Geochemistry of Mesoproterozoic Volcanic Rocks in the Western Kunlun Mountains: Evidence for Plate Tectonic Evolution

Mesoproterozoic volcanic rocks occurring in the north of the western Kunlun Mountains can be divided into two groups. The first group (north belt) is an reversely-evolved bimodal series. Petrochemistry shows that the alkalinity of the rocks decreases from early to late: alkaline→calc-alkaline→tholeiite, and geochemistry proves that the volcanic rocks were formed in rifting tectonic systems. The sedimentary facies shows characteristics of back-arc basins. The second (south belt) group, which occurs to the south of Yutian-Minfeng-Cele, is composed of calc-alkaline island arc (basaltic) andesite and minor rhyolite. The space distribution, age and geochemistry of the two volcanite groups indicate that they were formed in a back-arc basin (the first group) and an island arc (the second group) respectively and indicate the plate evolution during the Mesoproterozoic. The orogeny took place at -1.05 Ga, which was coeval with the Grenville orogeny. This study has provided important geological data for exploring the position of the Paleo-Tarim plate in the Rodinia super-continent.

Rock Series and Genetic Types of Granitoids in the Western Kunlun Orogenic Belt,China

A systematic geological and geochemical study was conducted for the granitoids of different periods in the western Kunlun orogenic belt. The study indicates that the granitoids belong to tholeiitic, calc-alkaline, high-K calc-alkaline, alkaline and shoshonitic series, and that there are 5 genetic types, i.e., I-, S-, M-, A- and SH-type, of which SH-type is first put forward in this paper, which corresponds to shoshonitic granitoids.

Constraints of molybdenite Re-Os and scheelite Sm-Nd ages on mineralization time of the Kukaazi Pb-Zn-Cu-W deposit, Western Kunlun, NW China

The Kukaazi Pb-Zn-Cu-W polymetallic deposit, located in the Western Kunlun orogenic belt, is a newly discovered skarn-type deposit. Ore bodies mainly occur in the forms of lenses and veins along beddings of the Mesoproterozoic metamorphic rocks. Three ore blocks, KⅠ,KⅡ, and KⅢ, have been outlined in different parts of the Kukaazi deposit in terms of mineral assemblages. The KⅠ ore block is mainly composed of chalcopyrite, scheelite,pyrrhotite, sphalerite, galena and minor pyrite, arsenopyrite,and molybdenite, whereas the other two ore blocks are made up of galena, sphalerite, magnetite and minor arsenopyrite and pyrite. In this study, we obtained a molybdenite isochron Re–Os age of 450.5 ± 6.4 Ma(2σ,MSWD = 0.057) and a scheelite Sm–Nd isochron age of 426 ± 59 Ma(2σ, MSWD = 0.49) for the KⅠ ore block.They are broadly comparable to the ages of granitoid in the region. Scheelite grains from the KⅠ ore block contain high abundances of rare earth elements(REE, 42.0–95.7 ppm)and are enriched in light REE compared to heavy REE, with negative Eu anomalies(δEu = 0.13–0.55). They display similar REE patterns and Sm/Nd ratios to those of the coeval granitoids in the region. Moreover, they also have similar Sr and Nd isotopes [ ^(87)Sr/ ^(86)Sr = 0.7107–0.7118;ε_(Nd)(t) =-4.1 to-4.0] to those of such granitoids, implying that the tungsten-bearing fluids in the Kukaazi deposit probably originate from the granitic magmas. Our results first defined that the Early Paleozoic granitoids could lead to economic Mo–W–(Cu) mineralization at some favorable districts in the Western Kunlun orogenic belt and could be prospecting exploration targets.

Integrated geophysical evidence for a new style of continent-continent collision beneath the western Kunlun in the northwestern China

Along the western Kunlun-Tarim-Tianshan geoscience transect in the northwestern China, an integrated geophysical investigation was carried out. Owing to the abominable natural conditions there, the sounding profile could not cross the whole transect, consequentially, a variety of velocity structures in the transverse and vertical orientations beneath the whole transect were not obtained, such as the case within the western Kunlun orogenic belt. To supply a gap of deep seismic soundings within the western Kunlun orogenic belt, we used the Bouguer gravity anomaly data and the relationship between the compressive wave and the density to obtain the density structure of the crust beneath the western Kunlun and the southern Tarim basin by a forward fitting of gravity anomalies within the two-dimensional polygonal model of uniform medium. The crust of the Tarim basin with a rigid basement was like an asymmetrical arc, whose surface feature was the Bachu uplift in the middle of the Tarim basin. Beneath the conjoint area between the Tarim basin and the western Kunlun belt, there was a V-shape structure located just up to the top of the uplifted Moho. The multi-seismological structures jointly revealed that the face-to-face continent-continent collision beneath the western Kunlun is a new structural style within the continent-continent collision zone, which is a real model proved by the numerical modeling.

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.

Petrogenesis of I-type granites from Yuqikapa granite pluton of the western Kunlun orogen: Deformation-driven filter-pressing differentiation

1 Introduction The western Kunlun orogen,located in the northwest Tibet Plateau,and is a conjunction between the Pan-Asian and the Tethys tectonic domains.From north to south,the Western Kunlun orogen includes four

Source process of the 14 November 2001 western Kunlun Mountain M_S=8.1 earth-quake

Based on digital teleseismic P-wave seismograms recorded by 28 long-period seismograph stations of the global seismic network, source process of the November 14, 2001 western Kunlun Mountain MS=8.1 (MW=7.8) earth- quake is estimated by a new inversion method. The result shows that the earthquake is a very complex rupture event. The source rupture initiated at the hypocenter (35.95°N, 90.54°E, focal depth 10 km, by USGS NEIC), and propagated to the west at first. Then, in several minutes to a hundred minutes and over a large spatial range, several rupture growth points emerged in succession at the eastern end and in the central part of the finite fault. And then the source rupture propagated from these rupture growth points successively and, finally, stopped in the area within 50 km to the east of the centroid position (35.80°N, 92.91°E, focal depth 15 km, by Harvard CMT). The entire rupture lasted for 142 s, and the source process could be roughly separated into three stages: The first stage started at the 0 s and ended at the 52 s, lasting for 52 s and releasing approximately 24.4% of the total moment; The sec- ond stage started at the 55 s and ended at the 113 s, lasting for 58 s and releasing approximately 56.5% of the total moment; The third stage started at the 122 s and ended at the 142 s, lasting for 20 s and releasing approximately 19.1% of the total moment. The length of the ruptured fault plane is about 490 km. The maximum width of the ruptured fault plane is about 45 km. The rupture mainly occurred within 30 km in depth under the surface of the Earth. The average static slip in the underground rocky crust is about 1.2 m with the maximum static slip 3.6 m. The average static stress drop is about 5 MPa with the maximum static stress drop 18 MPa. The maximum static slip and the maximum stress drop occurred in an area within 50 km to the east of the centroid position.

Estimation of the stress levels in the focal region before and after the 2001 M=8.1 Western Kunlun Mountain Pass earth-quake

A method estimating the stress level in the focal region of an earthquake is proposed here. Taking the 2001 M=8.1 Western Kunlun Mountain Pass earthquake as an example, we estimate its stress level in the focal region before and after it by this method. The results show that the stress level in the focal region just prior to the initiation of this event is approximately 6.3-8 MPa, and about 5-6.7 MPa remained in the focal region after its occurrence. The stress in the focal region decreased by roughly twenty percent after this event.

Chemical Characteristics of Cr-spinel of Chromites in Kuda Ophiolite of Western Kunlun and Its Geological Implications

The Kuda ophiolite belongs to the early Paleozoic ophiolite belt in Western Kunlun Mountains and is composed of metamorphic peridotites,cumulate peridotites,mafic volcanic rocks and quartzites(Li et al.,

Ordovician Granitoids and Silurian Mafic Dikes in the Western Kunlun Orogen, Northwest China:Implications for Evolution of the Proto-Tethys

The western Kunlun orogen in the northwest Tibet Plateau is related to subduction and collision of Proto-and Paleo-Tethys from early Paleozoic to early Mesozoic. This paper presents new LA-ICPMS zircon U-Pb ages and Lu-Hf isotopes, whole-rock major and trace elements, and Sr–Nd isotopes of two Ordovician granitoid plutons(466–455 Ma) and their Silurian mafic dikes(～436 Ma) in the western Kunlun orogen. These granitoids show peraluminous high-K calcalkaline characteristics, with(^(87)Sr/^(86)Sr)_i value of 0.7129–0.7224, ε_(Nd)(t) values of -9.3 to -7.0 and zircon ε_(Hf)(t) values of -17.3 to -0.2, indicating that they were formed by partial melting of ancient lower-crust(metaigneous rocks mixed with metasedimentary rocks) with some mantle materials in response to subduction of the Proto-Tethyan Ocean and following collision. The Silurian mafic dikes were considered to have been derived from a low degree of partial melting of primary mafic magma. These mafic dikes show initial ^(87)Sr/^(86)Sr ratios of 0.7101–0.7152 and ε_(Nd)(t) values of -3.8 to -3.4 and zircon ε_(Hf)(t) values of -8.8 to -4.9, indicating that they were derived from enriched mantle in response to post-collisional slab break-off. Combined with regional geology, our new data provide valuable insight into late evolution of the Proto-Tethys.

Dual influence of the rejuvenation of Southern Tianshan and Western Kunlun orogen on the Cenozoic structure deformation of Tarim Basin,northwestern China:A superposition deformation model from Bachu Uplift

Based on new high-resolution seismic profiles and existing structural and sedimentary results,a superposition de-formation model for Cenozoic Bachu Uplift of northwestern Tarim Basin,northwestern China is proposed. The model presents the idea that the Bachu Uplift suffered structure superposition deformation under the dual influences of the Cenozoic uplifting of Southern Tianshan and Western Kunlun orogen,northwestern China. In the end of the Eocene (early Himalayan movement),Bachu Uplift started to be formed with the uplifting of Western Kunlun,and extended NNW into the interior of Kalpin Uplift. In the end of the Miocene (middle Himalayan movement),Bachu Uplift suffered not only the NNW structure deformation caused by the Western Kunlun uplifting,but also the NE structure deformation caused by the Southern Tianshan uplifting,and the thrust front fault of Kalpin thrust system related to the Southern Tianshan orogen intrudes southeastward into the hinterland of Bachu Uplift and extends NNE from well Pi1 to Xiaohaizi reservoir and Gudongshan mountain,which resulted in the strata folded and denuded strongly. In the end of the Pliocene (late Himalayan movement),the impact of Southern Tianshan orogen decreased because of the stress released with the breakthrough upward of Kalpin fault extending NE,and Bachu Uplift suffered mainly the structure deformation extending NW-NNW caused by the uplifting of Western Kunlun orogen.

TECTONIC EVOLUTION OF THE WESTERN KUNLUN AND KARAKORAM MOUNTAINS—SOME NEW OBSERVATIONS FROM A MULTI-DISCIPLINARY GEOSCIENTIFIC TRANSECT (MGT) IN NW TIBET

New results from deep seismic sounding reveal that the Tarim block (basin) was subducted, with gently dipping, to the W.Kunlun Mts. The depths of Moho from north to south are 50km in S.Tarim to 65km in the W. Kunlun Mts. A low velocity and low resistivity layer occurs at the depth of 15～30km, which may be the decollement in the upper crust. A “delamination Crack" might have been formed in the lower crust\|mantle lid, causing the intrusion of alkaline magma along the southern margin of the W. Kunlun Mts. A detailed research for forming and evolving of the Kudi ophiolite in the central part of the W. Kunlun Mts.,which has been given the age of 700～450Ma by using the Re\|Os isotopic dating, and the Radiolarits in chert intercalated with pillow lava are determined to be the late Ordovician to Silurian. Until now we did not find any complete ophiolite suite except Kudi ophiolite. So we suggest that there was no “vast protero\|ocean", and a “limited oceanic basin" exists only during the late Proterozoic to Early Paleozoic, which might have been subducted in Silurian\|Devonian and formed the oldest suture in the north of the Tibet plateau.A suture zone consisting of H. P/T metamorphic blueschist, ophiolitic melange and olistostrome in the north\|western Tibet has been revealed recently, which may extend to NW. connecting with MKT in N. Pakistan.A study of the Fission track thermochronology indicates that the W.Kunlun Mts. has been undergone a pulsatory uplift since 20～18Ma,and a rapid uplift after 3Ma.A model of collision between the Indo\|Pakistan subcontinent\|N.Tibet and the Eurasian blocks of Tarim has been illustrated.

EARLY PALEOZOIC SUBDUCTION-RELATED ACCRETIONARY COMPLEX IN WESTERN KUNLUN

Previous studies on ophiolite and granite revealed that a paleo\|ocean which was referred to as “Proto\|Tethys" developed in north part of western Kunlun in Pt 3～Pz 1.The geotectonic unit in Kuda, western Kunlun is composed of early Paleozoic accretionary complexes and the metamorphic equivalents due to subduction of Proto\|Tethys: Along the Xinzang highway (from Yecheng, Xinjiang to Shiquanhe, Tibet), about 30km north of Kuda, a schistose granodiorite intrudes a thick unit of deformed mafic rocks; Non to weakly metamorphosed early Paleozoic fore\|arc turbidite exposed in Yixiekegou; A set of metamorphic rocks about 20km wide in Kuda which was thought of the Precambrian basement is a large\|scale ductile shear zone determined by detailed field observation and laboratory research; About 10km north of Kuda, there is an undeformed potassic granite batholith intrudes the shear zone.The shear zone mainly consists of parametamorphic rocks and orthometamorphte rocks. Moreover, it contains various kinds of oceanic materials of different origins including fragments of seamounts, oceanic reef limestone and dismembered ophiolite suite, such as dunite, peridotite, gabbro and anorthisite with high number up to 91. The dismembered ophiolite suite may probably represents a remnant oceanic crust that related to the breakup of Rodinia. The parametamorphite rocks is composed of mica\|schist, quartzite and gamete\|mica schist,etc., of which protoliths are sedimentary rocks such as mudstone, sandstone, siliciclastic rocks and impure carbonates that most likely accumulated along the passive continental margin of pelagic sedimentary materials. The orthometamorphic rocks is composed of granitic gneiss and amphibolite,etc. The protoliths of amphibolite is oceanic island basalt of MORB basalt that was accretioned in accretionary complex. The accretionary wedge complex suffers intense ductile strain and metamorphism due to the subduction of Proto\|Tethys with the metamorphic grade reaching the greenschist facies to amphibolite facies. The 40 Ar/ 39 Ar age 451Ma of metamorphic homoblende in amphibolite is interpreted to closely date attainment of maximum metamorphic conditions representing the age of initial subduction at the latest, while the 40 Ar/ 39 Ar age 425Ma of biotite is interpreted as dating the post metamorphic cooling through temperatures required for intracrystalline retention of argon representing the cease of subduction. These data provide significant information on the timing of accretion and the age of the subducted Proto\|Tethys. The whole rock Sm\|Nd isochronal age of amphibolite is problematic 737Ma that maybe represent the rifting age of the Rodinia.

Two granitic plutons in Central Western Kunlun Belt: Their ages and possible sources

Geochronological and isotope studies have been carried out for two important plutons in the Central Western Kunlun Belt. U-Pb single grain zircon dating results show that the North Kudi Pluton (404 Ma) was produced near the end of Caledonian; whereas the previously labeled Hercynian Arkarz Mountain Pluton (215 Ma) is either an Indosinian pluton or a product

Geochronological evidence of Indosinian(high-pressure) metamorphic event and its tectonic significance in Taxkorgan area of the Western Kunlun Mountains,NW China

The CL images,LA-ICP-MS in situ trace elements analysis,and U-Pb dating for zircons indicate that the metamorphic ages of the sillimanite-garnet-biotite gneiss and the garnet-amphibole gneiss from eastern Taxkorgan of the Western Kunlun Mountains are 220±2 and 220±3 Ma respectively,and their protolith ages are younger than 253±2 and 480±8 Ma respectively.Two samples were collected at the same outcrops with HP mafic granulite and HP pelitic granulite.Mineral assemblage of the sillimanite-garnet-biotite gneiss(Grt+Sill+Per+Q) is consistent with that of HP pelitic granulite at early high amphibolite-granulite facies stage.Mineral assemblage of the garnet-amphibole gneiss(Grt+Amp+Pl+Q) is consistent with retro-metamorphic assemblage of HP mafic granulite at amphibolite facies stage.The dating results suggest that these HP granulites underwent peak metamorphism at 220±2 to 253±2 Ma.Thus,the Kangxiwar tectonic zone was probably formed by subduction and collision of the Paleo-Tethys Ocean during Indosinian.Protolith ages of the two samples,together with previously published U-Pb zircon dating age,suggest that the sillimanite-garnet schist-quartzite unit is a late Paleozoic unit,not a part of the Paleoproterozoic Bulunkuole Group.

History of tectono-magmatic evolution in the Western Kunlun Orogen

Based on the statistical and analytical data on more than 170 isotopic ages published since the 1980s of magmatic rocks, metamorphic rocks, tectonites and ores from the Western Kunlun Orogen, and the characteristics of sedimentation, magmatism, metamorphism and tectonic activities in the region studied in conjunction with geological field investigations and necessary supplementary isotope data, five stages of tectono-magmatic evolution, i.e. Ar3-Pt21 tectono-magmatic active stage (Ⅰ) , Pt22 stable stage (Ⅱ), Pt31-P2 active stage (Ⅲ), T1-T2 stable stage (Ⅳ) , and T3-Q active stage (Ⅴ) can be distinguished in the Western Kunlun Orogen. Moreover, the tectono-magmatic active style and general trend, the characteristics of tectonic settings, etc. of each stage and substage in the region studied are also discussed .

High-pressure granulite from Western Kunlun,northwestern China:Its metamorphic evolution,zircon SHRIMP U-Pb ages and tectonic implication

High-pressure mafic granulites occurring as lenticular bodies within garnet-amphibolites in Kangxiwar Fault have been first reported in this paper. The P-T conditions of two metamorphic stages were ob-tained using calibrated geothermal barometers and ThermoCalc Program. The peak metamorphic con-dition of these high-pressure granulites is about 760―820℃,1.0―1.2 GPa and the retrograde meta-morphic condition is about 620―720℃,0.7―0.8 GPa. The petrological studies show that they have a near-isobaric cooling P-T path which suggests that the Western Kunlun underwent initial crustal thickening,subsequent exhumation and cooling. The SHRIMP zircon U-Pb dating gives two groups of ages for high-pressure granulites. One is 177±6 Ma which is obtained from the rim of the zircon. We consider this age should be the metamorphic age. And the other is 456±30 Ma which is obtained from the core of the zircon and should be the protolith age. The formation of these high-pressure granulites in western Kunlun is closely correlated with the evolution of the Paleo-Tethys and has important im-plications for the research on Tethys and Paleo-Asian tectonic zone.

Platinum-group elements for the Kudi ophiolite, western Kunlun

The analyses of PGE characteristics for the Kudi ophiolite, western Kunlun, are reported in this note. The Ir-depleted anomalous ancient upper mantle represented by the Kudi mantle peridotites had been percolated and mixed by remelted oceanic crust material from a supra-subduction zone, leaving the rocks relatively enriched in Pd. The PGE contents in the Kudi cumulate pyroxenites and gabbros vary from 2.4 to 23.94 ng/g, and the highest values are in pyroxenites. The Pd/lr ratios increase from the pyroxenites to the gabbros. The PGE patterns for the Kudi cumulate rocks are steeply positively sloped. The Pd enrichment in these rocks is similar to that in pyroxenites from the Thetford ophiolite and the Troodos ophiolite. The Kudi diabase and basalts have low PGE contents and positively sloped PGE patterns, and are comparable with the upper pillow lava?from the Troodos ophiolite, indicative of the environment of back-arc spreading center in a supra-subduction zone.

The large-scale ductile shear zone in Kuda, western Kunlun

A set of metamorphic suites, previously known as the Precambrian metamorphic basement in Kuda, western Kunlun has been determined as a large nappe ductile shear zone which develops penetrative foliation, oriented stretching lineation, various but consistent kinematic indicators of thrusting from north to south. The microstructure and the

Thrust Propagation in the Aqqikkol Lake Area, the East Kunlun Mountains, Northwestern China

The western segment of the East Kunlun Mountains is one of the poorly studied regions in northwestern China. Through a structural analysis of the typical sections, we have the following views: (1) There is a very well developed fault system in the western segment of the East Kunlun Mountains and thrust propagation, normal slip and decoupling are the chief deformation events in this area. (2) Although the thrusting started in the Late Carboniferous and Late Triassic-Early Jurassic, strong activity took place in the Miocene-Quaternary when the Kumkol basin was strongly downwarped. (3) The tectonic pattern of coexistence of N-directed thrust propagation and S-directed normal slip in this area is consistent with the general tectonic pattern of the northern Qinghai-Tibet plateau and also very similar to that of the Himalayan region on the southern margin of the Qinghai-Tibet plateau, but their directions between the thrust propagation are opposite and all the strong thrust propagations occurred from the Miocene-Pliocene to Quaternary, a period featuring strong collision between the Indian plate and the Eurasian plate and abrupt uplift of the Qinghai-Tibet plateau. This oppositely directed thrust propagation and normal slip reveal such kinematic characteristics as symmetric propagations of deep-seated materials towards the north and south beneath the Qinghai-Tibet plateau and gravitational sliding of superficial materials towards the interior of the plateau. Therefore, the establishment of the fault system in the study area may provide an approach to the study of deep processes of the northern Qinghai-Tibet plateau and the construction of a unified geodynamic model for the uplift of the Qinghai-Tibet plateau.