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.

Genesis of Pyroxenite Veins in the Zedang Ophiolite,Southern Tibetan Plateau

Understanding the nature of parental melts for pyroxenite veins in supra-subduction zone(SSZ)ophiolites provides vibrant constraints on melt infiltration processes operating in subduction zones.The Zedang ophiolitic massif in the eastern Yarlung–Zangbo suture zone in Tibet consists of mantle peridotites and a crustal section of gabbro,diabase,and basalt.Veins of two pyroxenite varieties cut the southern part of the Zedang massif.These pyroxenite rocks have different geochemical characteristics,where the first variety(type-I)has relatively higher contents of SiO_(2)(51.82–53.08 wt%),MgO(20.08–23.23 wt%),andΣPGE(3.42–13.97 ppb),and lower Al_(2)O_(3)(1.59–2.28 wt%)andΣREE(1.63–2.94 ppm).The second pyroxenite variety(type-II)is characterized by SiO_(2)(45.44–49.61 wt%),Mg O(16.68–19.78 wt%),Al_(2)O_(3)(4.24–8.77 wt%),ΣPGE(14.46–322.06 ppb),andΣREE(5.82–7.44 ppm).Pyroxenite type-I shows N-MORB-like chondritenormalized REE patterns.Zircon U-Pb ages of pyroxenite type-I(194±10 Ma),associated ophiolitic gabbro(135.3±2.0 Ma),and plagiogranite(124.2±2.3 Ma)evidently imply episodic evolution of the Zedang ophiolites.The mineralogical and geochemical characteristics of the investigated pyroxenites can be explained by subduction-initiated hydrous melting of metasomatized sub-arc mantle,later overprinted by sub-slab mantle melting triggered by upwelling asthenosphere during the Jurassic–Early Cretaceous times.The geochemical variations in pyroxenite vein composition,coupled with age differences amongst the other ophiolite units,may correspond to intermittent emplacement of pyroxenite dikes and isotropic gabbroic intrusions where the geodynamic setting progressed from arc maturation and slab rollback to slab tearing and delamination.

Identification of clayey altered ophiolite in the Nujiang tectonic belt and new understanding of its impacts on engineering stability

1. Objectives Ophiolites from the oceanic crust are important indicators for identifying tectonic suture zones. Recently, a continuous ophiolite belt was found near the Guola Mountain in the Nujiang tectonic belt. Due to intensive hydrothermal alteration during tectonic evolution, clayey altered ophiolite with special engineering geological characteristics was formed, which has an extremely adverse impact on engineering stability. However, the adverse properties of clayey altered ophiolite are still not well understood in engineering practices(Zhang YS, et al., 2011).

Genesis and Geodynamic Significance of Chromitites from the Fuchuan Ophiolite,Southern China,as Evidenced by Trace Element Fingerprints of Chromite,Olivine and Pyroxene

The Fuchuan ophiolite is located in the northeasternmost segment of the Neoproterozoic Jiangnan orogen and consists mainly of harzburgites,with minor dunites,pyroxenite and gabbro veins and dykes.In order to investigate the genesis and tectonic setting of the Fuchuan ophiolite and chromitites,in situ analyses of unaltered chromites and silicates were carried out.Trace element analyses of unaltered chromites from the Fuchuan chromitites indicate the parental magma is of mid-ocean ridge basalt(MORB)-like origin,with the Ti/Fe^(3+#)–Ga/Fe^(3+#)diagram of chromites showing that the chromitites are a result of melt/rock interaction of MORB melts with mantle peridotites,and that the Fuchuan harzburgites present the dual features of MORB and supra-subduction zone peridotites(SSZP).Trace and rare earth element(REE)analyses of olivines and orthopyroxenes from the Fuchuan harzburgites hint at the possibility of mantle metasomatism influenced by SSZ-subducted fluids.Finally,integrating with previous study,the Fuchuan ophiolite and chromitites might have been formed in a back-arc spreading ridge between the Yangtze and Cathaysia blocks during the Neoproterozoic.

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）.

SSZ Semail Ophiolite vs MORB Masirah Ophiolite: A Perspective from Podiform Chromitites

One of the major topics of debate in ophiolite geology is the original tectonic setting of ophiolites. New studies show that most ophiolites are formed more frequently in a suprasubduction zone(SSZ) environment and that only a very small number of ophiolites have formed in an oceanic range(MOR). The Masirah ophiolite is one of the few oceanic ridge ophiolites that have been preserved, and the evidence that was formed in a subduction environment is missing(Moseley and Abbotts 1979, Dilek and Furnes, 2011;Rollinson, 2017). Masirah Island, the Batain and Ras Madrah areas of eastern Oman are almost entirely composed of a well-developed ophiolite, known as the Masirah ophiolite(Fig. 1), which is, however, completely unrelated to the nearby Semail Ophiolite in the northern Oman Mountains(Fig. 2). The Masirah ophiolite is Jurassic in age and represents oceanic lithosphere derived from the Indian Ocean, but is about 15–20 Myr later than emplacement of midCretaceous Semail ophiolite in northern Oman. The presence of basaltic to rhyolitic lavas of calc-alkaline affinity and boninites in the lava sequence of the Semail ophiolite led several researchers to propose a back-arc basin model for this ophiolite(e.g. Tamura and Arai, 2006;Godard et al., 2008;Rollinson and Adetunji, 2015). The Masirah Ophiolite shows close affinities with MORB peridotites in general. Most of the olivine from the Masirah harzburgites show Fo contents that are similar to those of olivine from MORB. Both pyroxenes in these harzburgites have similar Mg# values, Al2O3 and Cr2O3 contents to those of pyroxenes from MORB peridotites. The observed primitive mantlenormalized REE patterns showing enrichment in LREEs indicate that the Masirah peridotites have been modified by fluids or melts enriched in LREEs in a MORB environment. Podiform chromitites housed in ophiolites today interpreted as magmatic deposits formed during the reaction of molten rock in environments spike in the middle of the ocean(MOR) or suprasubduccion zone(SSZ)(Arai and Matsukage, 1998;Rollinson and Adetunji, 2015). The Masirah chromitites has a mineral chemistry similar to the mineral chemistry of chromite crystallized from MOR magmas. The Cr# values of chromite in the Masirah chromatite are similar to those of MOR peridotites. These findings suggest that the ultramafic and mafic cumulate rock assemblages overlying the upper mantle peridotites in the Masirah ophiolite represent the products of magma evolution in a MOR initiation stage within the proto Indian Ocean. Coexisting high-and low-Cr# associations of chromitite and dunite have been found in the Semail ophiolite, which illustrates the common situation of ophiolites having both SSZ and MOR geochemical signatures. Cr# varies from 40–60 for shallow chromite bodies, and over the range 70–80 for the deep locations. This diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis, in response to a switch of tectonic setting. The first is residual from lower degree, partial melting of peridotite, which produced lowCr# chromitites at the Moho transition zone, possibly in a midocean-ridge setting. The second chromitite-forming event involves higher degree partial melting, which produced high-Cr# discordant chromitite in the upper mantle, possibly in a suprasubduction zone setting. Assemblages of mono-and poly-phase silicate inclusions(including olivine, orthopyroxene, clinopyroxene, amphibole, phlogopite, serpentine, native Fe, FeO, alloy, sulfide, calcite, laurite, celestine and halite) within chromite have been observed in the low Cr# podiform chromitites from the Semail and Masirah ophiolites. The existence of hydrous silicate inclusions in the chromite calls for a role of hydration during chromite genesis. High-T bright green hornblende–edenite included in the chromites is evidence of the introduction of water in the magma at the end of the chromite crystallization. Such paragenesis points to the presence of hydrous fluids during the activity of the shear bands.

Comparison of Redox States between the Ultramafic Bodies of Xigaze and Luobusha Ophiolites, Tibet, China

The tectonic setting of podiform chromitite formation still remains highly debated. There is a close correlation between tectonic settings and oxygen fugacity(fO2)(e.g., Ballhaus, 1993;Dare et al., 2009;Parkinson and Arculus, 1999). Here we present results of fO2 of chromites determined by M?ssbauer spectroscopy from both the Luobusha and Dazhuqu areas along Yarlung Zangbo suture zone, Southern Tibet. The fO2 values(-1.02~0.04 log units against the FMQ buffer) and Cr#(22~54%) in chromites from lherzolites and harzburgites of both areas are similar to those of abyssal peridotites, indicating that they may be residues after partial melting at spreading centers. However, both dunite envelopes and chromitites from Luobusha have high fO2 values(0.04~2.25 log units) and Cr#(73~84%), showing an affinity to boninitic melts, and thus form in a suprasubduction zone. Dazhuqu dunites show diverse fO2 values(-0.22~2.19 log units) and Cr#(22~82%), indicating that they form in distinct settings. Chromitites and chromite dunites from Dazhuqu have low fO2 values(-0.3~0.71 log units) and Cr#(16~63%), both of which are similar to those of MORB-like basalts, inferring that they form in an extensional setting. Both high-Cr and high-Al chromitites from other typical podiform chromite ore deposits, such as Kempirsai, Oman, and Albania ophiolites, also show high fO2 values(e.g., Chashchukhin and Votyakov, 2009;Melcher et al., 1997;Quintiliani et al., 2006;Rollinson and Adetunji, 2015), while the distribution-limited small chromitites and chromite dunites from Dazhuqu exhibit low fO2 values. The phenomenon infers that the suprasubduction zone is more beneficial to the formation of podiform chromitites.

Neotethyan Ophiolites and Their Geodynamic Evolution During the Mesozoic: A Global Overview

Neotethyan ophiolites evolved in multiple seaways separated by Gondwana–derived ribbon continents within an eastward widening, latitudinal oceanic realm(Neotethys) throughout the Mesozoic. Opening and closure of these seaways were diachronous events, resulting in E–W variations in the timing of oceanic crust production and ophiolite emplacement. The Neotethyan ophiolites are highly diverse in their crustal–mantle structures and compositions, isotopic fingerprints, and sedimentary cover types, pointing to major differences in their mantle melt sources and tectonic and paleogeographic settings of magmatic construction(Dilek and Furnes, 2019). The Jurassic Western Alpine and Ligurian ophiolites in Europe and their counterparts in southern and northern Iberia formed in a narrow basin(Western Tethys) that developed between Europe and North Africa–Adria–Iberia. Their peridotites represent exhumed, continental lithospheric mantle, and the ophiolites display a Hess–type oceanic crustal architecture with MORB geochemical signatures(Dilek and Furnes, 2011). All these ophiolites were incorporated into continental margins from the downgoing oceanic lithosphere of the Western Tethys. Triassic, Jurassic and Cretaceous ophiolites east of Adria formed in different Neotethyan seaways(Dilek et al., 1990), and their rift–drift, seafloor spreading and suprasubduction zone(SSZ) magmatic construction involved multiple episodes of melting, depletion and refertilization of previously or actively subduction metasomatized mantle sources. Deep mantle recycling processes through subduction zone tectonics and/or plume activities played a major role in their melt evolution, and in the incorporation of mantle transition zone(MTZ) materials into their peridotites(Fig. 1;Dilek and Yang, 2018;Xiong et al., 2019). Tectonic mélanges structurally beneath these ophiolites include Permo–Triassic, OIB–type extrusive rocks, indicating that the initial dismantling of the Pangea supercontinent that led to the opening of the Triassic and Jurassic ocean basins within the Neotethyan realm was associated with plume magmatism(Dilek, 2003 a;Yang and Dilek, 2015). This plume signature is absent in the Permo–Triassic magmatic record of the Western Tethys to the west. The Cretaceous ophiolites around the Arabia(Dilek et al., 1990;Dilek and Delaloye, 1992;Dilek and Eddy,1992) and India sub-continents(Fareeduddin and Dilek, 2015) occur discontinuously along a ~9000-km-long belt from SW Anatolia to SE Tibet and Indo-China. The majority of these ophiolites have a Penrose–type oceanic crustal architecture(Dilek, 2003 b) and display SSZ geochemical affinities, complete with a MORB–IAT–BON progression of their chemo-stratigraphy(Fig. 1;Dilek and Thy, 1998;Dilek et al., 1999;Dilek and Furnes, 2014;Saccani et al., 2018). They evolved above a N–dipping, Trans–Tethyan subduction–accretion system that was situated in sub-tropical latitudes within the Neotethyan realm. The Trans–Tethyan subduction–accretion system was segmented into two major domains(Western and Eastern domains) by the NNE–SSW–oriented, sinistral Chaman–Omach–Nal transform fault plate boundary. This Cretaceous intraoceanic arc–trench system was analogous to the modern Izu–Bonin–Mariana(IBM) and Tonga arc–trench systems in the western Pacific in terms of its size. Diachronous collisions of the Arabia and India sub-continents with this segmented Trans-Tethyan arc–trench system resulted in the southward emplacement of the SSZ Neotethyan ophiolites onto their passive margins in the latest Mesozoic(Dilek and Furnes, 2019). A separate N–dipping subduction system, dipping beneath Eurasia to the north during much of the Jurassic and Cretaceous, was consuming the Neotethyan oceanic lithosphere and was responsible for the construction of a composite magmatic arc belt extending discontinuously from Southern Tibet to Northern Iran. Slab rollback along this northern subduction system produced locally developed forearc–backarc oceanic lithosphere that was subsequently collapsed into the southern margin of Eurasia. The existence of these two contemporaneous, Ndipping subduction systems within Neotethys led to its rapid contraction and the fast convergence of India towards Eurasia during the late Mesozoic–early Cenozoic(Dilek and Furnes, 2019). It was the collision with Eurasia of the India sub-continent with the accreted ophiolites around its periphery in the Late Paleogene that produced the Himalayan orogeny.

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.

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.

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.

The Discovery of Diamonds in Chromitites of the Hegenshan Ophiolite,Inner Mongolia,China

Diamond, moissanite and a variety of other minerals, similar to those reported from ophiolites in Tibet and northern Russia, have recently been discovered in chromitites of the Hegenshan ophiolite of the Central Asian Orogenic Belt, north China. The chromitites are small, podiform and vein-like bodies hosted in dunite, clinopyroxene-bearing peridotite, troctolite and gabbro. All of the analysed chromite grains are relatively Al-rich, with Cr^# [100Cr/（Cr＋Al）] of about 47-53. Preliminary studies of mainly disseminated chromitite from ore body No. 3756 have identified more than 30 mineral species in addition to diamond and moissanite. These include oxides （mostly hematite, magnetite, ruffle, anatase, cassiterite, and quartz）, sulfides （pyrite, marcasite and others）, silicates （magnesian olivine, enstatite, augite, diopside, uvarovite, pyrope, orthoclase, zircon, sphene, vesuvianite, chlorite and serpentine） and others （e.g., calcite, monazite, glauberite, iowaite and a range of metallic alloys）. This study demonstrates that diamond, moissanite and other exotic minerals can occur in high-Al, as well as high-Cr chromites, and significantly extends the geographic and age range of known diamond-bearing ophiolites.

Geochronology and petrogenesis of the mafic dykes from the Purang ophiolite:Implications for evolution of the western Yarlung-Tsangpo suture zone,southwestern Tibet

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 of144.2±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 midoceanic 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.

In-situ Moissanite in Dunite: Deep Mantle Origin of Mantle Peridotite in Luobusa Ophiolite, Tibet

We report the discovery of an in-situ natural moissanite as an inclusion in the Cr-spinel from the dunite envelope of a chromitite deposit in Luobusa ophiolite,Tibet.The moissanite occurs as a twin crystal interpenetrated by two quadrilateral signal crystals with sizes of 17 pm × 10 μm and 20 μm × 7 μm,respectively.The moissanite is green with parallel extinction.The absorption peaks in its Raman spectra are at 967-971 cm-1,787-788 cm-1,and 766 cm-1.The absorption peaks in the infrared spectra are at 696 cm-1,767 cm-1,1450 cm-1,and 1551 cm-1,which are distinctly different from the peaks for synthetic silicon carbide.Moissanites have been documented to form in ultra-high pressure,high temperature,and extremely low fO2 environments and their 13C-depleted compositions indicate a lower mantle origin.Combined with previous studies about other ultra-high pressure and highly reduced minerals in Luobusa ophiolite,the in-situ natural moissanite we found indicates a deep mantle origin of some materials in the mantle sequence of Luobusa ophiolite.Further,we proposed a transformation model to explain the transfer process of UHP materials from the deep mantle to ophiolite sequence and then to the supra-subduction zone environment.Interactions between the crown of the mantle plume and mid-ocean ridge are suggested to be the dominant mechanism.

Geochronological constraints on the metamorphic sole of the Semail ophiolite in the United Arab Emirates

The Semail ophiolite of Oman and the United Arab Emirates （UAE） provides the best preserved large slice of oceanic lithosphere exposed on the continental crust, and offers unique opportunities to study processes of ocean crust formation, subduction initiation and obduction. Metamorphic rocks exposed in the eastern UAE have traditionally been interpreted as a metamorphic sole to the Semail ophiolite. However, there has been some debate over the possibility that the exposures contain components of older Arabian continental crust. To help answer this question, presented here are new zircon and futile U-Pb geochronological data from various units of the metamorphic rocks. Zircon was absent in most samples. Those that yielded zircon and futile provide dominant single age populations that are 95-93 Ma, partially overlapping with the known age of oceanic crust formation （96.5-94.5 Ma）, and partially overlapping with cooling ages of the metamorphic rocks （95 90 Ma）. The data are interpreted as dating high-grade metamorphism during subduction burial of the sediments into hot mantle lithosphere, and rapid cooling during their subsequent exhumation. A few discordant zircon ages, interpreted as late Neoproterozoic and younger, represent minor detrital input from the continent. No evidence is found in favour of the existence of older Arabian continental crust within the metamorphic rocks of the UAE.

Petrogenesis of Volcanic Rocks in the Khabr-Marvast Tectonized Ophiolite:Evidence for Subduction Processes in the South-Western Margin of Central Iranian Microcontinent

The Late Cretaceous Khabr-Marvast tectonized ophiolite is located in the middle part of the Nain-Baft ophiolite belt, at the south-western edge of the central Iranian microcontinent. Although all the volcanic rocks in the study area indicate subduction-related magmatism （e.g. high LILE （large ion lithophile elements） / HFSE （high field strenght elements） ratios and negative anomalies in Nb and Ta）, geological and geochemical data clearly distinguish two distinct groups of volcanic rocks in the tectonized association： （1） group 1 is comprised of hyaloclastic breccias, basaltic pillow lavas, and andesite sheet flows. These rocks represent the Nain-Baft oceanic crust; and （2） group 2 is alkaline lavas from the top section of the ophiolite suite. These lavas show shoshonite affinity, but do not support the propensity of ophiolite.

Petrology and PGE Abundances of High-Cr and High-Al Podiform Chromitites and Peridotites from the Bulqiza Ultramafic Massif, Eastern Mirdita Ophiolite, Albania

The Bulqiza ultramafic massif, which is part of the eastern Mirdita ophiolite of northern Albania, is world renowned for its high-Cr chromitite deposits. High-Cr chromitites hosted in the mantle section are the crystallized products of boninitic melts in a supra-subduction zone(SSZ). However,economically important high-Al chromitites are also present in massive dunite of the mantle-crust transition zone(MTZ). Chromian-spinel in the high-Al chromitites and dunites of the MTZ have much lower Cr~# values(100 Cr/(Cr+Al))(47.7-55.1 and 46.5-51.7, respectively) than those in the high-Cr chromitites(78.2-80.4), harzburgites(72.6-77.9) and mantle dunites(79.4-84.3). The chemical differences in these two types of chromitites are reflected in the behaviors of their platinum-group elements(PGE).The high-Cr chromitites are rich in IPGE relative to PPGE with 0.10-0.45 PPGE/IPGE ratios, whereas the high-Al chromitites have relatively higher PPGE/IPGE ratios between 1.20 and 7.80. The calculated melts in equilibrium with the high-Cr chromitites are boninitic-like, and those associated with the high-Al chromitites are MORB-like but with hydrous, oxidized and TiO-poor features. We propose that the coexistence of both types of chromitites in the Bulqiza ultramafic massif may indicates a change in magma composition from MORB-like to boninitic-like in a proto-forearc setting during subduction initiation.

On the Geotectonics of Southern China

The tectonic nature of southern China has changed again and again in the Phanerozoic. In the Caledoniancycle, there existed three tectonic units——the Yangtze paraplatform, Indosinian-South China Sea paraplatformand Caledonian South China fold belt, of which the last unit is not a collisional orogenic belt but ascissor-shaped aulacogen-type geosyncline opening towards Yunnan and Vietnam. In the Indosinian cycle,South China belonged to the Tethyan tectonic domain, and no abyssal oceanic basin existed there. Since theLate Triassic, especially in the Yanshanian orogenic stage, it became a component part of the peri-Pacificcontinental-margin activation belt of eastern Asia. No Alpinc-type orogenic belt occurs in the interior of thecontinent of southern China.

An Early Aged Ophiolite in the Western Kunlun Mts., NW Tibetan Plateau and Its Tectonic Implications

The early aged ophiolites have attracted attention of many geologists in recent decades, because the early aged ophiolites can provide the information about the ancient oceanic processes relevant to the evolution of plate tectonics in the early period of the earth, and also concern such problems as whether there existed a ＂Proto-Tethys＂ and the break-up and convergence of the Rodinian Supercontinent. This paper reveals a definite complete ophiolite of Neoproterozoic-Early Paleozoic, named Kuda ophiolite in the western Kunlun Mrs., NW Tibetan Plateau, and reports the recent reasonable SHRIMP zircon U-Pb ages of 510±4 Ma, and 502±13 Ma for the cumulates of the Kuda ophiolite, using the most powerful dating tool, the SHRIMP-Ⅱ. The geochemical and geochronology data integrating with the geological setting suggest that the Kuda ophiolite might have formed in an archipelago oceanic basin, not in a vast ocean, the so-called ＂Proto-Tethys＂, and was tectonically emplaced during the Early Paleozoic.