Texture and Geochemistry of Multi-stage Hydrothermal Scheelite in the Mamupu Cu-Au-Mo(-W)Deposit,Eastern Tibet:Implications for Tungsten Mineralization in the Yulong Belt

Multistage tungsten mineralization was recently discovered in the Mamupu copper-polymetallic deposit in the southern Yulong porphyry copper belt(YPCB),Tibet.This study reports the results of cathodoluminescence,trace element and Sr isotope analyses of Mamupu scheelite samples,undertaken in order to better constrain the mechanism of W mineralization and the sources of the ore-forming fluids.Three different types of scheelite are identified in the Mamupu deposit:scheelite A(Sch A)mainly occurs in breccias during the prograde stage,scheelite B(Sch B)forms in the chlorite-epidote alteration zone in the retrograde stage,while scheelite C(Sch C)occurs in distal quartz sulfide veins.The extremely high Mo content and negative Eu anomaly in Sch A represent high oxygen fugacity in the prograde stage.Compared with ore-related porphyries,Sch A has a similar REE pattern,but with higher ΣREE,more depleted HREE and slightly lower(^(87)Sr/^(86)Sr)i ratios.These features suggest that Sch A is genetically related to ore-related porphyries,but extensive interaction with carbonate surrounding rocks affects the final REE and Sr isotopic composition.Sch B shows dark(Sch B-I)and light(Sch B-II)domains under CL imaging.From Sch B-I to Sch B-II,LREEs are gradually depleted,with MREEs being gradually enriched.Sch C has the highest LREE/HREE ratio,which indicates that it inherited the geochemical characteristics of fluids after the precipitation of HREE-rich minerals,such as diopside and garnet,in the early prograde stage.The Mo content in Sch B and Sch C gradually decreased,indicating that the oxygen fugacity of the fluids changed from oxidative in the early stages to reductive in the later,the turbulent Eu anomaly in Sch B and Sch C indicating that the Eu anomaly in the Mamupu scheelite is not solely controlled by oxygen fugacity.The extensive interaction of magmatic-hydrothermal fluids and carbonate provides the necessary Ca^(2+)for the precipitation of scheelite in the Mamupu deposit.

Petrology, geochemistry and Ar-Ar geochronology of eclogites in Jinshajiang orogenic belt, Gonjo area, eastern Tibet and restriction on Paleo-Tethyan evolution

As one of the important Paleo-Tethys suture zones in eastern Tibet,the Jinshajiang orogenic belt is of great significance to study the tectonic evolution of the main suture zone of Paleo-Tethys.In this paper,eclogites developed in the Jinshajiang suture zone in Gonjo area,eastern Tibet,are selected as specific research objects,and petrological,geochemical and Ar-Ar geochronological analyses are carried out.The major element data of the whole rock reveals that the eclogite samples have the characteristics of picritic basalt-basalt and belong to the oceanic low potassium tholeiites.The results of rare earth elements and trace elements of the samples show that the protoliths of eclogites have characteristics similar to oceanic island basalt(OIB)or normal mid ocean ridge basalt(N-MORB).Muscovite(phengite)from two eclogite samples yield the Ar-Ar plateau ages of 247±2 Ma and 248±2 Ma respectively,representing the peak metamorphic age of eclogite facies and the timing of complete closure of the Jinshajiang Paleo-Tethys Ocean.Muscovite and biotite selected from the hosting rocks of eclogite yield the Ar-Ar plateau ages are 238±2 Ma and 225±2 Ma respectively,reflecting the exhumation age of eclogites and their hosting rocks.Combined with the zircon U-Pb dating data(244 Ma)of eclogites obtained in previous work,it can be concluded that the Jinshajiang Paleo-Tethys ocean was completely closed and arc-continent collision was initiated at about 248-244 Ma(T21).Subsequently,due to the large-scale arc(continent)-collision orogeney between Deqin-Weixi continental margin arc and Zhongza block(T31-T32),the eclogites were rapidly uplifted to the shallow crust.

Geochemistry of the Eocene Felsic Porphyric Rocks and High-Mg Potassic Rocks along JARSZ:Implication for the Tectonic Evolution in Eastern Tibet

Eocene felsic porphyric rocks and the high-Mg potassic volcanic rocks（HMPR） occur along the Jinshajiang-Ailao Shan-Red River shear zone（JARSZ） in eastern Tibet.Compared with the HMPR,which are generally believed to be sourced from an enriched mantle,the felsic porphyric rocks show similar K_2O contents,enrichment in LREE and LILE,particularly radiogenic isotope（e.g.Sr and Nd） features much similar to the former,implying generation of the felsic porphyric rocks most likely related to the HMPR,although they both have clearly different major and trace element compositions. The close relationship in spatial-temporal distribution and similar Sr-Nd characteristics between the felsic porphyric rocks and HMPR in eastern Tibet indicate that both of them were possibly formed by a similar tectonic process（event）.Combining the basic dikes in southern and eastern Tibet,we suggest that the break-off of north-dipping Neo-Tethyan slab in southern Tibet during 50-40 Ma,triggered formation of high-Mg potassic magma.This led to developing felsic porphyric magma production by partial melting of underplating HMPR in the lower crust,or fractionation crystallization of the high-Mg potassic magmas.The break-off of slab in the Eocene may also have contributed to the abundant ore-forming material related to earlier subduction events,resulting in formation of the porphyric deposits along JARSZ in eastern Tibet.

Discovery of eclogites in Jinsha River suture zone, Gonjo County, eastern Tibet and its restriction on Paleo-Tethyan evolution

As the important component of the eastern Tethys tectonic region,the Jinsha River-Ailao Mountain suture zone is often considered to be an ophiolitic mé1ange belt.However,the P-T-t path and chronological framework of the metamorphic evolution in the collisional orogenetic process of this zone are still poorly understood owing to the lack of metamorphism research of symbolic high-pressure rocks.During a regional geological survey on a scale of 1:50000 in Gonjo County,Tibet Autonomous Region involved in this paper,(retrograde)eclogites lenses of different scales were found in Jinsha River suture zone,eastern Tibet for the first time.The(retrograde)eclogites can be divided into garnet-albite-chlorite-actinolite schists and eclogites according to retrograde degrees.The mainly mineral components of eclogites include garnet(45%-50%),clinopyroxene(about 25%),and hornblende(5%-10%)primarily,and biotite,quartz,rutile,and muscovite secondarily.According to the data of electron probe micro analysis(EPMA),clinopyroxenes feature high content of Na2O(5.6%-6%)and corresponding jadeite(Jd)molecules of 37%-44%,and they fall within the omphacite region in Quad-Jd-Ae diagram.The temperature and pressure of the metamorphism at peak are P≈2.2-2.34 GPa and T≈622-688℃respectively as measured with geobarometry Grt-Omp-Phe and geothermometer Grt-Omp.This will provide a new reference for the understanding of Paleo-Tethyan evolution.In this paper,two samples of eclogites were chosen for LA-ICP-MS zircon U-Pb dating and their 206pb/238U ages obtained are 240±3 Ma and 244±1 Ma respectively.Furthermore,the zircons feature extremely low Th/U ratio(<0.01),extremely low content ofNb,Ta,and HREE,and invisible negative Eu anomaly.Therefore,the genesis of the zircons shall be eclogites-facies metamorphism,indicating that the 240-245 Ma determined in this paper shall be the age of eclogites-facies metamorphism and may represent the westward subduction-collision epoch of Paleo-Tethys Ocean located between Zhongza Block and Qamdo Block.

Lithosphere density structure beneath the eastern margin of the Tibetan Plateau and its surrounding areas derived from GOCE gradients data

A three-dimensional density model of the crust and uppermost mantle is determined by the inversion of a set of GOCE gravity and gradients residual anomalies beneath the eastern margin of the Tibetan Plateau and its surrounding areas. In our work, we choose five independent gravity gradients （Txx, Tzz, Txy, Txz, Tyz） to perform density inversion. Objective function is given based on Tikhonov regularization theory. Seismic S-wave velocities play the role of initial constraint for the inversion based on a relationship between density and S-wave velocity. Damped Least Square method is used during the inversion. The final density results offer some insights into understanding the underlying geodynamic processes： （1） Low densities in the margin of the Tibet, along with low wave velocity and resistivity results, yield conversions from soft and weak Tibet to the hard and rigid cratons. （2）The lowest densities are found in the boundary of the plateau, instead of the whole Tibet indicates that the effects of extrusion stress environment in the margin affect the changes of the substance there. The substances and environments conditioning for the earthquake preparations and strong deformation in this transitional zone. （3） Evident low-D anomaly in the upper and middle crust in the Lasha terrane and Songpan-Ganzi terrane illustrated the eastward sub-ducted of southeastern Tibet, which could be accounts for the frequent volcano and earthouakes there.

Estimation of Seismic Landslide Hazard in the Eastern Himalayan Syntaxis Region of Tibetan Plateau

The eastern Himalayan syntaxis is one of the most tectonically active and earthquake-prone regions on Earth where earthquake-induced geological disasters occur frequently and caused great damages. With the planning and construction of Sichuan-Tibet highway, Sichuan-Tibet railway and hydropower development on the Yarlung Zangbo River etc. in recent years, it is very important to evaluate the seismic landslide hazard of this region. In this paper, a seismic landslide hazard map is produced based on seismic geological background analysis and field investigation using Newmark method with 10% PGA exceedance probabilities in future 50 years by considering the influence of river erosion, active faults and seismic amplification for the first time. The results show that the areas prone to seismic landslides are distributed on steep slopes along the drainages and the glacier horns as well as ridges on the mountains. The seismic landslide hazard map produced in this study not only predicts the most prone seismic landslide areas in the future 50 years but also provides a reference for mitigation strategies to reduce the exposure of the new building and planning projects to seismic landslides.

A new species of the genus Scutiger （Anura： Mego- phryidae） from Medog of southeastern Tibet, China

A new species of Scutiger Theobald, 1868 is described from Medog, southeastern Tibet, China, based on morphological and molecular data. The new species was previously identified as Scutiger nyingchiensis, but it can be differentiated from the latter and all other congeners by the following combination of characters： （1） medium adult body size, SVL 50.5-55.6 mm in males and 53.8-57.2 mm in females; （2） maxillary teeth absent; （3） web rudimen- tary between toes; （4） prominent, conical-shaped tubercles on dorsal and lateral surfaces of body and limbs; （5） tubercles covered by black spines in both sexes in breeding condition; （6） a pair of pectoral glands and a pair of axillary glands present and covered by black spines in males in breeding condition, width of axillary gland less than 50% of pectoral gland; （7） nuptial spines present on dorsal surface of first and second fingers, and inner side of third finger in males in breeding condition; （8） spines absent on the abdominal region; （9） vocal sac absent. In addition, the distribution and conservation status of the new species are also discussed

TECTONO-SEDIMENTARY EVOLUTION OF THE TERTIARY BASINS IN EASTERN TIBET: CONSTRAINING THE RAISING OF TIBETAN PLATEAU

The Tertiary basins are distributed in the eastern margin of the Tibetan Plateau along the large\|scale arc\|shaped strike\|slip belt of the Qiangtang—Hengduan Shan—Jinshajiang—Red River belt. Most of basins were controlled by regional northwest\| and south\|north treading faults, and a few basins were constrained by northwest\| or north\|north\|east\|treading local faults.The detailed field mapping and sedimentologic observations for 6 typical Tertiary basins in eastern Tibet show that their types include depressive basins, rifting basins, pull\|apart and extensional basins related to strike\|slip faulting, and erosion\|residual basins. Recently, most of them are controlled by compression\| or thrust\|related margin faults in single side or double laterals of these basins. Not\|well\|developed strata in the Tertiary basins were deformed to form various tight folds and thrusts. High\|K magma intruded widely into the basins. The tectono\|sedimentary evolution of the Tertiary basins appears tempo\|spatially inhomogeneous. The preliminary model to interpret the Tertiary basin evolution is described as below.

New Zircon U-Pb Age of the Ore-bearing Porphyry from the Kuga Copper Deposit in the Eastern Bangongco–Nujiang Matallogenic Belt, Tibet

Objective As the third most important copper polymetallic metallogenic belt in Tibet, the Bangongco-Nujiang metallogenic belt （BNMB） has attracted much attention among geoscientists all over the world （Lin Bin et al., 2017a）. There are two ore clusters in the westem of BNMB, the Duolong giant porphyry-epitherrnal Cu （Au, Ag） ore cluster and the Ga＇erqiong-Galalelarge porphyry- skarn Cu （Au） ore cluster （Lin Bin et al., 2017a; 2017b）. Now, the latest exploration advances show that the Kuga project is the first economic porphyry-skam copper deposit in the eastern of BNMB, with over 0.4 Mt melt copper （333＋334） @ 0.9%. However, the Kuga deposit is poorly studied about its diagenetic age. In this study, we present a zircon U-Pb LA-ICP-MS dating of ore-bearing biotite granite, in order to identify the time of the ore- related magmatism and reveal the relationship with the westem of BNMB.

Petrogenesis and Tectonic Implications of the Latest Cretaceous Intrusive Rocks from the Eastern Gangdese Belt,Southeast Tibet

The latest Cretaceous magmatic activity in the eastern segment of the Lhasa terrane provides important insights for tracking the magma source and geodynamic setting of the eastern Gangdese batholith,eastward of eastern Himalayan Syntaxis.Detailed petrological,geochemical and geochronological studies of the intrusive rocks(monzodiorites and granodiorites)of the eastern Gangdese batholith are presented with monzodiorites and granodiorites giving zircon U-Pb crystallization dates of 70-66 Ma and 71-66 Ma withεHf(t)values of−4.8 to+6.2 and−1.9 to+5.3,respectively.These rocks are metaluminous to weakly peraluminous I-type granites showing geochemically arc-related features of enrichment in LREEs and some LILEs,e.g.,Rb,Th,and U,and depletion in HREEs and some HFSEs,e.g.,Nb,Ta,and Ti.The rocks are interpreted to be derived from partial melting of mantle material and juvenile crust,respectively,which are proposed to be triggered by Neo-Tethyan slab rollback during northward subduction,with both experiencing ancient crustal contamination.The studied intrusive rocks formed in a transitional geodynamic setting caused by Neo-Tethyan oceanic flat subduction to slab rollback beneath the eastern Gangdese belt during the latest Cretaceous.

Geochemical Characteristics of Cenozoic Basaltic High-K Volcanic Rocks from Maguan Area, Eastern Tibet

The major element, trace element and Nd-Sr isotopic composition of Cenozoic basaltic volcanic rocks from the Maguan area, eastern Tibet, indicates that the volcanic rocks are enriched in alkalis, especially K (K-2O up to {3.81%}) and depleted in Ti (TiO-2={1.27%}-{2.00%}). These rocks may be classified as two groups, based on their Mg+# numbers: one may represent primary magma (Mg+# numbers from 68 to 69), and the other, the evolved magma (Mg+# numbers from 49 to 57). Their REE contents are very high (∑REE={155.06}-{239.04}μg/g). Their REE distribution patterns are of the right-inclined type, characterized by LREE enrichment [(La/Yb)-N={12.0}-{19.2}], no Ce anomaly (Ce/Ce+*={1.0}), and weak negative Eu anomaly (Eu/Eu+*={0.9}). The rocks are highly enriched in Rb, Sr and Ba ({59.5}-{93.8} μg/g, 732-999 μg/g, and 450-632 μg/g, respectively), high in U and Th ({1.59}-{2.31} μg/g and {4.73}-{8.16} μg/g, respectively), and high in Nb, Ta, Zr and Hf (70-118 μg/g, {3.72}-{5.93} μg/g, 215-381 μg/g, and {5.47}-{9.03} μg/g, respectively). In the primitive mantle-normalized incompatible element spidergram, Nb, Ta, Zr, Hf and P show positive anomalies, whereas Ba, Ti and Y show negative anomalies. The {{}+{87}Sr/+{86}Sr} ratios range from {0.704029} to {0.704761}; {{}+{143}Nd/+{144}Nd} from {0.512769} to {0.512949}; and εNd from {+2.6} to {+6.1}. These geochemical features might suggest that the potential source of the basaltic high-K volcanic rocks in the Maguan area is similar to the OIB-source mantle of Hawaii and Kergeulen volcanic rocks.

Climatic Signals in the Last 200 Years from Stable Isotope Record in the Shells of Freshwater Snails in Lake Xingcuo, Eastern Tibet Plateau, China

Lake Xingcuo is a small closed, hard\|water lake, situated on eastern Tibet Plateau. Stable isotope data (\{δ\{\}\+\{18\}O\} and \{δ\{\}\+\{13\}C\}) from the freshwater snail Gyraulus sibirica (Dunker) in a 34 cm long, radioactive isotope\|dated sediment core represent the last 200 years of Lake Xingcuo environmental history. Carbon and oxygen isotope ratios in the shells of the freshwater snail bear information on the isotopic composition of the water in which the shells were formed, which in turn characterizes the climatic conditions prevailing during the snail’s life span. Whole\|shell and incremental growth data were collected from modern and fossil shells from Lake Xingcuo. The \{δ\{\}\+\{18\}O\} values of modern shells from Lake Xingcuo are in equilibrium with those of modern waters, while \{δ\{\}\+\{13\}C\} values are indicative of snail dietary carbon and a lake with high \{δ\{\}\+\{13\}C\}\-\{TDIC\}. By calibrating \{δ\{\}\+\{18\}O\} and \{δ\{\}\+\{13\}C\} in the shell Gyraulus sibirica (Dunker) with instrument\|measured data for the period 1954-1992, we found that the \{δ\{\}\+\{18\}O\} of the snail shells is an efficient indicator to reveal air temperature in the warmer half year instead of that around the whole year, and that there is a certain positive correlation between index\{δ\{\}\+\{18\}O\} and the running average temperature in the warmer half\|year period. Climatic variability on eastern Tibet Plateau, for the last two centuries, has been successfully inferred from the\{δ\{\}\+\{18\}O\} record in freshwater snails in the sediments of Lake Xingcuo. As such, the last 200 years of palaeoclimatic record for this region can be separated into three periods representing oscillations between warming and cooling, which are confirmed by the Guliya ice record on the Tibet Plateau.

TRIASSIC SEQUENCE STRATIGRAPHY AND CORRELATION FROM POLY-ARC AND BACK-ARC SYSTEM IN EASTERN TIBET

It is dominantly characterized that arc and back\|arc basin occur mutually in arc\|basin systems, Jomda arc and Sinda back\|arc basin located in Qamdo area are typical(Fig.1). Jomda arc covered by Triassic with thickness about 10000m.Marine sediments predominate, and are Characterized by the largely thick turbidity rocks and arc volcanic rocks of calc\|alkali suite which frequent volcanic events raised. Sinda back\|arc basin with a land\|crust basement is filled by the Upper Triassic sedimentary strata of more than 5000m. The active marginal sediments are composed of terrigenous,basinal and volcanic turbidity deposits of bathyal to deep\|sea facies,Characteristic extensive alkali basalts and submarine spout hot water deposits. Based on investigation of the cutcrop pro\|file, isotime grillwork for Triassic sequence stratigraphy has been established. for example Jomda arc basin, including 6 type 1 and 4 type 2 boundaries.10 three\|order sequences are divided, including 6 type 1 and 4 type 2 sequences. These sequences evolves during 37Ma,the estimated average time of a sequences is 3～7Ma,with being generally 1～2Ma,the longest being about 10Ma. type 1 and type 2 sequence boundaries are identified, in Upper Triassic Sinda back\|arc basin two type 1 and one type 2 sequences are divided. Relations have been first concluded among mainly controlled factors of sequence stratigraphies in arc and back\|arc basin systems. First, more sequences product and three\|units of its structures are complete in arc systems of complicated tectonic\|volcanic topographies, for example Jomda arc area of island\|sea framework, because lots of deposits are supplied and accumulations are allowed. Secondly, tectonic control is very clear, type 1 boundaries of SQ1 and SQ7 are clearly unconformity interfaces stressed by tectonism.Thirdly,volcanic rocks is greatly related with sequences, and there are various volcanic rocks in mang sequences,for example Jomda sequence stratigraphies,volcanic rocks of calc alkaline suite occurs in LST which thickness occupying in LST is middle; both tholeiite and calc\|alkaline,alkaline suite (Sinda back\|arc area) in TST, thickness smallest; calc\|alkaline suite in HST, Thickness biggest. It proves that intra\|genetic association between volcanic and relative sea\|level changes, which is gained by the concordance of thick curves of volcanic rock and sea\|level in different systematic traces. When thickness of volcanic rocks growing or progressing, relative sea\|level falls; otherwise, when reducing, it rises. There are larger differences in quantities, structures and types of sequences by comparing sequence stratigraphies of arc, back\|arc with craton area. Those varieties are responses for interactions among tectonic\|volcano, sediment and eustacy. It is pointed out that tectonic and arc volcanic processes are crucial.

Deep-derived enclaves (belonging to middle-lower crust metamorphic rocks) in the Liuhe-Xiangduo area, eastern Tibet: Evidence from petrogeochemistry

Petrological and geochemical studies of deep-derived enclaves from the Liuhe-Xiangduo area, eastern Tibet, showed that the enclaves involve five types of rocks, i.e., garnet diopsidite, garnet amphibolite, garnet hornblendite, amphibolite and hornblendite, whose main mineral assemblages are Grt+Di+Hbl, Grt+Pl+Hbl+Di, Grt+Hbl+Pl, Pl+Hbl, and Hbl+Bt, respectively. The enclaves exhibit typical crystalloblastic texture, and growth zones are well developed in garnet (Grt) in the enclaves. In view of major element geochemistry, the deep|derived enclaves are characterized by high MgO and FeO+*, ranging from {12.00%} to {12.30%} and {8.15%} to {10.94%}, respectively. The protolith restoration of metamorphic rocks revealed that the enclaves belong to ortho-metamorphic rocks. The REE abundances vary over a wide range, and ∑REE ranges from {53.39} to {129.04} μg/g. The REE patterns slightly incline toward the HREE side with weak LREE enrichment. The contents of Rb, Sr, and Ba range from {8.34} to 101μg/g, 165 to 1485 μg/g, and 105 to 721 μg/g, respectively. The primitive mantle-normalized spider diagrams of trace elements show obvious negative Nb, Ta, Zr and Hf anomalies. Sr-Nd isotopic compositions of the enclaves indicated that the potential source of deep-derived enclaves is similar to the depleted|mantle, and their {({}+{87}Sr/+{86}Sr)-i} ratios vary from {0.706314} to {0.707198}, {({}+{147}Nd/+{144}Nd)-i} ratios from {0.512947} to {0.513046}, and {ε-{Nd}(T)} values from {+7.0} to {+9.0}, respectively. The potential source of the enclaves is obviously different from the EM2-type mantle from which high-K igneous rocks stemmed (the host rocks), i.e., there is no direct genetic relationship between the enclaves and the host rocks. Deep-derived enclaves in the host rocks belong to mafic xenoliths, and those in the Liuhe-Xiangduo area, eastern Tibet, are some middle-lower crust ortho-metamorphic rocks which were accidentally captured at 20-50 km level by rapidly entrained high-temperature high-K magma, whose source is considered to be located at 50-km depth or so.

Geochronology and Geochemistry of the Magmatic Rocks from Zedong Ophiolite, Eastern Yarlung-Zangbo Suture Zone, Tibet

The Yarlung Zangbo suture zone extends more than2000 km along southern Tibet and marks the boundary between the Indian subcontinent and Eurasia.The Zedong terrane has been not suggested to represent the vestige of such an intra-oceanic arc developed within the Neo-Tethys Ocean,as a result of the northward subduction of the Neo-Tethys Ocean during the Late Jurassic.In this study,we present detailed geochemical and geochronological data of various types of magmatic rocks widely exposed in the Zedong terrane to constrain the formation age and tectonic setting of the Zedong terrane.We found that the Zedong volcanic rocks belong to high K2O calc-alkaline series,whereas the diabase and gabbro plotted in the low-K calcalkline.The basalt rocks are highly enriched in LREE and LILE,but strongly depleted in HFSE,indicating they were derived from a metasomatized mantle.Both gabbros and diabase have similar N-MORB geochemistry indicates that the cumulates were produced from MOR setting.Zircons from four samples,including the basalt rocks(158-161Ma)are older than the gabbro(131 Ma),certificate the gabbro are as the vein intrude into the basalt rocks.This suggests that the volcanic eruption and plutonic emplacement were coevally developed in the Zedonghave similar positiveεHf(t)values(+2.0 to+15.6)and(+8.6 to+18.4),indicating they were stemmed from similarly depleted mantle sources,same with the gabbro and granitic rocks from the Gangdese arc.Therefore,we proposed that the basalt rocks in the Zedong terrane were formed through partial melting of the mantle wedge metasomatized by slab-released fluids/melts.A part of hydrous basalts were underplated in the thickened lower crust beneath the Zedong terrane,which gave rise to the cumulate and granitic rocks.This suggests that the Zedong terrane represents a slice of the active continental margin developed on the southern margin of the Lhasa terrane as a result of the northward subduction of the Neo-Tethys Ocean during the Late Jurassic,although a possible intra-oceanic arc setting cannot be excluded.

Geochemical characteristics of Cenozoic high-K igneous rocks from Liuhe-Xiangduo area, eastern Tibet

The major elements, trace elements and Nd-Sr isotopic composition of Cenozoic high-K igneous rocks and mafic deep-derived enclaves from the Liuhe-Xiangduo area, eastern Tibet, indicate the high-K igneous rocks are characterized as being enriched in Ca (CaO={1.20%}-{8.80%}), alkali (Na-2O+K-2O={3.47%}-{10.65%}), especially K (K-2O up to {5.96%}) and depleted in Ti (TiO-2={0.27%}-{1.50%}). Their REE contents are very high (REE={91.29}-{231.11} μg/g). Their REE distribution patterns are of the right-inclined type, characterized by intense LREE enrichment [(La/Yb)-N={7.44}-{15.73}]. The rocks are distinctly enriched in Rb, Sr and Ba ({46.3}-316 μg/g, 349-1220 μg/g and 386-2394 μg/g, respectively), high in U and Th ({1.17}-{8.10} μg/g and {2.58}-{27.0} μg/g, respectively), moderate in Zr and Hf ({87.5}-241 μg/g and {2.83}-{7.52} μg/g, respectively), and depleted in Nb and Ta ({4.81}-{16.8} μg/g and {0.332}-{1.04} μg/g, respectively). In the primitive mantle-normalized incompatible element spidergram, U, K, Sr and Hf show positive anomalies, whereas Th, Nb, Ta, P, and Ti show negative anomalies. The rocks are strongly depleted in Cr and Ni ({21.4}-1470 μg/g and {7.79}-562 μg/g, respectively), and their transition element distribution curves are obviously of type-W. The ({{}+{87}Sr/+{86}Sr)-i} ratios range from {0.704184} to {0.707539}; ({{}+{143}Nd}/{{}+{144}Nd)-i} from {0.512265} to {0.512564}; and ε-{Nd}(t) from {-6.3} to {-0.4}. These geochemical features might suggest that the potential source of the high-K igneous rocks in the Liuhe-Xiangduo area is similar to the EM2, which may be similar to the material enriched K that is located under the crust-mantle mixed layer. The mafic deep-derived enclaves in the high-K igneous rocks belong to chance xenoliths. Their ({{}+{87}Sr/+{86}Sr)-i} ratios range from {0.706314} to {0.707198}; ({{}+{143}Nd}/{{}+{144}Nd)-i} from {0.512947} to {0.513046}; and ε-{Nd}(t) from {+7.0} to {+9.0}. These geochemical features might indicate that the enclaves probably came from the depleted mantle. The P-T conditions of the enclaves also showed that the enclaves are middle-lower crust metamorphic rocks, which were accidentally captured at 20-50 km level by rapidly entrained high-K magma, whose source is over 50 km in depth.

Provenance and tectonic setting of the Triassic Yidun Group, the Yidun Terrane, Tibet

The Yidun Group extends from the Shangri-La region to the south and the Changtai region to the north,and is an important component of the Triassic Yidun arc in the eastern Tibetan plateau.It is composed of the Lieyi,Qugasi,Tumugou and Lanashan Formations from the base upward.Both the Lieyi and Lanashan Formations consist dominantly of black or gray slate and sandstone,whereas the Qugasi and Tumugou Formations have variable amounts of mafic to felsic volcanic rocks and turfs accompanied with gray slate and sandstone.Sandstone from the Yidun Group has variable CIA values from 55 to 76,indicative of mild to moderate weathering condition for the source rocks.All the sandstones define a general weathering trend nearly parallel to the A-CN boundary in the A-CN-K triangular diagram,implying limited effect of diagenetic and post-depositional K-metasomatism.Dominant detrital quartz and feldspar grains of the sandstones suggest predominantly felsic sources.Relatively high Y/Ni and low Cr/V ratios of sandstones from the Yidun Group indicate more contribution from felsic than mafic sources.Similarly,the Yidun sandstones have Co/Th and La/Sc ratios generally similar to upper continental crust （UCC） and cluster between UCC and felsic sources,indicating felsic rocks as primary sources.Granodiorite represents the average chemical composition of sources as evaluated by extending the predicted weathering trend back to the feldspar join in A-CN-K diagram.Prominently high Zr/Sc ratio or Hf concentration and Paleoproterozoic Nd modal ages （1.94-2.21 Ga）point to input of recycling components derived from old sedimentary source in a relatively stable tectonic setting.

Three-Dimensional S-Wave Velocity Structure in Eastern Tibet from Ambient Noise Rayleigh and Love Wave Tomography

We apply ambient noise tomography to continuous three-component broadband seismic data between January 1,2008 and December 31,2008 from the regional networks of 76 stations de-ployed by China Earthquake Administration.Ambient noise cross-correlations were performed to produce the Green＇s functions of each station-pair.Within the period from 6 to 50 s,Rayleigh and Love wave dispersion curves were measured using the multiple filter analysis method.Then three-dimensional（3-D） S-wave velocity structures from the surface down to 70 km are inverted from both Rayleigh and Love wave dispersion results.The obtained S-wave velocity maps show strong lateral variations and correlate well with the distinct geological and tectonic features in the study area.The Sichuan basin displays low velocity in shallow depth due to thick sedimentary deposits but high velocity in the mid-lower crust;the eastern Tibetan plateau is clearly featured with a low-velocity zone in its mid-to-lower crust which is consistent with the crustal flow model proposed to explain the mechanism of uplift and pattern of deformation for the Tibetan plateau.Meanwhile,our results also exhibit that the crustal thickness decreased from the eastern Tibetan plateau to the Sichuan basin.