Sedimentary Record of the Planation Surface in the Hoh Xil Region of the Northern Tibet Plateau

Miocene marl is the most widespread Tertiary stratigraphic record in the northern Tibet Plateau, termed the Wudaoliang Group in the Hoh Xil region and the correlative Suonahu Formation in the Qiangtang region. The uniform marl overlies red beds of the Eocene-Oligocene Fenghuoshan Group. The Wudaoliang Group is generally 100-400 m thick, but the thickest strata are 700-1300 m, located in the Haidinghu (Maiding Lake) and Tuotuohe (Tuotuo River) regions respectively. Based on observations from eight measured sections and outcrops, the thin-bedded marl, which varies in colour from grey-white to light brown-grey, is explained as a large-scale or serial lacustrine deposit stretching throughout northern Tibet.The Wudaoliang Group commonly crops out on geographic lowland at an average elevation of 4600 m above sea level within the mountain chains, showing concordant summit levels, e.g. the Fenghuoshan and Bairizhajia Mountains. These mountains with a flat ridge are considered to be remains of the palaeo-planation surface. However, the spatial distribution of the Wudaoliang Group is not confined by the current mountain-basin landform configuration. We have observed the Miocene Wudaoliang Group marl exposed on a 5233 m-high mountain peak. The largest difference in height between the current lake level and the mesa crest is 600 m; the maximum dip angle is 25°, but usually below 10°, which is obviously different from the Fenghuoshan Group red beds with moderate to strong structural deformation. The horizon of the Wudaoliang Group thin-bedded marl and its widespread occurrence throughout the northern Tibetan Plateau can only be reasonably inferred to a sedimentary record relevant to the palaeo-planation. Its deposition occurred on primary land floor by erosion at 20 Ma and its bed surface is a typical sign of geomorphic rise and collapse after the erosion.

Chemo-and Biostratigraphy of the Jurassic Oil Shales from the Qiangtang Basin,Northern Tibet,China:A Case Study of the Toarcian Oceanic Anoxic Event

The marine oil shales of the Qiangtang Basin, northern Tibet, exposed in the Biluo Co, Tuonamu, Shenglihe and Changsheshan areas are believed to be important petroleum source rocks. This work comprehensively analyzed the carbon isotopes, trace elements, and calcareous nannofosills, ammonites and bivalves of the Biluo Co section in the Qiangtang Basin. The organic carbon isotopes show a positive excursion close to 2.17‰（relative to PDB）, which, albeit significantly smaller, may also be associated with other Early Toarcian Oceanic Anoxic Events（T-OAE） in the European epicontinental seas and the Tethyan continental margins. Coinciding with the Early Toarcian transgression, the oxygen deficiency in bottom water had led to dysoxic-anoxic conditions and deposition of black shales lacking benthic fauna. Under such condition, the redox-sensitive trace metals such as Mo, V, Ni, Cr, and U were enriched, in conjunction with high planktonic productivity of Watznaueriaceae calcareous nannofossils. Comparison of the results with the records of chemo-and biostratigraphy, as well as the palaeogeography during the Early Jurassic suggests that the anoxia linked to the Early Toarcian oceanic anoxic event was mainly caused by the high surface water temperature, sea-level rise and an increase of surface water productivity.

Jurassic Black Shales Facies from Qiangtang Basin (Northern Tibet): Rare Earth and Trace Elements for Paleoceanographic Implications

The Biluo Co and Amdo 114 station, northern Tibet, cropping out the Early Toarcian and Middle-Late Tithonian （Jurassic） organic-rich black shales, have been a focus to petroleum geologists in discussing their oil-producing potential. This paper first reports the trace elements and rare earth elements to discuss the paleoenvironments, redox conditions and sedimentary mechanisms of those black shales. Both sections exhibit variation in trace element abundances with concentrations 〈0.1 ppm to 760 ppm, mostly enriched in V, Cr, Ni, Cu, Zn, Mo, Ba and U. Element ratios of Ni/Co, V/Cr, U/Th and V/（V＋Ni） plus U were used to identify redox conditions. The shale-normalized rare earth element （REE） patterns are characterized by the flat-shale type with instable Ce anomalies and very weekly positive Eu anomalies. Positive Ceanom values are significant with values varying between - 0.064 and 0.029 in Biluo Co, which may be interpreted as release of REE and input of riverine terrestrial matter with rich Ce （resulting in pH change） during the anoxic conditions. In the middle parts of Amdo 114 station, distinct negative Ceanom values are observed （-0.238 to -0.111） and associated surface water warming were interpreted as being related to a major sea level rise. In contrast, the formation of the black shales in the lower and upper part of the studied succession took place during a cooler （Ceanom values 〉-0.10）, lower surface water productivity, and lower sea-level stage. Thus, we emphasize the role of different factors that control the formation of local and regional black shales. The most important factors are sea-level fluctuations and increasing productivity.

Organic Geochemistry of the Early Jurassic Oil Shale from the Shuanghu Area in Northern Tibet and the Early Toarcian Oceanic Anoxic Event

This paper presents new geological and geochemical data from the Shuanghu area in northern Tibet, which recorded the Early Toarcian Oceanic Anoxic Event. The stratigraphic succession in the Shuanghu area consists mostly of grey to dark-colored alternating oil shales, marls and mudstones. Ammonite beds are found at the top of the Shuanghu oil shale section, which are principally of early Toarcian age, roughly within the Harplocearas falciferrum Zone. Therefore, the oil shale strata at Shuanghu can be correlated with early Toarcian black shales distributing extensively in the European epicontinental seas that contain the records of an Oceanic Anoxic Event. Sedimentary organic matter of laminated shale anomalously rich in organic carbon across the Shuanghu area is characterized by high organic carbon contents, ranging from 1.8% to 26.1%. The carbon isotope curve displays the δ 13C values of the kerogen (δ 13Ckerogen) fluctuating from –26.22 to –23.53‰ PDB with a positive excursion close to 2.17‰, which, albeit significantly smaller, may also have been associated with other Early Toarcian Oceanic Anoxic Events (OAEs) in Europe. The organic atomic C/N ratios range between 6 and 43, and the curve of C/N ratios is consistent with that of the δ 13Ckerogen values. The biological assemblage, characterized by scarcity of benthic organisms and bloom of calcareous nannofossils (coccoliths), reveals high biological productivity in the surface water and an unfavorable environment for the benthic fauna in the bottom water during the Oceanic Anoxic Event. On the basis of organic geochemistry and characteristics of the biological assemblage, this study suggests that the carbon-isotope excursion is caused by the changes of sea level and productivity, and that the black shale deposition, especially oil shales, is related to the bloom and high productivity of coccoliths.

Late Cambrian SSZ-type Ophiolites in Acite Zone, East Kunlun Orogen of Northern Tibet Plateau: Insights from Zircon U-Pb Isotopes and Geochemistry of Oceanic Crust Rocks

1 Introduction East Kunlun orogen(EKO)stretching more than 1000km in E-W extension is located in the western segment of Central Orogen Belt(COB),China(Xu et al.,2006,Li et al.,2014).There outcropped Cambrian ophiolites

Fluid Inclusion and Stable Isotope Geochemistry of the Shangxu Gold Deposit, Northern Tibet

Objective The Shangxu gold deposit is located in the south of the middle Bangong-Nujiang suture zone in northern Tibet. The origin of this deposit as an orogenic gold deposit is debatable. The study of the Shangxu deposit has a profound implication on gold exploration in the Bangong- Nujiang metallogenic belt and can also improve our understanding of gold mineralization in northern Tibet.

Geochronology and Genetic Model for Early Cretaceous Volcanic Rocks from the Southern Qiangtang Terrane,Northern Tibet,China: Constraints from U-Pb Zircon Dating, Whole-Rock Geochemical and Sr-Nd Isotopic Data

Post-collisional volcanic rocks of Mesozoic age occur in the regions adjacent to Gerze, part of the southern Qiangtang Terrane of northern Tibet, China. Geochronological, geochemical, and wholerock Sr-Nd isotopic analyses were performed on the volcanic rocks to better characterize their emplacement age and models for their origin. Laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS) U-Pb zircon analyses yielded consistent ages ranging from 123.1±0.94 Ma to 124.5±0.89 Ma for six volcanic rocks from the study area. The intermediate volcanic rocks belong to the alkaline and sub-alkaline magma series in terms of K2 O+Na2 O contents(5.9%–9.0%), and to the shoshonitic and calc-alkaline series on the basis of their high K2 O contents(1.4%–3.3%). The Gerze volcanic rocks are characterized by the enrichment of light rare earth elements [(La/Yb)N=34.9–49.5] and large–ion lithophile elements(e.g., Rb, Ba, Th, U, K, Pb, and Sr), slightly negative Eu anomalies(Eu/Eu*=0.19–0.24), and negative anomalies in high field strength elements(e.g., Nb, Ta, Hf and Ti), relative to primitive mantle. The samples show slightly elevated(87 Sr/86 Sr)i values that range from 0.7049 to 0.7057, and low εNd(t) values from-0.89 to-2.89. These results suggest that the volcanic rocks studied derived from a compositionally heterogeneous mantle source and that their parent magmas were basaltic. The more mafic, parental magmas to the Gerze volcanic rocks likely underwent fractional crystallization of clinopyroxene, hornblende, biotite, and potassium feldspar, during ascent, with little to no crustal contamination, prior to their eruption/emplacement. While these volcanic rocks exhibit geochemical signatures typical of magmas formed in a destructive plate-margin setting, it is plausible that their mantle source might also have acquired such characteristics in an earlier episode of subduction.

The Major Two-stage Shortening Deformation of the Northern Tibet and Tian Shan Area Since the Latest Oligocene

It seems to be progressively recognized that the stress of the India-Asia convergent front can be transferred rapidly through the southern and central Tibetan lithosphere to the northern Tibet, hence leading to the crustal thickening deformation there during or immediately after the onset of the India-Asia collision（ca.55 Ma）.This study focuses on the late Cenozoic deformation and tectonic uplift of the northern Tibet and Tian Shan area.Detailed compilations of a variety of proxy data from sediments and bedrocks suggest that the northern Tibet and Tian Shan area underwent one stage of approximately synchronous widespread contractile deformation since 25–20 Ma, which seemed to decrease at circa 18 Ma as revealed by low-temperature thermochronological data.The latest Oligocene-early Miocene was also significant basin-forming episodes when many intermontane subbasins began to receive syntectonic sedimentation in the northeastern Tibet.Subsequently, the other phase of compressional deformation began to encroach more widely into the northern Tibet and Tian Shan area in episodic steps or continuously from 16–12 Ma to present.

Organic geochemistry characteristics of Jurassic black shales from the Amdo area, northern Tibet, China

The Jurassic black shales of the Qiangtang Basin,northern Tibet are known to be amongst the most important source rocks.To date,there is no detailed study on the organic geochemistry of the Bathonian/Callovian coccolith-bearing sediments and their source-rock characteristics.Here,we focus on the black limestones,marls,and shales at the Amdo 114 station section using Rock–Eval pyrolysis and biomarkers for sources,types,and thermal maturity of organic matter and the reconstructed depositional environment.All samples are characterized by(1)relatively high TOC values(0.20–2.56 wt%)and relatively low Tmax values between 428 and 452℃;(2)shortchain n-alkanes with a maximum at C_(14) to C_(18) and Pr/Ph>1;(3)31–35 homohopanes and gammacerane with lower abundance;and (4)C_(27)–C_(28)–C_(29) regular steranes with C_(29) being slightly dominant.The organic matter is TypeⅡand thus dominantly derived from phytoplankton,especially algae.Thermal maturity indicators include the Ts/(Ts+Tm),C_(31)22S/(22S+2R),C_(29)aaa20S/(20S+0R),C_(29)ββ/(ββ+aa)and C_(27)Dia/(Dia+Reg),which are regarded as immature to very early mature.Additionally,the high Pr/Ph,Pr/nC_(17),and Ph/nC_(18) ratios,Pr/Ph vs Gammacerane index suggest that these black shales were deposited in unstratified marine waters of normal salinity under dysoxic to weakly reduced conditions.

A VISIT TO Northern Tibet Plateau

TIDBITS ABOUT VISIT TONORTHERN TIBETNyainqentanglha Mountain,whichis majestic enough to make othermountains feel shy. Nam Co Lake,which is as pretty as a young girl.Changtang Grasslands,which arevast in xize and house muchwildlife. Lava caves, which are mys-terious. Rock caves where manyBuddhist masters have practicedmediation over the years.No-man’sland where human footprints arerarely seen......

Dynamics of Above-and Below-ground Biomass and C,N,P Accumulation in the Alpine Steppe of Northern Tibet

The temporal dynamics of the biomass,as well as the carbon (C),nitrogen (N),phosphorus (P) concentrations and accumulation contents,in above-and below-ground vegetation components were determined in the alpine steppe vegetation of Northern Tibet during the growing season of 2010.The highest levels of total biomass (311.68 g m-2),total C (115.95 g m-2),total N (2.60 g m-2),and total P (0.90 g m-2) accumulation contents were obtained in August in 2010.Further,biomass and nutrient stocks in the below-ground components were higher than those of the above-ground components.The dominant species viz.,Stipa purpurea and Carex moorcrofti had lower biomass and C,N,P accumulations than the companion species which including Oxytropis.spp.,Artemisia capillaris Thunb.,Aster tataricus L.,and so on.

THE METAMORPHIC FEATURE,AGE AND MECHANISM OF AMUGANG GROUP IN NORTHERN TIBET

Amugang Group is distributed mainly over Amugang, Jiangai Mountain, Mayigangri,Gemuri and Qiagela,etc. lt includes Gemuri Formation and Qiagela Formation. They had even been thought of the crystalline basement of Qiangtang terrain. However, Gemuri Formation and Qiagela Formation are not alike in the protolithes, metamorphism and deformation, rock association,etc.In fact, they are different in mechanism.Gemuri Formation is composed of high\|Pressure, low\|temperature glaucophane greenschist\|faci metamorphic rocks. The petrolithes of the blueschists are glaciomarine conglomerates and basalts from the Southern Qiagtang area. The typical mineralogy include: glaucophane+ epidote + calcite +stilpnomelane and stilpnomelane+chlorite+sericite+ quartz+ glaucophane. P.T conditions for the metamorphism of blueschist are estimated to be 0 6～0 7GPa and 320～400℃.The 40 Ar/ 39 Ar dating of the crossite has yielded good plateau age of (222 5±3 7)Ma,which represents the formation of Gemuri Formation.Qiagela Formation comprises schist series, marbles, Gneisses and plagioamphiboles. The protolithes of them are a suit of argillaceous sandstones, arkoses, carbonates and mafic volcanic rocks. The Sm\|Nd isochron age of the metamorphic mafic volcanic rocks is (268 0±5 6)Ma, which shows that The age of the protolithes is early Dias. The typical mineralogy include: Muscovite+biotite+plagioclase+quartz; garnet+kyanite+ staurolite+ biotite+ muscovite+plagioclase+quartz; amphibole+ plagioclase±garnet+ quartz. They are meos\|pressure,meso\|temperature low amphibole\|faci metamorphic rocks. Qiagela Formation is coexistent with the late Triassic and the early Jurassic volcano\|magmatic arc in space and overlapped by the triassic limestones of Xiaocaka Formation. So,it is suggested that the formation of Qiagela Formation be between the late Dias and the late Triassic period. Its genesis is relative to the thermal current provided by magmatic activity.

BIOLOGICAL MARKER OF MIDDLE JURASSIC OIL SHALE SEQUENCE FROM SHUANGHUI AREA,NORTHERN TIBETAN PLATEAU

The oil shale with marine origin was first reported in 1987 from Shuanghui of the Qiangtang region. Its depositional sequence consists of brown\|black oil shale interbedded massive to thin limestone. Eleven oil shale beds occur and aggregated thickness is up to 47 38m. It deposit age is confined in middle Jurassic by fossils identification. Nine samples selected from horizons with high\|organic contents have been examined by organic geochemistry approach. The oil\|shale range widely in organic carbon content (Toc), average in 8 34%, maximum values reaching 26.12%. Toc are markedly varied in vertical section. The Upper and lower members are slightly low and increase in the middle. The oil\|shale sediments are characterized by high concentration in chloroform bitumen“A”(608～18707)×10 -6 )and total hydrocarbon ((311～5272)×10 -6 ).The Rock\|Eval T \|max data (434～440℃) and vitrinite reflectance values (0.88%～1.26%) indicate that oil\|shale sequence are mature in all samples. The organic matter is predominantly made up of typeⅡ kerogen.

BASIN-RANGE TRANSITION AND GENETIC TYPES OF SEQUENCE BOUNDARY OF THE QIANGTANG BASIN IN NORTHERN TIBET

The surface of sequence boundary is a negative record. Its recognition largely depends on the physics of the sediments below and above the boundaries, or on the different sedimentary structures are synthetic marks for the sedimentation and tectonic movements in the sedimentary basin. The Qiangtang Basin that is in 5000m above the sea level is located in Northern Tibet. The Lazhulung—Jinshajiang suture zone now bound it to the north and the Bangong—Nujiang suture zone to the south. Three second\|order tectonic units have been distinguished, i.e. North Qiangtang depression, Central rise and South Qiangtang depression from north to south.The Upper Permian Riejuichaka Formation is built up of mudstone and mud\|limestone, which is represented by sediments in seamarsh. The Lower Triassic Kuanglu Formation, which exhibits the structure unconformable contact with the overlying Upper Permian strata, is characterized by terrigenous clastic rocks in the lower area and is carbonate rocks in the upwarding area and the Middle Triassic Kuangnan Formation. The Upper Triassic Xiachaka Formation consisting of terrigenous clastic rocks, carbonates rocks and mixed sediments, is confined to the uplift zones. The lower Jurassic volcanic rocks are deposited in continental rift. The middle and Upper Jurassic Yangshiping Group are conformable contact and assembled by the gypsum\|bearing terrigenous clastic rock formations and carbonate rock formation. The Middle Cretaceous and the Paleocene strata is built up of the terrigenous clastic rock formations.

Origin of Qushenla Formation Volcanic Rocks in the Nawucuo Area,Northern Tibet,and Constraints on the Subduction Polarity of the Bangong-Nujiang Tethys Ocean

The Qushenla Formation volcanic rocks are widely exposed in the northern margin of the Bangong-Nujiang suture zone(BNSZ).Research on these rocks is of great significance for understanding the tectonic evolution of the Bangong-Nujiang Tethys Ocean(BNTO).In this study,a systematic geological survey was conducted on the Qushenla Formation volcanic rocks that are widely exposed in the Nawucuo area,in the northern margin of the western segment along the BNSZ.The whole-rock geochemistry,zircon U-Pb dating,and in situ zircon Lu-Hf isotopes were carried out in this study,aiming to constrain the formation age,rock genesis,magma source and tectonic setting of the volcanic rocks.The zircon U-Pb dating shows that the Qushenla Formation volcanic rocks in the western BNSZ erupted during the period of 120–108 Ma,i.e.,Early Cretaceous.The Qushenla Formation volcanic rocks are a suite of intermediate-basic volcanic and pyroclastic rocks belonging to the medium-K calc-alkaline series.They are relatively enriched in light rare earth elements(LREEs)and incompatible elements such as Rb,K,La,Th,Sm,and Hf,whereas depleted in heavy REEs(HREEs)and high field strength elements(HFSEs)such as Nb,P,Zr,and Ti.The in situ zirconεHf(t)values of the volcanic rocks range from 8.95 to 12.57,with an average of 10.40.The Mg#,Th/La and Th/Ce values are between those of the mantle-derived magma and the continental crust.The formation of the Qushenla Formation volcanic rocks can be explained through the following process:(1)As the nospheric materials that upwelled during the rollback of the subducting Tethys Ocean slab induced the large-scale partial melting of the mantle wedge and the formation of the initial basaltic magma.(2)These mantle-derived magmas ascended and induced the partial melting of the lower crust to generate peraluminous melts.(3)The mixing of the peraluminous melts and mantle-derived melts generated the initial magma with homogeneous Sr-Nd isotope compositions.(4)Last,the eruption of the magma produced the widespread Qushenla Formation volcanic rocks at the surface.When combining this information with the regional geological background,it is believed that the Qushenla Formation volcanic rocks,the Meiriqicuo Formation volcanic rocks and the Late Jurassic–Early Cretaceous intrusive rocks together constitute the tectonic magmatic arc of the active continental margin on the southern margin of Qiangtang,which was formed in the tectonic setting of the northward subduction of the Bangong-Nujiang oceanic crust beneath the Qiangtang Block.

SHRIMP U-Pb dating and Hfisotopic analyses of zircons from the mafic dyke swarms in central Qiangtang area,Northern Tibet

In the "Central Uplift" of Qiangtang,Northern Tibet,a large scale of mafic dyke swarms extended in E-W direction.Zircon SHRIMP U-Pb dating on two representative samples of diabase dyke yield weighted mean ages of 284 ± 3 Ma and 302 ± 4 Ma,respectively,suggesting they emplaced in the Late Carboniferous to Early Permian.Zircon 176Hf/177Hf ratios range from 0.282852 to 0.283041,with εHf(t) values of +12.1 to +12.2 and Hf modal(TDM) ages of ～440―460 Ma.These data indicate that these mafic dykes were mainly derived from the depleted mantle.In addition,geochemical data suggest that these mafic dyke swarms are ofintra-plate origin.Therefore,the mafic dyke swarms represent products during the extensional process associated with the opening of the Paleo-Tethys Ocean in the area.

Petrology and ^(40)Ar/^(39)Ar iso-topic ages of blueschists in Gangmar, central Qiangtang, northern Tibet

The petrology and mineral assemblages of blueschists in the Gangmar, central Qiangtang, northern Tibet were examined, which indicates that the metamorphic condition is high-pressure low-temperature. In this note, we reported the 40Ar/39Ar dating results of glaucophane from two blueschist samples. Their apparent ages are (275.0±1.3) Ma and (287.6±2.3) Ma and similar isochron ages are (275.0±0.9) Ma and (282.4±0.8) Ma, respectively. These iso-topic datings show the high-pressure metamorphism occurring in Lower Permian. The ages also coincide with the evolution of Palaeo-Tethys ocean in Late Palaeozoic.

East-west crustal structure and “down-bowing” Moho under the northern Tibet revealed by wide-angle seismic profile

We recognized 6 sets of reflecting P- and S-wave events from Moho and other interfaces within the crust, respectively, with the wide-angle seismic data acquired from 510 km-long Selincuo-Ya'anduo profile in the northern Tibet, fitted the observed events with forward modeling, and interpreted crustal structure of P- and S-wave velocities and Poisson's ratio under the profile. The results demonstrate that the crustal structure between Yarlungzangbo and Bangong-Nujiang sutures changes abruptly, and the crust is the thickest at the middle part of the profile with thickness of 80 km or more. The 'down-bowing' Moho is the striking feature for the crustal variation along the west-east direction. The Moho uplifts with steps, and the uplifting rate westward is greater than that eastward. The heterogeneity of P- and S-wave velocities exists both vertically and horizontally, and one lower velocity layer (LVL) exists with the depth range of 27-34 km and the thickness range of 5-7 km. For the upper crust, Poisson's ratio is the lowest at the middle part of the profile; for the lower crust, the Poisson's ratio at the east segment is lower than that at west segment, which means that the crustal rigidity for the upper crust is different from the lower crust, and the lower crust under the east segment of the profile is more ductile. We infer that the substance in the lower crust endured eastward flow along with the collision between Eurasian and Indian plates, and the 'down-bowing' Moho is attributable to the multi-phase E-W tectonic processes.