diamond, moissanite, zircon, quartz, corundum, rutile, titanite, almandine garnet, kyanite, and andalusite have been recovered from the Dangqiong peridotites. More than 80 grains of diamond have been recovered, most of which are pale yellow to reddish-orange to colorless. The grains are all 100-200 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm^-1 and 1333 cm^-1, mostly at 1331.51 cm^-1 or 1326.96 cm^-1. Integration of the mineralogical, petrological and geochemical data for the Dongqiong peridotites suggests a multi-stage formation for this body and similar ophiolites in the Yarlung-Zangbo suture zone. Chromian spinel grains and perhaps small bodies of chromitite crystallized at various depths in the upper mantle, and encapsulated the UHP, highly reduced and crustal minerals. Some oceanic crustal slabs containing the chromian spinel and their inclusion were later trapped in suprasubduction zones（SSZ）, where they were modified by island arc tholeiitic and boninitic magmas, thus changing the chromian spinel compositions and depositing chromitite ores in melt channels.

Petrogenetic Evolution of the Bayan Gol Ophiolite— Geological Record of an Early Carboniferous "Red Sea Type"Ocean Basin in the Tianshan Mountains, Northwestern China

The Bayan Gol ophiolite fragment is a portion of the North Tianshan Early Carboniferous ophiolite belt. This ophiolite belt represents a geological record of an Early Carboniferous 'Red Sea type' ocean basin that was developed on the northern margin of the Tianshan Carboniferous-Permian rift system in northwestern China. The late Early Carboniferous Bayan Gol ophiolite suite was emplaced in an Early Carboniferous rift volcano- sedimentary succession of shallow-marine to continental facies (Volcanics Unit). Ophiolitic rocks in the Bayan Gol area comprise ultramafic rocks, gabbros with associated plagiogranite veins, diorite, diabase, pillow basalts and massive lavas. The Early Carboniferous rifting and the opening process of the North Tianshan ocean basin produced mafic magmas in composition of tholeiite and minor amounts of evolved magmas. Compositions of trace elements and Nd, Sr and Pb isotopes reveal the presence of two distinct mantle sources: (1) the Early Carboniferous rift mafic lavas from the Volcanics Unit were generated by a relatively low degree of partial melting of an asthenospheric OIB-type intraplate source; (2) younger (late Early Carboniferous, ~324.8 Ma ago) mafic lavas from the Ophiolite Unit were formed in a relatively depleted MORB-like mantle source, located in the uppermost asthenosphere and then gradually mixed with melts from the asthenospheric OIB-like mantle. A slight interaction between asthenosphere-derived magmas and lithospheric mantle took place during ascent to the surface. Subsequently, the most depleted mafic lavas of the ophiolite assemblage were contaminated by upper-crustal components (seawater or carbonate crust).

Ophiolites from the Mianlüe Suture in the Southern Qinling and Their Relationship with the Eastern Paleotethys Evolution

The Mianlue suture extends from Derni-Nanping-Pipasi-Kangxian to the Lueyang-Mianxian area, then traverses the Bashan arcuate structure eastward to the Huashan region, and finally to the Qingshuihe area of the southern Dabie Mountains. From east to west, with a length of over 1500 km, the ophiolitic melange associations are distributed discontinuously along the suture. The rock assemblages include ophiolite, island-arc and oceanic island rock series, indicating that there existed a suture zone and a vanished paleo-ocean basin. The Mianliie paleo-ocean basin experienced its main expansion and formation process during the Carboniferous-Permian and closed totally in the Triassic. It belongs to the northern branch of the eastern paleotethys, separated from the northern margin of the Yangtze block under the paleotethys mantle dynamic system.

Petrology, geochemistry and geochronology of the Zhongcang ophiolite, northern Tibet: implications for the evolution of the Bangong-Nujiang Ocean

Ophiolites are widespread along the Bangong-Nujiang suture zone, northern Tibet. However, it is still debated on the formation ages and tectonic evolution process of these ophiolites. The Zhongcang ophiolite is a typical ophiolite in the western part of the Bangong-Nujiang suture zone. It is composed of serpentinized peridotite, cumulate and isotropic gabbros, massive and pillow basalts, basaltic volcanic breccia, and minor red chert. Zircon SHRIMP Ue Pb dating for the isotropic gabbro yielded weighted mean age of 163.4 ± 1.8 Ma. Positive zircon ε Hf(t) values(+15.0 to +20.2) and mantle-like σ^(18)O values(5.29 ±0.21)% indicate that the isotropic gabbros were derived from a long-term depleted mantle source. The isotropic gabbros have normal mid-ocean ridge basalt(N-MORB) like immobile element patterns with high Mg O, low TiO_2 and moderate rare earth element(REE) abundances, and negative Nb,Ti, Zr and Hf anomalies. Basalts show typical oceanic island basalt(OIB) geochemical features, and they are similar to those of OIB-type rocks of the Early Cretaceous Zhongcang oceanic plateau within the Bangong-Nujiang Ocean. Together with these data, we suggest that the Zhongcang ophiolite was probably formed by the subduction of the Bangong-Nujiang Ocean during the Middle Jurassic. The subduction of the Bangong-Nujiang Tethyan Ocean could begin in the Earlye Middle Jurassic and continue to the Early Cretaceous, and finally continental collision between the Lhasa and Qiangtang terranes at the west Bangong-Nujiang suture zone probably has taken place later than the Early Cretaceous(ca. 110 Ma).

Exsolutions of Diopside and Magnetite in Olivine from Mantle Dunite,Luobusa Ophiolite,Tibet,China

The exsolutious of diopside and magnetite occur as intergrowth and orient within olivine from the mantle dunite, Luobusa ophiolite, Tibet. The dunite is very fresh with a mineral assemblage of olivine （〉95%） ＋ chromite （1%-4%） ＋ diopside （〈1%）. Two types of olivine are found in thin sections： one （Fo = 94） is coarse-grained, elongated with development of kink bands, wavy extinction and irregular margins; and the other （Fo = 96） is fine-grained and poly-angied. Some of the olivine grains contain minor Ca, Cr and Ni. Besides the exsolutions in olivine, three micron-size inclusions are also discovered. Analyzed through energy dispersive system （EDS） with unitary analytical method, the average compositions of the inclusions are： Na20, 3.12%-3.84%; MgO, 19.51%-23.79%; Al2O3, 9.33%-11.31%; SiO2, 44.89%-46.29%; CaO, 11.46%-12.90%; Cr2O3, 0.74%-2.29%; FeO, 4.26%- 5.27%, which is quite similar to those of amphibole. Diopside is anhedral f＇dling between olivines, or as micro-inclusions oriented in olivines. Chromite appears euhedral distributed between olivines, sometimes with apparent compositional zone. From core to rim of the chromite, Fe content increases and Cr decreases; and A! and Mg drop greatly on the rim. There is always incomplete magnetite zone around the chromite. Compared with the nodular chromite in the same section, the euhedral chromite has higher Fe3O4 and lower MgCr2O4 and MgAI2O4 end member contents, which means it formed under higher oxygen fugacity environment. With a geothermometer estimation, the equilibrium crystalline temperature is 820℃-960℃ for olivine and nodular chromite, 630℃-770℃ for olivine and euhedral chromite, and 350℃-550℃ for olivine and exsoluted magnetite, showing that the exsolutions occurred late at low temperature. Thus we propose that previously depleted mantle harzburgite reacted with the melt containing Na, Al and Ca, and produced an olivine solid solution added with Na^＋, Al^3＋, Ca^2＋, Fe^3＋, Cr^3＋. With temperature decreasing, the olivine solid solution decomposed; and Fe^3＋, Cr^3＋ diffused into magnetite and Ca^2＋ and Na^＋ into clinopyroxene, both of which formed intergrowth textures. A few Fe^3＋ and Cr^3＋ entered interstitial chromite. Through later tectonism, the peridotite recrystallized and formed deformational coarse grained olivine, fine grained and poly-angled olivine, and euhedral grained chromite. Due to the fast cooling rate of the rock or rapid tectonic emplacement, the exsolution textures in olivine and compositional zones of chromite are preserved.

Origin of the Zedang and Luobusa Ophiolites, Tibet

The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt,and they share similar geological tectonic setting and age.Thus,an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean.There is no complete ophiolite assemblage in the Zedang ophiolite.The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks,with some blocks of olivinepyroxenite.The mantle peridotite mainly consists of Cpx-harzburgite,harzburgite,some lherzolite,and some dunite.A suite of volcanic rocks is mainly composed of caic-aikaline pyroclastic rocks and secondly of tholeiitic pillow lavas,basaltic andesites,and some boninitic rocks with a lower TiO2 content （TiO2 ＜ 0.6％）.The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched （compared to HFSE） spider diagram,demonstrating an island-arc origin.The tholeiitic volcanic rock has a LREE-depleted REE pattern and a LILE-depleted （compared to HFSE） spider diagram,indicative of an origin from MORB.The boninitic rock was generated from fore-arc extension.The Luobusa ophiolite consists of mantle peridotite and mafic-ultramaflc cumulate units,without dike swarms and volcanic rocks.The mantle peridotite mainly consists of dunite,harzburgite with low-Opx （Opx ＜ 25％）,and harzburgite （Opx ＞ 25％）,which can be divided into two facies belts.The upper is a dunite-harzburgite （Opx ＜ 25％） belt,containing many dunite lenses and a large-scale chromite deposit with high Cr203; the lower is a harzburgite （Opx ＞25％） belt with small amounts of dunite and lherzolite.The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower.Many mantle peridotites are highly depleted,with a characteristic U-shaped REE pattern peculiar to fore-arc peridotite.The Luobusa cumulates are composed of wehrlite and olivine-pyroxenite,of the P-P-G ophiolite series.This study indicates that the Luobusa ophiolite was formed in a fore-arc basin environment on the basis of the occurrence of highly depleted mantle peridotite,a high-Cr2O3 chromite deposit,and cumulates of the P-P-G ophiolite series.We conclude that the evolution of the Eastern Tethys Ocean involved three stages：the initial ocean stage （formation of MORB volcanic rock and dikes）,the forearc extension stage （formation of high-Cr203 chromite deposits and P-P-G cumulates）,and the islandarc stage （formation of caic-alkaline pyroclastic rocks）.

Discovery and Significance of Diamonds and Moissanites in Chromitite within the Skenderbeu Massif of the Mirdita Zone Ophiolite,West Albania

In recent years diamonds and other unusual minerals （carbides, nitrides, metal alloys and native elements） have been recovered from mantle peridotites and chromitites （both high-Cr chromitites and high-Al chromitites） from a number of ophiolites of different ages and tectonic settings. Here we report a similar assemblage of minerals from the Skenderbeu massif of the Mirdita zone ophiolite, west Albania. So far, more than 20 grains of microdiamonds and 30 grains of moissanites （SIC） have been separated from the podiform chromitite. The diamonds are mostly light yellow, transparent, euhedral crystals, 200-300μm across, with a range of morphologies; some are octahedral and cuboctahedron and others are elongate and irregular. Secondary electron images show that some grains have well-developed striations. All the diamond grains have been analyzed and yielded typical Raman spectra with a shift at -1325 cm^-1. The moissanite grains recovered from the Skenderben chromitites are mainly light blue to dark blue, but some are yellow to light yellow. All the analyzed grains have typical Raman spectra with shifts at 766 cm^-1, 787 cm^-1, and 967 cm^-1. The energy spectrums of the moissanites confirm that the grains are composed entirely of silicon and carbon. This investigation expands the occurrence of diamonds and moissanites to Mesozoic ophiolites in the Neo-Tethys. Our new findings suggest that diamonds and moissanites are present, and probably ubiquitous in the oceanic mantle and can provide new perspectives and avenues for research on the origin of ophiolites and podiform chromitites.

Study on the Tectonic Setting for the Ophiolites in Xigaze, Tibet

The Xigaze ophiolite is located in the middle section of the Yarlung Zangbo River ophiolite belt and includes a well-preserved sequence section of seven ophiolite blocks. The relatively complete ophiolitic sequence sections are represented by Jiding, Dejixiang, Baigang, and Dazhuqu ophiolites and consist of three-four units. The complete ophiolite sequence in order from the bottom to top consists of mantle peridotite, cumulates, sheeted sill dike swarms, and basic lavas±radiolarian chert. These cumulates are absent in the remaining blocks of Dejixiang and Luqu. The age of radiolaria in the radiolarian chert is Late Jurassic-Cretaceous. The basalt and ultramafic rock of the ophiolite also are overlaid by Tertiary Liuqu conglomerate, which contains numerous pebble components of ophiolite, indicating that the Tethys Ocean began to close at the end of Cretaceous Period. The isotopic data of gabbro, diabase, and albite granite in the Xigaze ophiolite are approximately 126-139 Ma, which indicates that the ophiolite formed in the Early Cretaceous. The K-Ar age of amphibole in garnet amphibolite in the ophiolite melange is 81 Ma, indicating that tectonic ophiolite emplacement occurred at the end of Late Cretaceous. Research in petrology, petrological chemistry, mineralogy, and geochemistry of volcanic rocks and dikes of the Xigaze ophiolite indicate the following characteristics： （1） They are mainly composed of basalt, basaltic andesite, dolerite, and diabase and are characterized by high TiO2 （0.7-1.47%）, low MgO （mostly less than 8%）, and low SiO2 （mostly less than 53%）. （2） The volcanic rocks and dikes of the Xigaze ophiolite show light rare earth element （LREE）-depleted rare earth element （REE） patterns. （3） The spider diagrams of the volcanic rocks and dikes of the Xigaze ophiolite exhibit LILE depletion relative to high-field-strength element （HFSE） patterns with left oblique features. （4） No protogenetic olivine and clinoenstatite was detected. （5） Some dikes show low TiO2 and high MgO, in which a few of Cr-enriched spinels and a very few pseudomorphs of olivine, orthopyroxene can be seen. They show more distinctive affinity as boninitic rock and canbe classified to boninite series rock. The previously mentioned features of the volcanic rocks and dikes in the Xigaze ophiolite implies that these ophiolites formed in a mid-ocean ridge （MOR） in the earlier stage and than forearc extension of subduction initiation occurred once at the later stage of the evolution of the Xigaze ophiolite. The forearc extention caused further melting of the residue-depleted mantle, resulting in the formation of melts with lower TiO2 and higher MgO. These melts formed as dikes and intruded into the oceanic crust formed in the earlier stage, resulting in a close association of mid-ocean ridge basalt and the boninite rock of the Xigaze ophiolite.

Diamonds Discovered from High–Cr Podiform Chromitites of Bulqiza,Eastern Mirdita Ophiolite,Albania

Various combinations of diamond, moissanite, zircon, corundum, rutile and titanitehave been recovered from the Bulqiza chromitites. More than 10 grains of diamond have been recovered, most of which are pale yellow to reddish–orange to colorless. The grains are all 100–300 μm in size and mostly anhedral, but with a range of morphologies including elongated, octahedral and subhedral varieties. Their identification was confirmed by a characteristic shift in the Raman spectra between 1325 cm-1 and 1333 cm-1, mostly at 1331.51 cm-1 or 1326.96 cm-1. This investigation extends the occurrence of diamond and moissanite to the Bulqiza chromitites in the Eastern Mirdita Ophiolite. Integration of the mineralogical, petrological and geochemical data of the Bulqiza chromitites suggests their multi–stage formation. Magnesiochromite grains and perhaps small bodies of chromitite formed at various depths in the upper mantle, and encapsulated the ultra–high pressure, highly reduced and crustal minerals. Some oceanic crustal slabs containing the magnesiochromite and their inclusion were later trapped in suprasubduction zones, where they were modified by tholeiitic and boninitic arc magmas, thus changing the magnesiochromite compositions and depositing chromitite ores in melt channels.

Recognition of Ophiolite Belt and Granulite in Northern Area of Mian－Lue， Southern Qinling， China and Their Implication

A ophiolite belt associated with the tectonic melange is recoghzed in northern area of Mian-Lue, southern Qinling. The ophiolite is considered to originate in a island arc. The occurrence of the ophiolite indicates that a paleo-ocean or finite oceanic basin existed POSt-Prot6rozoic in southern Qiuling, implying the difference of the continental basement of Southern Qinling from the Yangtze craton. The ophiolitc and themelange may mark the paleo-suture zoic between the two plates. The basic granulite is found in eastern area (Anzishan) of the ophiolite belt. The p-T path for metamorphism of the granulite demonstrates a process of continental collision.

Discovery of the Plagiogranites in the Diyanmiao Ophiolite,Southeastern Central Asian Orogenic Belt,Inner Mongolia,China and Its Tectonic Significance

In this study, plagiogranites in the Diyanmiao ophiolite of the southeastern Central Asian Orogenic Belt （Altaids） were investigated for the first time. The plagiogranites are composed predominantly of albite and quartz, and occur as irregular intrusive veins in pillow basalts. The plagiogranites have high SiO2 （74.37-76.68wt%） and low A1203 （11.99-13.30wt%）, and intensively high Na20 （4.52-5.49wt%） and low K20 （0.03-0.40wt%） resulting in high Na20/K20 ratios （11.3-183）. These rocks are classified as part of the low-K tholeiitic series. The plagiogranites have low total rare earth element contents （∑REE）（23.62-39.77ppm）, small negative Eu anomalies （JEu=0.44-0.62）, and flat to slightly LREE-depleted chondrite-normalized REE patterns （（La/Yb）N=0.68-0.76）, similar to N-MORB. The plagiogranites are also characterized by Th, U, Zr, and Hf enrichment, and Nb, P, and Ti depletion, have overall flat primitivemantle-normalized trace element patterns. Field and petrological observations and geochemical data suggest that the plagiogranites in the Diyanmiao ophiolite are similar to fractionation-type plagiogranites. Furthermore, the REE patterns of the plagiogranites are similar to those of the gabbros and pillow basalts in the ophiolite. In plots of SREE-SiO2, La-SiO2, and Yb-SiO2, the plagiogranites, pillow basalts, and gabbros show trends typical of crystal fractionation. As such, the plagiogranites are oceanic in origin, formed by crystal fractionation from basaltic magmas derived from depleted mantle, and are part of the Diyanmiao ophiolite. LA-ICP-MS U-Pb dating of zircons from the plagiogranites yielded ages of 328.6±2.1 and 327.1±2.1Ma, indicating an early Carboniferous age for the Diyanmiao ophiolite. These results provide petrological and geochronological evidence for the identification of the Erenhot-Hegenshan oceanic basin and Hegenshan suture of the Paleo-Asian Ocean.