Early Triassic Volcanic Rocks in the Western Yarlung Zangbo Suture Zone:Implications for the Opening of the Neo-Tethys Ocean

In this study,zircon U-Pb dating of volcanic rocks from the Zhongba ophiolite of the Yarlung Zangbo Suture Zone(YZSZ)in southern Xizang(Tibet)yielded an age of 247±3 Ma.According to whole rock geochemical and Sr-NdPb isotopic data,the Early Triassic samples could be divided into two groups:Group 1 with P-MORB affinity,showing initial^(87)Sr/^(86)Sr ratios of 0.70253–0.70602,ε_(Nd)(t)values of 4.2–5.3,(^(206)Pb/^(204)Pb)_(t)ratios of 16.353–18.222,(^(207)Pb/^(204)Pb)_(t)ratios of 15.454–15.564,and(^(208)Pb/^(204)Pb)_(t)ratios of 35.665–38.136;Group 2 with OIB affinity,showing initial^(87)Sr/^(86)Sr ratios of 0.70249–0.70513,ε_(Nd)(t)values of 4.4–4.9,(^(206)Pb/^(204)Pb)_(t)ratios of 17.140–18.328,(^(207)Pb/^(204)Pb)_(t)ratios of 15.491–15.575,and(^(208)Pb/^(204)Pb)_(t)ratios of 36.051–38.247.Group 2 rocks formed by partial melting of the mantle source enriched by a former plume,and assimilated continental crustal material during melt ascension.The formation of Group 1 rocks corresponds to the mixing of OIB melts,with the same components as Group 2 and N-MORBs.The Zhongba Early Triassic rocks belong to the continental margin type ophiolite and formed in the continental–oceanic transition zone during the initial opening of the Neo-Tethys in southern Xizang(Tibet).

Summer Atmospheric Water Cycle under the Transition Influence of the Westerly and Summer Monsoon over the Yarlung Zangbo River Basin in the Southern Tibetan Plateau

This study compares the summer atmospheric water cycle,including moisture sources and consumption,in the upstream,midstream,and downstream regions of the Yarlung Zangbo River Basin in the southern Tibetan Plateau.The evolutions of moisture properties under the influence of the westerly and summer southerly monsoon are examined using 5-yr multi-source measurements and ERA5 reanalysis data.Note that moisture consumption in this study is associated with clouds,precipitation,and diabatic heating.Compared to the midstream and downstream regions,the upstream region has less moisture,clouds,and precipitation,where the moisture is brought by the westerly.In early August,the vertical wet advection over this region becomes enhanced and generates more high clouds and precipitation.The midstream region has moisture carried by the westerly in June and by the southerly monsoon from July to August.The higher vertical wet advection maximum here forms more high clouds,with a precipitation peak in early July.The downstream region is mainly affected by the southerly-driven wet advection.The rich moisture and strong vertical wet advection here produce the most clouds and precipitation among the three regions,with a precipitation peak in late June.The height of the maximum moisture condensation is different between the midstream region(325 hPa)and the other two regions(375 hPa),due to the higher upward motion maximum in the midstream region.The diabatic heating structures show that stratiform clouds dominate the upstream region,stratiform clouds and deep convection co-exist in the midstream region,and deep convection systems characterize the downstream region.

Wind regime features and their impacts on the middle reaches of the Yarlung Zangbo River on the Tibetan Plateau, China

The wide valley of the Yarlung Zangbo River is one of the most intense areas in terms of aeolian activity on the Tibetan Plateau,China.In the past,the evaluation of the intensity of aeolian activity in the Quxu–Sangri section of the Yarlung Zangbo River Valley was mainly based on data from the old meteorological stations,especially in non-sandy areas.In 2020,six new meteorological stations,which are closest to the new meteorological stations,were built in the wind erosion source regions(i.e.,sandy areas)in the Quxu–Sangri section.In this study,based on mathematical statistics and empirical orthogonal function(EOF)decomposition analysis,we compared the difference of the wind regime between new meteorological stations and old meteorological stations from December 2020 to November 2021,and discussed the reasons for the discrepancy.The results showed that sandy and non-sandy areas differed significantly regarding the mean velocity(8.3(±0.3)versus 7.7(±0.3)m/s,respectively),frequency(12.9%(±6.2%)versus 2.9%(±1.9%),respectively),and dominant direction(nearly east or west versus nearly north or south,respectively)of sand-driving winds,drift potential(168.1(±77.3)versus 24.0(±17.9)VU(where VU is the vector unit),respectively),resultant drift potential(92.3(±78.5)versus 8.7(±9.2)VU,respectively),and resultant drift direction(nearly westward or eastward versus nearly southward or northward,respectively).This indicated an obvious spatial variation in the wind regime between sandy and non-sandy areas and suggested that there exist problems when using wind velocity data from non-sandy areas to evaluate the wind regime in sandy areas.The wind regime between sandy and non-sandy areas differed due to the differences in topography,heat flows,and their coupling with underlying surface,thereby affecting the local atmospheric circulation.Affected by large-scale circulations(westerly jet and Indian monsoon systems),both sandy and non-sandy areas showed similar seasonal variations in their respective wind regime.These findings provide a credible reference for re-understanding the wind regime and scientific wind-sand control in the middle reaches of the Yarlung Zangbo River Valley.

Compound extreme events in Yarlung Zangbo River Basin from 1977 to 2018

Extreme climate events threaten human health,economic development,and ecosystems.Many studies have been conducted on extreme precipitation and temperature changes in the Yarlung Zangbo River Basin(YZRB).However,little attention has been paid to compound climate extremes.In this study,the variations of wet/warm compound extreme events in summer and dry/cold compound extreme events in winter over the past 42 years in the YZRB were investigated using eight extreme climate indices that were estimated using monthly temperature and precipitation observations.The results showed that the numbers of frost days and ice days tended to decrease on the spatiotemporal scale,while the maximum values of daily maximum temperature and daily minimum temperature exhibited increasing trends.The frequency of wet/warm compound extreme events was significantly higher from 1998 to 2018 than from 1977 to 1997.Dry/cold compound extreme events became less frequent from 1998 to 2018 than from 1977 to 1997.The rate of increase of wet/warm compound extreme events was about ten times the absolute rate of decrease of dry/cold compound extreme events.With regard to the spatial pattern,the frequency of wet/warm compound extreme events increased significantly in almost all parts of the YZRB,while that of dry/cold compound extreme events decreased across the basin.This study helps to improve our understanding of the changes in compound precipitation and temperature extremes in the YZRB from a multivariable perspective.

Middle Jurassic-early Cretaceous radiolarian assemblages of the western Yarlung Zangbo Suture Zone: Implications for the evolution of the Neo-Tethys

Cherts in the Zhongba melange of the western Yarlung Zangbo Suture Zone(YZSZ) contain well preserved radiolarian assemblages. These radiolarian assemblages indicate that the Zhongba melange has middle Jurassic-early Cretaceous remnant, are coeval with those from the central and eastern parts of the YZSZ. Cherts from the Najiu area yield Aalenian to Aptian radiolarians, while cherts interbedded with siliceous mudstones from the Bielongjiala area yield Aptian radiolarians, indicating that terrigenousderived sediments were deposited during early Aptian. The above observations indicate that the entire YZSZ have a similar geochronological framework and thus they underwent similar geological evolution:(1) during the Jurassic, the Neo-Tethys was a wide ocean with pelagic sediments distal from continents;(2) during the Cretaceous(around 130-120 Ma), the Neo-Tethys started to subduct along the southern margin of the Lhasa block, and terrigenous-derived siliceous mudstone began deposition.

Climate change over the Yarlung Zangbo River Basin during 1961-2005

The Yarlung Zangbo River （YR） is the highest great river in the world, and its basin is one of the centers of human economic activity in Tibet. Using 10 meteorological stations over the YR basin in 1961-2005, the spatial and temporal characteristics of temperature and precipitation as well as potential evapotranspiration are analyzed. The results are as follows. （1） The annual and four seasonal mean air temperature shows statistically significant increasing trend, the tendency is more significant in winter and fall. The warming in Lhasa river basin is most significant. （2） The precipitation is decreasing from the 1960s to the 1980s and increasing since the 1980s. From 1961 to 2005, the annual and four seasonal mean precipitation is increasing but not statistically significant, especially in fall and spring. The increasing precipitation rates are more pronounced in Niyangqu and Palong Zangbo river basins, the closer to the upper YR is, the less precipitation increasing rate would be. （3） The annual and four seasonal mean potential evapotranspiration has decreased, especially after the 1980s, and most of it happens in winter and spring. The decreasing trend is most significant in the middle YR and Nianchu river basin. （4） Compared with the Mt. Qomolangma region, Tibetan Plateau, China and global average, the magnitudes of warming trend over the YR basin since the 1970s exceed those areas in the same period, and compared with the Tibetan Plateau, the magnitudes of precipitation increasing and potential evapotranspiration decreasing are larger, suggesting that the YR basin is one of the most sensitive areas to global warming.

THE CAUSES AND DEVELOPMENTAL TREND OF DESERTIFICATION IN THE MIDDLE REACHES OF THE YARLUNG ZANGBO RIVER AND ITS TWO TRIBUTARIES IN XIZANG

The middle reaches of the Yarlung Zangbo River and its two tributaries are an important development and construction region in Xizang(Tibet) in recent years, but the fast development of desertification has caused immense damages to its social and economic development, so the study on desertification in this region is significant. The paper studies the causes and development trend of desertification in this region through analysis of natural and human factors as well as their relationship. It can be concluded that desertification in the study area is one process which is accelerated and intensified by human activities on the basis of the slow natural desertification process. It is resulted from the dislocation of irrational and intensive human activities, fragile eec-environment and meagre natural resources, and tends to become more wide and severe.

Temporal and spatial variations of δ^18O in precipitation of the Yarlung Zangbo River Basin

This paper reveals the temporal and spatial variations of stable isotope in precipitation of the Yarlung Zangbo River Basin based on the variations of δ^18O in precipitation at four stations （Lhaze, Nugesha, Yangcun and Nuxia） in 2005. The results show that δ^18O of precipitation has distinct seasonal changes in the Yarlung Zangbo River Basin. The higher value of δ^18O occurs in spring prior to monsoon precipitation, and the lower value occurs during monsoon precipitation. From the spatial variations, with the altitude-effect and rainout process during moisture transport along the Yarlung Zangbo River Valley, δ^18O of precipitation is gradually depleted. Thus, δ^18O of precipitation decreases gradually from the downstream to the upstream, and the lapse rate of δ^18O in precipitation is approximately 0.34‰/100m and 0.7%J‰/100km for the two reasons. During monsoon precipitation, spatial variation of δ^18O in precipitation is dominated by the amount effect in the large scale synoptic condition.

PRESENT STATUS AND CAUSE OF LAND DESERTIFICATION IN THE YARLUNG ZANGBO RIVER BASIN

The Yarlung Zangbo River basin is a spacial zone in the south of Xizang, the types, distribution and causes of desertified lands have special features. The type, area, distribution and damage of desertification land in the Yarlung Zangbo River basin are firstly analysed in detail by using latest investigation information. According to the classification criteria of desertified land types and grades, the desertified land in the Yarlung Zangbo River basin can be divided into three grades and five types: fixed sand (dune) land, semi bare sand and gravel land, semi fixed sand (dune) land, base sand and gravel land, and shifting sand (dune) land. The desertified lands in the basin are mainly distributed in the wide valley floor and tributary junction area. The main factors affecting desertification are fragile eco environment, climate warming and drying and over exploitation of land resources. The man made factors leading to desertification in the Yarlung Zangbo River basin are over cutting and over grazing, dominated by over grazing.

Grain-size Characteristics of Sediments Formed Since 8600 yr B.P. in Middle Reaches of Yarlung Zangbo River in Tibet and Their Paleoenvironmental Significance

Widespread aeolian sediments have been found in the middle reaches of the Yarlung Zangbo River, China, The grain-size characteristics of sediments from Cha＇er Section in the area were analyzed. The results show that the section include one stratum of paleo-mobile dunes, four strata of paleo-semi-fixed dunes, two strata of paleo-fixed dunes, one stratum of sandy immature soils. The paleo-mobile and paleo-semi-fixed dune sand in this section are similar to modem aeolian sand in either grain-size composition or Mz and c distribution. Compared the above types of dunes each other, the content of sand substance decreases, while the content of silt and clay increases for paleo-fixed dunes and sandy immature soils. Combined with age data for each stratum, the analysis shows that these strata are the products of climate changes and the evolution of aeolian landforms. The evolutionary sequence of the paleoclimate and of aeolian activities in the valley since 8600 yr B.P. reveals four stages： 8600-5700 yr B.P., when the paleoclimate was cold and dry, with strong winds, thereby activating dunes; 5700-3600 yr B.P., when it was warm and wet, with weak winds, causing dunes to undergo soil-forming processes; 3600-1900 yr B.P., when climate shifted from cold-dry with strong winds to warm-wet with weak winds, and activated dunes were fixed again; and 1900 yr B.P. -present, when the climate became fine, with weak winds, fixing dunes again.

CHRONOSTRATIGRAPHY,SEDIMENTATION AND EVOLUTION OF THE XIGAZE FOREARC BASIN: IMPLICATIONS FOR DYNAMIC EVOLUTION OF THE YARLUNG ZANGBO SUTURE ZONE

The Yarlung Zangbo Suture Zone (YZSZ), a major lineament in Tibet geological framework, which is accepted as the collision site between India and Asia (Allègre et al., 1984; Coulon et al., 1986; Dewey et al., 1990; Yin et al., 1994), is an extremely complicated tectonic zone. It includes seven different tectonic\|sedimentary units from north to south as follows: Gangdese arc complex keeping the Sangri Group inside, the Qiuwu Formation, the Giabulin Formation, the Xigaze Group, ophiolitic massifs, the Liuqu Group, and melange zones (Wang et al., 1999). Current models, which mainly focus on researches at the unit of ophiolitic massifs, propose that most of the Tethyan oceanic lithosphere was subducted into one single subduction zone active during the Middle Cretaceous or the Late Cretaceous, and closed during the Paleogene India\|Asia collision. In this report, we present latest research results on units in the Xigaze forearc basin and others in YZSZ after 6\|year\|period of comprehensive investigations. Chronostratigraphy framework, sedimentology, and evolution of the Xigaze forearc basin are discussed in details. Four thrust systems in YZSZ are named. Dynamic evolution of the YZSZ including two subductions of Tethys is presented.

Mineralogy and Geochemistry of the High-Cr Podiform Chromitite from the Cuobuzha Ophiolite, Yarlung Zangbo Suture Zone, Western Tibet, China: Implication for its Origin

The Cuobuzha high-Cr chromitites in the western segment of Yarlung Zangbo Suture Zone of Tibet are mainly hosted in the harzburgites as massive type, which are characterized by high concentrations of platinum group elements(PGE) ranging from 380 to 577 ppb, and low Pd/Ir ratios(<0.1). In mid-ocean ridge basalts(MORB)-normalized spidergrams, chromites of the Cuobuzha chromitites are depleted in Al, Ga, V, Mg and Zn, and enriched in Mn and Cr, sharing similar patterns with those of ophiolitic boninites in the Bonin and Thetford Mines. Approximately 20 platinum group mineral(PGM) grains were discovered from the samples, including laurite, erlichmanite, Os-Fe alloy, cuproiridsite, and irarsite. The PGM assemblages indicate that sulfur fugacity was initially low enough to allow the precipitation of Os-Fe alloy and increased thereafter, with the fall in temperature. Primary Fe-Ni and Fe-Cr alloys, which are stable in a highly reduced environment, occur as inclusions within chromites or clinopyroxenes. Calculated results show that the parental magma has an intimate affinity with boninites. Based on our observations, a model is proposed wherein the Cuobuzha chromitites contain high-pressure and low-pressure chromites. Low-pressure chromites were formed via reaction between boninitic melts and peridotites, during which the high-pressure chromites hosting highly reduced minerals were mobilized by melts and were reallocated to podiform chromitites.

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.

Near-surface wind environment in the Yarlung Zangbo River basin,southern Tibetan Plateau

Aeolian processes have been studied extensively at low elevations,but have been relatively little studied at high elevations.Aeolian sediments are widely distributed in the Yarlung Zangbo River basin,southern Tibetan Plateau,which is characterized by low pressure and low temperature.Here,we comprehensively analyzed the wind regime using data since 1980 from 11 meteorological stations in the study area,and examined the interaction between the near-surface wind and aeolian environment.The wind environment exhibited significant spatial and temporal variation,and mean wind speed has generally decreased on both annual and seasonal bases since 1980,at an average of 0.181 m/(s•10a).This decrease resulted from the reduced contribution of maximum wind speed,and depended strongly on variations of the frequency of sand-driving winds.The drift potential and related parameters also showed obvious spatial and temporal variation,with similar driving forces for the wind environment.The strength of the wind regime affected the formation and development of the aeolian geomorphological pattern,but with variation caused by local topography and sediment sources.The drift potential and resultant drift direction were two key parameters,as they quantify the dynamic conditions and depositional orientation of the aeolian sediments.Wind affected the spatial variation in sediment grain size,but the source material and complex topographic effects on the near-surface wind were the underlying causes for the grain size distribution of aeolian sands.These results will support efforts to control aeolian desertification in the basin and improve our understanding of aeolian processes in high-elevation environments.

ACCRETION OF AN EARLY CRETACEOUS INTRA- OCEANIC ISLAND ARC TO INDIA: EVIDENCE FROM THE YARLUNG ZANGBO SUTURE ZONE

A discontinuous line of ophiolitic bodies occurs along the Yarlung Zangbo Suture Zone (YTSZ), which stretches across southern Tibet and beyond. This zone marks the locus of collision between Eurasia and India in the Early Cenozoic. Should we assume that the entire Tethyan Ocean basin that lay between these two continental blocks was oceanic or might it have been more complex? Fragments of any terranes that developed within this once extensive ocean potentially lie within the YTSZ. Detailed investigations over the past three field seasons reveal the presence of several terranes distributed along this zone. Work is currently underway to analyze the nature of individual terranes and the timing of any inter\|relationships.

A long-term record(1995-2019)of the dynamics of land desertification in the middle reaches of Yarlung Zangbo River basin derived from Landsat data

Widespread desertification in the middle part of the Yarlung Zangbo River(YZR)basin is threatening the sustain-able development of this region.To capture this process,a method was proposed for large-scale desertification monitoring by using Landsat images from 1995 to 2019.The method used an integrated classification method combined with a hierarchical decision tree and nearest neighbor classifiers.The spatio-temporal dynamics of the desertification pattern were analyzed to assist in the detection of possible driving forces.Using validation samples collected from Google Earth high-resolution images and field investigations,the overall accuracy of the classification in 2019 was 92.3%with a Kappa coefficient of 0.84.The major results were:(1)total sandy land area in 2019 was 734.1 km^(2),which accounted for 3.7%of the study area,prominently distributed along the wide river valleys and inlets of tributaries with a strip and discontinuous pattern.Sandy land tends to be distributed in the southern aspect regions with lower elevations and that are closer to rivers;(2)sandy land areas showed two temporal stages:a gradual increase of 102.4 km^(2)from 1995 to 2015 and a large decrease of 106.8 km^(2)from 2015 to 2019;(3)newly increased sandy land was distributed in the YZR Valley,while the revegetation on sandy land occurred mainly in the Lhasa River basin and some regions in the YZR Valley;and(4)increased sandy land area of 142.1 km^(2)was mainly distributed in the southern band of the two rivers.Correspondingly,revegetation on sandy land was more effective on the northern banks of the river valleys.These findings provide guidance for implementing vegetation recovery on sandy lands and provide important insights for maintaining sustainable development.

Petrological and Re-Os Isotopic Constraints on the Origin and Tectonic Setting of the Cuobuzha Peridotite, Yarlung Zangbo Suture Zone, SW Tibet, China

The Yarlung Zangbo Suture Zone（YZSZ）in southern Tibet includes the remnants of Neotethyan oceanic lithosphere and marks a major suture between the Indian Plate to the south and the Lhasa Terrane of Tibet to the north（Dupuis et al.,2005;Yang et al.,2011）.In the western part of the YZSZ,the Northern and the Southern sub-belts form two sub-parallel zones of mafic–ultramaficrockassemblageswithoverlapping crystallization ages（Xiong et al.,2011;Hebért et al.,2012;Liu et al.,2015）.The upper mantle section of the Cuobuzha ophiolite in the northern sub-belt of the Yarlung–Zangbo Suture Zone（YZSZ）in SW Tibet comprises mainly clinopyroxene（cpx）–rich and depleted harzburgites.Spinels in the cpx-harzburgites show lower Cr#values（12.6–15.1）than the spinels in the harzburgites（26.1–34.5）,and the cpx-harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites.The harzburgites have subchondritic Os isotopic compositions（0.11624–0.11699）,whereas the cpx-harzburgites have suprachondritic 187Os/188Os ratios（0.12831–0.13125）with higher Re concentrations（0.380-0.575 ppb）.The cpx-harzburgites plot in a Re vs.Al2O3 diagram as a result of subsequent addition of Re following the last partial melting event that occurred during mid-ocean ridge melt evolution processes（Uysal et al.,2015）.Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid-ocean ridge type upper mantle units,their melt evolution trends reflect different mantle processes.The cpx-harzburgites formed from low-degree partial melting（;%）of a primitive mantle source,and they weresubsequently modified by melt–rock interactions in a mid-ocean ridge environment.The depleted harzburgites,on the other hand,were produced by re-melting of the cpx-harzburgites,which later interacted with MORB-or island arc tholeiite（IAT）-like melts（Fig.1）possibly in a trench-distal backarc spreading center.Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neotethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.

YARLUNG ZANGBO OPHIOLITES,SOUTHERN TIBET REVISITED

Field work conducted in September 1998 and Summer 1999 aimed to reassess the ophiolitic segment of the Yarlung Zangbo suture zone (YZS) and shed new light on the preserved fragments of Neo\|Tethys ocean\|floor. This eastern ophiolitic segment was partly surveyed during the 1980 Sino\|French Cooperative Investigation of Himalayas, but little work has been done since that time. Progress in ophiolite research field and new developments in modern ocean crust guided us in the recent field work investigation. Mantle peridotites and associated minor crustal units are assumed Early Cretaceous in age, while diabase interbedded with phyllites and radiolarian sediments of presumed seamount origin are attributed to Late Jurassic—Early Cretaceous age. Six different massifs were visited that are from west to east: Jiding, Qunrang, Beimarang, Dazhuqu, Luobusa, and Zedang. Each massif presents specific characteristics summarized below. The Jiding massif is made of partly to totally serpentinized granular upper mantle harzburgites with orthopyroxenite banding, a transitional Moho zone, a thick diabase sill\|dike complex intruded into heterogeneous gabbro, and pillow lavas.. High\|temperature plastic foliation, although generally oriented NW—SE, and lineation show folding. Numerous gabbroic and diabasic intrusions are observed in peridotites. The orientations of the mafic rocks foliation and lineation do not fit the structure of the host peridotites. The 350m thick transition zone is a syntectonically intrusive sequence of mantle peridotites cut by abundant different types of gabbro and diabase. In one case intrusion of gabbro postdates serpentinization of peridotites and the outer margin of the xenolith enclosed in fine\|grained gabbro has reacted to form pegmatitic hornblende gabbro. The crustal unit is made of gabbro intruded by multiple fine\|grained dikes. Hydrothermal circulation was locally intense and Cu mineralization and epidosite are observed close to shear zones.The Qunrang massif shows no transition zone overlying upper mantle unit, no significant gabbroic crustal unit and thick diabase and volcanic units. The foliation and lineation in granular lherzolite, harzburgite, and dunite show extremely wide variations and affected by late tectonics. The orientation of the structures is similar to the Jiding massif.

PALEOCEANOGRAPHY AND EVOLUTION OF THE CENO-TETHYS: MICROPALEONTOLOGICAL EVIDENCE FROM PELAGIC SEDIMENTS IN THE YARLUNG ZANGBO SUTURE ZONE, SOUTHERN TIBET

The Ceno\|Tethys is a southern branch of the Tethyan ocean and existed in the Mesozoic and early Cenozoic times. The evolution of the Ceno\|Tethys has been discussed based mainly on geological and paleontological data obtained from shallow marine sediments on the both sides of the ocean. Pelagic sediments deposited in a deep ocean basin of the Ceno\|Tethys are often incorporated in suture zones. However, geological and paleontological data from the pelagic sediments, which enable us to elucidate the paleoceanography and tectonic evolution of the Ceno\|Tethys, are still limited. The Xialu chert crops out about 30 km south of Xigaze and occupies the southern marginal part of the E—W trending Yarlung Zangbo Suture zone. As a result of radiolarian biostratigraphic research along two continuous sections, we identified seven different aged radiolarian assemblages from pelagic and hemipelagic sediments ranging early Middle Jurassic (Aalenian) to Early Cretaceous. The Aalenian radiolarian fauna is the oldest known record so far for the Xialu chert. It is noteworthy to point out that the fauna came from a chert sample which contains no terrigenous elements other than clay minerals. There is a possibility that radiolarian faunas older than Aalenian age will be discovered from the Xialu chert in the future.

Hydrogeochemical and isotopic characteristics of spring water in the Yarlung Zangbo River Basin, Qinghai-Tibet Plateau, Southwest China

The Yarlung Zangbo River Basin(YZRB)is situated in the southern part of the Tibetan Plateau and remains in a mostly natural state.To understand the chemical characteristics of spring water and its controlling factors in the YZRB,68 sets of spring water samples were analyzed using hydrochemical and isotopic techniques.The spring water was found to be slightly alkaline with total dissolved solids(TDS)below 1000 mg L−1.Major ions were Mg2+,Ca2+,SO42−,and HCO3−.The spring water types in this basin were determined to be HCO3-Ca·Mg and SO4·Cl-Ca·Mg.Ion exchange and dissolution of carbonate,gypsum,and silicate were identified as the prevalent hydrogeochemical processes contributing and defining spring water chemistry in this basin.Saturation indices(SI)of most major minerals studied in this region were below zero,indicating that these minerals remain under-saturated in the spring water in this area.Overall,the rank of different processes in terms of their contribution to the chemical composition of spring water in the YZRB was carbonate weathering>evaporate dissolution>silicate weathering>precipitation input.The content of 18O in spring water ranged from−22.22‰to−14.08‰with a mean of−18.15‰.Samples collected below and close to the local and global meteoric water lines indicated that spring water in this area is derived from meteoric water with chemistry affected by evaporation.