Magma source and tectonic setting of the Dunde granite in the Western Tianshan:constraints from geochronology,geochemistry,and Sr–Nd–Hf isotopes

The Dunde iron-zinc polymetallic deposit is one of large iron deposits occurred in the Awulale Metallogenetic Belt,Western Tianshan(NW-China).This study reports new geochronology and geochemistry for granite in the Dunde mining area in order to constrain the tectonicmagmatic activities and metallogenesis of this region.Granites in the southwest of Dunde mining area are mainly syenogranites intruded into volcanics of the Dahalajunshan Formation in the Early Carboniferous,and they are far from the area where ore bodies and mineralized altered rocks are widely developed.LA-ICP-MS U–Pb zircon dating indicates that Dunde syenogranite was at 306.8±1.0 Ma,which could constrain the upper limit of metallogenic age for this deposit.The Dunde granites are high SiO_(2)(73.41–80.07 wt%),high differentiation index(D.I.=89.7–95.0),weakly peraluminous to metaluminous(A/CNK=0.94–1.08),and they are enriched in LILE and LREE and depleted in Eu,Ba,Sr and P_(2)O_(5),indicating that they belong to highly fractionated Ⅰ-type granite.Based on εHf values(+9.2 to+10.5)for zircon and high εNd(-t)values(+4.7 to+5.8)for whole-rock,and the two-stage model ages for 601–735 Ma,suggest that the magma source could be the juvenile lower crust.Combined with regional geological setting,the 306.8 Ma Dunde granites are formed in post-collision extensional tectonic setting.

Redetermination of the Depositional Age of the Haerdaban Group in the Northern Margin of the Yili Block,Western Tianshan,NW China:Implications for Regional Tectonics and Pb-Zn Mineralization

The Yili Block in the Western Tianshan orogen is a key area for understanding the early crustal formation and evolution of the Central Asian orogenic belt,due to the widely-distributed Precambrian rocks.Also,it hosts a lot of medium–to large-scale sedimentary exhalative(SEDEX)Pb-Zn deposits that mainly occur in Proterozoic metamorphosed clasticcarbonate rocks.In this study,LA-ICP-MS U-Pb analyses were carried out on detrital zircons in siltstones of the Precambrian Haerdaban Group in the Haerdaban Pb-Zn deposit and magmatic zircons in the diorite dyke that cuts through the strata and orebodies.The maximum depositional age of the siltstones was determined to be about 604 Ma,the diorite having formed at approximately 500 Ma.As such,the Haerdaban Group was most likely formed in the Neoproterozoic Sinian,rather than the previously considered Mesoproterozoic Changchengian.Detrital materials of the Haerdaban Group were mostly derived from the Nanhua–Sinian mafic dykes and granitic rocks around Lake Sayram at the northern margin of the Yili Block.It is proposed that the Yili Block,together with the Kazakhstan and Central Tianshan blocks and the Tarim Craton,might all pertain to the same Rodinia supercontinent,which has great potential for targeting large to super-large SEDEX Pb-Zn deposits.

Abiotic Methane Reservoirs in the Western Tianshan HP–UHP Metamorphic Belt,China

Natural gas,consisting primarily of methane(CH_(4)),has become a major source of clean energy in modern society in many parts of the globe.Recent experimental observations and discoveries of deep-sourced abiotic CH_(4)in cold subduction zones indicate the important ability of cold subducted slabs to generate natural gas reservoirs.However,most CH_(4)flux and reservoirs remain unknown and their potential is overlooked in global carbon flux estimations.Massive abiotic CH_(4)-rich fluid inclusions(FIs)in garnet and omphacite from ultrahigh-pressure(UHP)eclogites have been found in the Western Tianshan(WT)UHP metamorphic belt,which provides one ideal case for quantification of abiotic CH_(4)stored in the cold subducted crust.By two methods,we assess the abiotic CH_(4)content stored in the Chinese WT HP–UHP metamorphic belt.Our calculations show that at least 113 Mt CH_(4)is stored in the WT eclogites.We also discuss the implications for CH_(4)reservoirs in subduction zones worldwide and speculate that the cold subduction zones may represent one of the largest,yet overlooked,sources of abiotic CH_(4)on Earth,which should not be ignored in the global natural resource and carbon flux estimations.

Geological Characteristics and Zircon U-Pb Dating of Volcanic Rocks from the Beizhan Iron Deposit in Western Tianshan Mountains, Xinjiang, NW China

The Beizhan large iron deposit located in the east part of the Awulale metallogenic belt in the western Tianshan Mountains is hosted in the Unit 2 of the Dahalajunshan Formation as lens, veinlets and stratoid, and both of the hanging wall and footwall are quartz-monzonite; the dip is to the north with thick and high-grade ore bodies downwards. Ore minerals are mainly magnetite with minor sulfides, such as pyrite, pyrrhotite, chalcopyrite and sphalerite. Skarnization is widespread around the ore bodies, and garnet, diopside, wollastonite, actinolite, epidote, uralite, tourmaline sericite and calcite are ubiquitous as gangues. Radiating outwards from the center of the ore body the deposit can be classified into skarn, calcite, serpentinite and marble zones. LA-ICP-MS zircon U-Pb dating of the rhyolite and dacite from the Dahalajunshan Formation indicates that they were formed at 301.3±0.8 Ma and 303.7±0.9 Ma, respectively, which might have been related to the continental arc magmatism during the late stage of subduction in the western Tianshan Mountains. Iron formation is genetically related with volcanic eruption during this interval. The Dahalajunshan Formation and the quartz-monzonite intrusion jointly control the distribution of ore bodies. Both ore textures and wall rock alteration indicate that the Beizhan iron deposit is probably skarn type.

Permo-Carboniferous Radiolarians from the Wupata'erkan Group,Western South Tianshan, Xinjiang, China

The Wupata'erkan Group, also called Wupata'erkan Formation, distributed in the South Tianshan, Xinjiang, China, mainly consists of gray and dark gray fine-grained clastic rocks, interlayered with volcanic rocks, carbonates and cherts. Some ultra-basic rocks (blocks) punctuate the formation. The formation was variously assigned to Silurian-Middle Devonian, Silurian-Lower Devonian, and pre-Devonian, mainly based on Atrypa bodini Mansuy, Hypothyridina parallelepipedia (Brour.) and Prismatophyllum hexagonum Yoh collected from the limestone interlayers, respectively. However, radiolarian fossils obtained from 24 chert specimens of the Wupata'erkan Group, mainly include Albaillella sp. cf. A. undulata Deflandre, Albaillella sp. cf. A. paradoxa Deflandre, Albaillella cf. A. deflandrei Gourmelon, Albaillella sp. cf. A. indensis Won, Albaillella sp. cf. A. excelsa Ishiga, Kito and Imoto, Albaillella sp. and Latentifistulidae gen. et. sp. indet., are earliest Carboniferous and Late Permian. The earliest Carboniferous assemblage is characterized by Albaillella sp. cf. A. undulata Deflandre, Albaillella sp. cf. A. paradoxa Deflandre, Albaillella cf. A. deflandrei Gourmelon and Albaillella sp. cf. A. indensis Won, and the Late Permian assemblage by Albaillella sp. cf. A. excelsa Ishiga, Kito and Imoto. This new stratigraphic evidence indicates that the Wupata'erkan Group is possibly composed of rocks with different ages from Silurian to Permian, and therefore, it is probably an ophiolite melange. The discovery of Late Permian Albaillella sp. cf. A. excelsa provides more reliable evidence supporting the existence of a Permian relic ancient oceanic basin in the western part of Xinjiang South Tianshan.

Discovery of Radiolarian Fossils from the Aiketik Group at the Western End of the South Tianshan Mountains of Chinaand Its Implications

The Aiketik Group, distributed at the western end of the South TianshanMountains, China, is an important lithostratigraphic unit involved in the South Tianshan orogen. Itis separated from the adjacent rocks by faults. Generally, the geologists ascribed it to the UpperCarboniferous according to Pseudostaffella sp., Profusulinella sp. and Fusulinella sp. found fromthe limestone and sandy limestone of Aiketik. Our radiolarian fossils were obtained from the chertsamples collected from the Haladaok section located at the upper Tuoshihan River. The fossils mainlyinclude Albaillella undulata Deflandre, Albaillella paradoxa Deflandre, Albaillella sp. aff. A.paradoxa Deflandre, Albaillella sp. cf. A. deflandrei Gourmelon, Albaillella sp., Albaillellaexcelsa Ishiga, Kito and Imoto (?), Belowea variabilis (Ormiston et Lane), Callella cf. C.parvispinosa Won, Entactinia cf. E. tortispina Ormiston et Lane, Entactinia aff. E. tortispinaOrmiston et Lane, Entactinia variospina Won, Entactinia sp., Eostylodictya rota (Won),Latentifistula impella (Ormistone et Lane) (?), Latentifistula turgita Omiston et Lane,Latentifistulidae gen. et. sp. indet. and Polyentactinia cf. aranea Gourmelon. Among them,Albaillella excelsa Ishiga, Kito and Imoto (?) is a Late Permian species with some elementsuncertain as there is only one poorly-preserved fossil of this species found so far. And tworadiolarian assemblages can be identified from the other fossils. One is the early EarlyCarboniferous assemblage represented by Albaillella undulata Deflandre, Albaillella paradoxa andAlbaillella sp. cf. A. deflandrei Gourmelon. And the other is the late Early Carboniferousassemblage represented by Eostylodictya rota (Won). This is the first discovery of radiolarianfossils in the Aiketik Group, also the first discovery of Late Permian radiolarian fossils in theSouth Tianshan Mountains. Meanwhile, this is the current westernmost sampling site of radiolarianfossils in the South Tianshan Mountains.

Metallogenic Epoch and Ore-forming Environment of the Lamasu Skarn-porphyritic Cu-Zn deposit,western Tianshan,Xinjiang,NW China

Granitic rocks, widely developed in the Lamasu copper ore region, western Tianshan were formed at 390.5±7.7 Ma according to the SHRIMP zircon U-Pb dating of the plagioclase granite porphyry. Based on the regional tectonic evolution and published chronological data of both diagenesis and mineralization, the Biezhentao- Kokirqin region was rolled into the orogen associated with the closure of Yili Ocean during early Devonian. The N-S-trending thrust faults were formed during this period and accompanied by the intrusion of granitic rocks. On this stage, the paleo-Asian Ocean Plate entered into the early collision orogenic phase and the plagioclase granite porphyry intruded （390.5±7.7 Ma） and replaced with limestone of the Mesoproterozoic Kusongmuqieke Group, Jixianian System and formed the early phase of skarn-type copper mineralization in the Lamasu region. Furthermore, the subduction-melting of Bayingou Ocean Plate during Carboniferous generated a deep-seated magmatic chamber in the Lamasu copper ore region which located in the northwestern part of the Paleozoic Biezhentao-Kokirqin island arc. The magmatic chamber segregated Cu-bearing magmas, which transported upward to the shallow earth crust along the faults or fractures and formed the Cu-hosting porphyry. According to the research on the characteristics of the ore deposit and the ore-forming environment as mentioned above, the Lamasu Cu-Zn deposit was characterized by the superposing of mineralization at different geological settings and it was skarn-porphyritic type.

Association of Late Paleozoic Adakitic Rocks and Shoshonitic Volcanic Rocks in the Western Tianshan, China

The late Paleozoic adakitic rocks are closely associated with the shoshonitic volcanic rocks in the western Tianshan Mountains, China, both spatially and temporally. The magmatic rocks were formed during the period from the middle to the late Permian with isotopic ages of 248-268 Ma. The 87Sr/86Sr initial ratios of the rocks are low in a narrow variation range (-0.7050). The 143Nd/144Nd initial ratios are high (-0.51240) with positive εND(t) values (+1.28-+4.92). In the εNd(t)-(87Sr/86Sr)i diagram they fall in the first quadrant. The association of the shoshonitic and adakitic rocks can be interpreted by a two-stage model: the shoshonitic volcanic rocks were formed through long-term fractional crystallization of underplated basaltic magma, while the following partial melting of the residual phases formed the adakitic rocks.

Redox evolution of western Tianshan subduction zone and its effect on deep carbon cycle

Knowing the phase relations of carbon-bearing phases at high-pressure(HP) and high-temperature(HT) condition is essential for understanding the deep carbon cycle in the subduction zones.In particular,the phase relation of carbon-bearing phases is also strongly influenced by redox condition of subduction zones,which is poorly explored.Here we summarized the phase relations of carbon-bearing phases(calcite,aragonite,dolomite,magnesite,graphite,hydrocarbon) in HP metamorphic rocks(marble,metapelite,eclogite) from the Western Tianshan subduction zone and high-pressure experiments.During prograde progress of subduction,carbonates in altered oceanic crust change from Ca-carbonate(calcite) to Ca,Mg-carbonate(dolomite),then finally to Mgcarbonate(magnesite) via Mg-Ca cation exchange reaction between silicate and carbonate,while calcite in sedimentary calcareous ooze on oceanic crust directly transfers to high-pressure aragonite in marble or amorphous CaCO3 in subduction zones.Redox evolution also plays a significant effect on the carbon speciation in the Western Tianshan subduction zone.The prograde oxygen fugacity of the Western Tianshan subduction zone was constrained by mineral assemblage of garnet-omphacite from FMQ-1.9 to FMQ-2.5 at its metamorphic peak(maximum P-T) conditions.In comparison with redox conditions of other subduction zones,Western Tianshan has the lowest oxygen fugacity.Graphite and light hydrocarbon inclusions were ubiqutously identified in Western Tianshan HP metamorphic rocks and speculated to be formed from reduction of Fe-carbonate at low redox condition,which is also confirmed by high-pressure experimental simulation.Based on petrological observation and high-pressure simulation,a polarized redox model of reducing slab but oxidizing mantle wedge in subduction zone is proposed,and its effect on deep carbon cycle in subduction zones is further discussed.

Differential Evolution of High-Pressure and Ultrahigh-Pressure Metapelites from Habutengsu, Chinese Western Tianshan: Phase Equilibria Modelling and ^(40)Ar/^(39)Ar Geochronology

Metapelite is one of the predominant rock types in the high-pressure–ultrahigh-pressure（HP–UHP） metamorphic belt of western Tianshan, NW China; however, the spatial and temporal variations of this belt during metamorphism are poorly understood. In this study, we present comparative petrological studies and 40^Ar/39 ^Ar geochronology of HP and UHP pelitic schist exposed along the Habutengsu valley. The schist mainly comprises quartz, white mica, garnet, albite and bluish amphibole. In the Mn O–Na2O–Ca O–K2O–Fe O–Mg O–Al2O3–Si O2–H2O（Mn NCKFMASH） system, P–T pseudosections were constructed using THERMOCALC 333 for two representative pelitic schists. The results demonstrate that there was a break in the peak metamorphic pressures in the Habutengsu area. The northern schist has experienced UHP metamorphism, consistent with the presence of coesite in the same section, while the southern one formed at lower pressures that stabilized the quartz. This result supports the previous finding of a metamorphic gradient through the HP–UHP metamorphic belt of the Chinese western Tianshan by the authors. Additionally, phengite in the northern schist was modelled as having a Si content of 3.55–3.70（a.p.f.u.） at the peak stage, a value much higher than that of oriented matrix phengite（Si content 3.32–3.38 a.p.f.u.）. This indicates that the phengite flakes in the UHP schist were subjected to recrystallization during exhumation, which is consistent with the presence of phengite aggregates surrounding garnet porphyroblast. The 40^Ar/39^ Ar age spectra of white mica（dominantly phengite） from the two schists exhibit similar plateau ages of ca. 315 Ma, which is interpreted as the timing of a tectonometamorphic event that occurred during the exhumation of the HP–UHP metamorphic belt of the Chinese western Tianshan.

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.

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.

Variations of Snow Temperature and their Influence on Snow Cover Physical Parameters in the Western Tianshan Mountains,China

This article discussed about snow temperature variations and their impact on snow cover parameters.Automatic temperature recorders were used to sample at 10-minute intervals at the Tianshan Station for Snow-cover and Avalanche Research,Chinese Academy of Sciences.10-layer snow temperature and the snow cover parameters were measured by the snow property analyzer (Snow Fork) in its Stable period,Interim period and Snow melting period.Results indicate that the amplitude of the diurnal fluctuation in the temperature during Snow melting period is 1.62 times greater than that during Stable period.Time up to the peak temperature at the snow surface lags behind the peak solar radiation by more than 2.5 hours,and lags behind the peak atmospheric temperature by more than 0.2 hours during all three periods.The optimal fitted function of snow temperature profile becomes more complicated from Stable period to Snow melting period.22 h temperature profiles in Stable period are the optimal fitted by cubic polynomial equation.In Interim period and Snow melting period,temperature profiles are optimal fitted by exponential equation between sunset and sunrise,and by Fourier function when solar radiation is strong.The vertical gradient in the snow temperature reaches its maximum value at the snow surface for three periods.The peak of this maximum value occurs during Stable period,and is 4.46 times greater than during Interim period.The absolute value of temperature gradient is lower than 0.1°C cm-1 for 30 cm beneath snow surface.Snow temperature and temperature gradient in Stable period～Interim period indirectly cause increase (decrease) of snow density mainly by increasing (decreasing) permittivity.While it dramatically increases its water content to change its permittivity and snow density in Snow melting period.

The Spatial-Temporal Distribution Patterns of Dyke Swarms in Central Asia and their Tectonic Significance: Case Studies in Eastern Tianshan and Western Junggar

Dykes are a special kind of intrusive rocks which were formed by deep magma intruded into the existing brittle fractures in the crust.Dykes swarms in different tectonic environments are very significant to re-construct the

Constraints of REE and trace elements of high-pressure-veins and host rocks in western Tianshan on origin of deep fluids in paleosubduction zones

The western Tianshan high-pressure(HP) metamorphic belt represents the paleosubduction mélange of paleozoic south Tianshan ocean between the Yili-central Tianshan and Tarim plates. High-pressure veins are extensively developed in this HP belt. Compared with normal mid-ocean ridge basalt(N-MORB), the high-pressure veins and host rocks are enriched in light rare earth elements(LREE) and incompatible elements. But high-pressure veins show a larger variation than host rocks in total REE abundance. On the trace element spidergram, all the samples are enriched in incompatible elements and show strong positive Pb anomaly relative to N-MORB. The array tendency lines of plots of the HP rocks have apparent slopes on diagram of m(Sr) vs m(Sr)/m(Zr), m(Li) vs m(Li)/m(Y) and (m(Ce)) vs m(Ce)/m(Pb), which indicates that the enrichment in LREE and incompatible elements relative to N-MORB of the HP-metamorphic rocks from western Tianshan is not attributed to magma evolution process of the protolith. High field strength elements, such as Nb, Ta, Ti, Zr and Hf, do not show negative anomaly relative to N-MORB, which is dissimilar to that of island arc basalts. Therefore, the enrichment in LREE and incompatible elements of the HP rocks is not attributed to the enrichment of the source of the protolith. The mass ratios of Rb to Ba, Ce to Pb, Nb to U and Ta to U of high-pressure veins and host rocks are intermediate of mid-ocean ridge basalt or oceanic island basalt and continental crust. The fluids in western Tianshan paleosubduction zones are mixtures of two sources, dehydration or devolatilization of host rocks and of subducted sediments.

Analysis on Variations of the Temperature and Precipitation in North Slope Area of the Western Tianshan in Recent 50 Years

[Objective] The research aimed to analyze variation characteristics of the temperature and precipitation in north slope area of the Western Tianshan in recent 50 years.[Method] According to temperature and rainfall data from 1961 to 2010 at three meteorological stations in north slope area of the Western Tianshan,climate change in the zone in recent 50 years was analyzed by using linear trend analysis method and 5-year sliding average method.[Result] The temperature in north slope area of the Western Tianshan showed significant warming trend.The annual average temperature significantly increased at 0.3 ℃/10 a in recent 50 years.But rise rates of the temperature in different seasons were different:autumn> winter> spring> summer.The annual rainfall in north slope area of the Western Tianshan in recent 50 years overall presented relative increase trend.Seen from annual rainfall,it significantly increased at 16.9 mm/10 a.Seen from seasonal rainfall,except in autumn,rainfalls in other three seasons in the past 50 years all showed significant increase trends,with amplitude of 3.2-11.2 mm/10 a.The largest increase amplitude was in summer.After a small precipitation peak in the 1980s,autumn rainfall slightly declined in the past 20 years.[Conclusion] The climate in north slope area of the Western Tianshan become warm and wet.

Spatiotemporal variations in the growth status of declining wild apple trees in a narrow valley in the western Tianshan Mountains, China

Malus sieversii(wild apple tree),only distributed in the Tianshan Mountains in Central Asia,is a tertiary relic species and an ancestral species of cultivated apples.However,existing natural populations of wild apple trees have been declining.To date,spatiotemporal variations in the growth status of declining wild apple trees and influencing factors in the narrow valley areas in the Tianshan Mountains remain unclear.In this study,field investigation and sampling were carried out in three years(2016-2018)at four elevations(1300,1400,1500,and 1600 m)in the Qiaolakesai Valley(a typical longitudinal narrow valley in the Yili River Valley)of the western Tianshan Mountains in Xinyuan County,Xinjiang Uygur Autonomous Region,China.Projective coverage,dead branch percentage,and 18 twig traits(these 20 parameters were collectively referred to as plant traits)were determined to comprehensively reflect the growth status of declining wild apple trees.The values of dead branch percentage ranged from 36%to 59%,with a mean of 40%.Year generally showed higher impact on plant traits than elevation.In 2017 and 2018,projective coverage,leaf size,leaf nitrogen concentration,and nitrogen to phosphorous ratio were markedly higher than those in 2016.However,dead branch percentage and leaf and stem phosphorous concentrations showed the opposite trend.Most of the topological parameters of plant trait networks differed in the three years,but the strength of trait-trait association increased year by year.The mean difference between day and night temperatures(MDT),annual accumulative precipitation,soil electrical conductivity,and soil pH had the greatest impact on the plant trait matrix.The growth status of declining wild apple trees was directly and positively affected by MDT and leaf size.In conclusion,the growth of declining wild apple trees distributed in the narrow valley areas was more sensitive to interannual environmental changes than elevation changes.The results are of great significance for further revealing the decline mechanism of wild apple trees in the Tianshan Mountains.

THE CHARACTERISTICS OF TECTONIC FACIES IN THE WESTERN TIANSHAN MT. AND ITS ADJACENT AREA,NORTHWESTERN CHINA

Based on the theory of plate tectonics, the authors combined the concept and analytic methods of tectonic facies, and divided the western Tianshan Mt. and its adjacent area into eight tectonic facies. i. e.① Kuluktage rejuvenational foreland basement fold-thrust facie, ②Kalpin rejuvenational foreland basement fold-thrust facie,③ Kuqa rejuvenational foreland fold-thrust facie. ④ Southern Tianshan backarc foreland mollasse facie. ⑤ Southern Tianshan Late Paleozoic magmatic arc facie. ⑥ Southern Tianshan backarc melange facie. ⑦ Central Tianshan composite magmatic are facie, and ⑧ Northern Tianshan foreare melange facie. F1nally. we reconstructed the history of the western Tianshan Paleozoic tectonic evolution.

Fluid flow on centimeter-scale in deep paleosubduction zones in western Tianshan,China:Evidence from high-pressure veins

High-pressure(HP)veins were extensively developed in western Tianshan high-pressure(HP)metamorphic belt.The HP vein and host-rocks were analyzed by electronic microprobe to trace the origin of vein-forming fluids.Analytical data show that the immediately adjacent host-rocks of the studied HP vein are eclogites and gradually turned into blueschist as the distance from the veins increases,which indicates that the vein-forming fluid was derived from adjacent host-rocks;the boundaries between the vein and the host-rocks are sharp,which indicates that the fracture of the host-rocks is brittle during the vein-forming process.It is suggested that this type of HP veins is precipitated from the liquid formed by the dehydration of the host-rocks during the prograde metamorphism from blueschist to eclogite facies,which results in hydrofracturing of the rocks and provides the space for the vein to precipitate.The width of the eclogite-facies host-rocks is usually 1-2 cm,which provides the direct evidence that the fluid flow is on centimeter-scale.

西天山乌孙山花岗岩和闪长岩年代学、地球化学及岩石成因

笔者选取位于伊犁盆地内乌孙山北缘察布查尔林场花岗岩为研究对象,其地球化学、地质年代学和岩石成因研究对于探讨西天山南缘壳幔岩浆作用具有较重要的指示意义。岩石地球化学特征显示二长花岗岩属高钾钙碱性准铝质-弱过铝质岩石系列;轻稀土富集,弱的Eu负异常(δEu=0.74~0.84),富集大离子亲石元素,亏损Nb、Ta、P、Ti等高场强元素特征,Zr/Hf=42~44,部分样品中含有少量的刚玉标准矿物,显示出壳源花岗岩的特征。闪长岩Al_(2)O_(3)、FeOT、CaO含量明显高于二长花岗岩,Na_(2)O和K_(2)O含量低于二长花岗岩,属于高钾准铝质岩石系列;轻稀土富集,显示出Eu的轻微正异常(δEu=0.90~1.24),富集大离子亲石元素,亏损Nb、Ta等高场强元素,此外,闪长岩还显示出具有高Sr(Sr>400×10^(-6))低Y(12.83×10^(-6))及Yb(1.34×10^(-6))和高Mg^(#)特征,其源岩应为俯冲板片上覆地幔部分熔融产物。二者均显示出岛弧岩浆的特征。二长花岗岩锆石U-Pb年龄为(361.7±1.8)Ma;形成于晚泥盆世末期,结合前人的研究资料认为在~360 Ma由于南天山洋板片的回撤或俯冲流体参与,上覆地幔发生部分熔融产生了基性岩浆,岩浆上涌过程中提供大量的热导致地壳发生部分熔融形成了酸性岩浆,两种岩浆发生不均匀混合作用。上涌的岩浆引起地幔对流,导致伊犁地块内部出现一定的伸展作用(弧后伸展作用)。