Ore-forming event and geodynamic setting of molybdenum deposit at Wenquan in Gansu Province, Western Qinling

The Wenquan molybdenum deposit is associated with a Triassic granite in this area. The Wenquan granite is enriched in LILE and LREE, poor in HFSE, and has significantly higher contents of alkali （K2O＋Na2O） and Sr, Ba than those of the island arc volcanic rocks. These geochemical characteristics are similar to post-collisional granites in high K calc-alkaline series. Studies of major elements, trace elements, REEs and chronology of the Wenquan pluton show that, in the geodynamic transition stage of continent-continent convergence to extension, the partial melting of the enriched lithospheric mantle generated the basaltic magma and triggered the partial melting of the thickened lower crust which produceded the acidic magma, and the Wenquan pluton was formed by mixing of the two magmas. Molybdenite Re-Os isotopic dating gave Os model ages of 212.7±2.6 Ma to 215.1±2.6 Ma with a weighted mean of 214.1±1.1 Ma, and an Re-Os isochron age of 214.4±7.1 Ma. These ages are close to K-Ar ages （223 to 226 Ma） and a SHRIMP zircon U-Pb age （223±7 Ma） for the Wenquan granite within the error range, but relatively younger. This implies that the Mo mineralization occurred in a late stage of the magmatic intrusion, and the metallogenesis took place in the transition stage from syn-collision to post-collision in the tectonic setting of the Qinling Orogenic Belt （QOB） after continental collision between the North China Block （NCB） and the South China Block （SCB）. This process is also corresponding to the geological events of metamorphism and deformation in South Qinling, closure of the Mian-Lue oceanic basin, and exhumation of the Dabie-Sulu ultrahigh-pressure metamorphic rocks. The large-scale continent-continent collision between NCB and SCB in the middle Triassic triggered significant crustal thickening and exhumation of subducted slab. In the late Triassic, the tectonic setting was transformed to the transition stage from collision to extension. Materials from the asthenospheric mantle would ascend into the root of the lower crust, which could induce partial melting of the lower crust and generate Mo-enriched granitic magma. The ore-forming elements enriched in the fluid derived from the condensation and fraction of the magma resulted in the Mo mineralization. The Mo deposits in the QOB are mainly formed in two episodes, namely 220± Ma and 140± Ma. The two episodes of metallogenesis were developed in the tectonic transition settings from compression to extension, but they were in the different stages of the tectonic evolution. The occurrence of the Wenquan Mo-bearing pluton indicates that the Triassic tectonic-magmatic belt of Western Qinling is another favorable region for Mo mineralization in the QOB. Therefore, it is significant to pay more attention to evaluation of the ore-forming potentiality in the Triassic granites in Western Qinling.     

Paleoproterozoic reworking of ancient crust in the Cathaysia Block, South China: Evidence from zircon trace elements, U-Pb and Lu-Hf isotopes

A combined study of zircon LA-ICP-MS U-Pb dating, trace elements and Hf isotope was carried out for gneissic granite from the Sanzhishu area in Jingning, SW Zhejiang Province. Nearly all the zircons separated from the granite exhibited oscillatory zoning and high Th/U ratios (>0.1). The REE profile showed a pronounced positive Ce anomaly, negative Eu anomaly and an enrichment of HREE, which are typical characteristics of magmatic zircon. Thirteen concordant or nearly concordant analytical data yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1860±13 Ma (MSWD=0.084), representing the formation age of the granite. The magmatic zircons had negative εHf(t) values of −15.6 to −10.0 and two-stage Hf model ages of 3.1 to 3.4 Ga, indicating that the granites were formed by reworking of ancient crust. The major- and trace-element data indicate that the gneissic granites are metaluminous high-K calc-alkaline rocks and exhibit the same geochemical characteristics as aluminous A-type granites, implying the emplacement of the granite in a post-orogenic extensional tectonic setting. We conclude that the Paleoproterozoic crustal reworking event in the Cathaysia Block of South China marked the transition from assembly to break-up of the Columbia supercontinent. Supported by National Natural Science Foundation of China (Grant No. 40873004), Special Funds for National Scientific Research of Commonweal Industries, the Ministry of Land and Resources of China (Grant No. 2008110015), Opening Foundation of State Key Laboratory of Continental Dynamics, Northwest University (Grant No. 06LCD12) and the Project of Land and Resources Bureau of Zhejiang Province (Grant No. 2004005)     

A perspective view on ultrahigh-pressure metamorphism and continental collision in the Dabie-Sulu orogenic belt

The study of continental deep-subduction has been one of the forefront and core subjects to advance the plate tectonics theory in the twenty-first century. The Dabie-Sulu orogenic belt in China crops out the largest lithotectonic unit containing ultrahigh-pressure metamorphic rocks in the world. Much of our understanding of the world＇s most enigmatic processes in continental deep-subduction zones has been deduced from various records in the Dabie-Sulu rocks. By taking these rocks as the natural laboratory, earth scientists have made seminal contributions to understanding of ultrahigh-pressure metamorphism and continental collision. This paper outlines twelve aspects of outstanding progress, including spatial distribution of the UHP metamorphic rocks, timing of the UHP metamorphism, timescale of the UHP metamorphism, the protolith nature of deeply subducted continental crust, subduction erosion and crustal detachment during continental collision, the possible depths of continental subduction, fluid activity in the continental deep-subduction zone, partial melting during continental collision, element mobility in continental deep-subduction zone, recycling of subducted continental crust, geodynamic mechanism of postcollisional magmatism, and lithospheric architecture of collision orogen. Some intriguing questions and directions are also proposed for future studies.     

西藏措勤地区典中组火山岩地球化学特征及构造背景

措勤盆地位于西藏冈底斯构造-岩浆岩带的西段北侧,该区古新世火山活动十分强烈,形成了一套厚度大于1 000 m的中酸性火山岩地层,火山岩K-Ar和Rb-Sr同位素年龄为63.9～61.0 Ma,区域上可与古近纪林子宗群下部层位相对比,归为古新统典中组.火山岩岩石化学、地球化学特征分析表明,该火山岩系属高钾钙碱性-钙碱性系列,轻稀土富集,负Eu异常明显.与原始地幔相比,微量元素Rb,Ba,K,Th,U富集;Ti,P,Sr,Ta(Nb)亏损.锶、氧同位素组成显示岩浆来源与陆壳物质关系密切,推测是来自于俯冲带的幔源基性岩浆与陆壳重熔酸性岩浆以不同比例混合所形成.综合研究认为,这套火山岩形成于陆缘弧构造环境,与喜马拉雅特提斯洋壳向北俯冲消减作用有关.     

Caledonian reworking of Paleoproterozoic basement in the Cathaysia Block: Constraints from zircon U-Pb dating,Hf isotopes and trace elements

A combined study of zircon U-Pb dating, Hf isotopes and trace elements has been carried out for granodioritic neosomes of migmatites from the Tianjingping area in northwestern Fujian Province. Zircons are characterized by zoning, higher Th/U ratios （mostly≥0.1）, HREE enrichment, and positive Ce and negative Eu anomalies, and show features similar to magmatic or anatectic zircons. Apparent ^206Pb/^238U ages for the zircons are 447±2 Ma （95 % conf., MSWD=0.88）, corresponding to a Caledonian event. εHf（t） values are -13.3 to -9.7, indicating a crustal source. Two-stage Hf model ages are 1.7 to 1.9 Ga, suggesting that protolith of the migmates was probably formed in the Paleoproterozoic. The granodioritic neosomes have the characteristics of peraluminous calc-alkaline granite, and their REE patterns and trace elements spidergrams show features of middle to upper crustal rocks. Together with previous studies, we conclude that the protolith of the Cathaysia basement in the Tianjingping area was likely formed in the middle-late Paleoproterozoic and experienced partial melting during the Caledonian period. The recognition of Caledonian reworking of the Paleoproterozoic basement in the Cathaysia Block provides a new insight into the tectonic evolution of the Cathaysia Block in the Caledonian period and the interaction between the Cathaysia Block and the Yangtze Block.     

Multistage evolution of continental collision orogen: A case study for western Dabie orogen

The foramtion and evolution of collisional orogen is a prominent feature along convergent plate margins, and is generally a complex process. This article presents an integrated study of zircon genesis, U-Pb age and Lu-Hf isotope composition as well as geological characteristics for the western Dabie orogen to constrain its multi-stage evolution history. The results suggest that the formation of oceanic crust in the Huwan area was constrained at ca. 400--430 Ma, which was slightly later than the collision of the northern Qinling with the North China Block. It formed in a marginal basin in the northern margin of the Yangtze Block. The peak metamorphism of eclogite in the Huwan area occurred at ca. 310 Ma, and the timing of the initial exhumation of oceanic eclogite was about 270 Ma. The high to ultrahigh pressure （HP-UHP） metamorphic rocks in the Xinxian and the Hong＇an metamorphic zones have the same ages and natures as those of the HP-UHP metamorphic rocks in the other Dabie-Sulu terrains, and also have experienced multi-stage exhumation, and thus can be taken as a coherent part of the Dabie-Sulu orogen. Therefore, the Qinling-Dabie-Sulu orogen is a typical multi-stage continental collision orogen, with an amalgamation process extending more than 200 Ma.     

班公湖－怒江缝合带侵入岩地球化学和年代学特征及形成构造背景

对西藏纳屋错地区班公湖－怒江缝合带西段的侵入岩进行岩石地球化学和锆石 U-Pb年龄测定,并与缝合带以外南面岩浆弧侵入岩进行对比。岩石地球化学分析表明,缝合带内侵入岩样品均属于钙碱性和高钾钙碱性系列,隶属于岛弧型岩浆岩类。稀土元素球粒陨石标准化分布模式均表现出 LREE 富集、HREE 相对亏损的右倾模式,痕量元素原始地幔标准化蛛网图显示样品总体上富集大离子不相容元素（HFSE）Rb、K、Ba,亏损高场强元素（LILE）Nb、Ta、Ti、P,体现了钙碱性系列岩体的特征和演化趋势;年代学分析表明,带内侵入岩的成岩时代集中在早白垩世（104．5～141．7 Ma B．P．）;进一步的构造环境对比分析表明,所有侵入岩样品均属于岛弧型-大陆碰撞型花岗岩类,但缝合带内岩浆岩属性为同碰撞花岗岩,缝合带具由钙碱性向高钾钙碱性演化的特征和趋势,形成于受逆掩构造或地壳剪切作用的造山时期,处于威尔逊旋回的“陆陆碰撞造山-山带剥蚀”阶段,认为班-怒缝合带西段存在双向俯冲的碰撞造山构造背景。     

SHRIMP U-Pb zircon age of tuff from the Kunyang Group in central Yunnan： Evidence for Grenvillian orogeny in South China

Whether or not Grenvillian orogeny occurred in South China still remains highly controversial because high-quality,discriminating data are lacking,and therefore,the key to resolve this matter is to find datable volcanic and/or sedimentary rocks related to Grenvillian orogeny. Such rocks are apparently present in the Fuliangpeng Member from the lower-middle part of Kunyang Group in central Yunnan; here the unit is more than 100 m thick and consists of andesitic ignimbrite,tuffite,terrigeous clastic rocks and carbonates. These volcanic rocks,developed south of the Sibao fold-thrust belts,represent the earliest calc-alkaline volcanic activity in late Precambrian time from central Yunnan and are coeval with both a change in sedimentary facies from detritus to carbonates and the beginning of seismite development elsewhere. Two samples for SHRIMP analysis were collected from this volcanic unit. Sample G3-29-2,from the bottom of Fuliangpeng Member,is an ignimbrite,and about 100 zircon crys-tals recovered from it have euhedral shapes and display relatively simple sector zonation under cathodoluminescent (CL) imaging,suggesting a magmatogenic origin. Twenty-five of the zircons were analyzed and a weighed-mean U-Pb age of 1032±9 Ma was obtained. Sample G3-29-3 from uppermost part of Fuliangpeng Member is a tuffite,and many rounded,evidently detrital zircons were recovered. Nine of these zircons were analyzed,and the oldest single-grain U-Pb zircon age is 1938±26 Ma,im-plying that Paleoproterozoic basement developed in Cathaysia. The dating result,combined with the geotectonic research on the Fuliangpeng Member,leads us to conclude that late Mesoproterozoic orogenic volcanic activity occurred in the western part of South China,and that the related collision of Yangtze and Cathaysian cratons was an integral part of the assembly of Rodinia.     

Zircon SHRIMP U-Pb ages of the “Xinghuadukou Group” in Hanjiayuanzi and Xinlin areas and the “Zhalantun Group” in Inner Mongolia, Da Hinggan Mountains

A report is presented of SHRIMP zircon U-Pb dating data of meta-igneous and meta-sedimentary rocks of the Xinghuadukou Group(Xinlin-Hanjiayuanzi area,Heilongjiang Province)and meta-volcanic rocks of the Zhalantun Group(Zhalantun district,Inner Mongolia).The SHRIMP analyses show that the meta-igneous rocks from the Xinghuadukou Group formed at 506±10―547±46 Ma,belonging to Early-Middle Precambrian,whereas the meta-sedimentary rocks yielded detrital zircons,with ages of 1.0―1.2,1.6―1.8 and 2.5―2.6 Ga,indicative of deposition age at least<1.0 Ga. Meta-basic volcanic rocks from the Zhalantun Group have a formation age of 506±3 Ma.These data suggest that both the Xinghuadukou and Zhalantun Groups formed during Cambrian and/or Neoproterozoic time,rather than Paleoproterozoic time as previously thought.Early Precambrian inherited zircons in the meta-igneous rocks and numerous Precambrian detrital zircons in the meta-sedimentary rocks imply that these rocks were formed proximal to older crust.It is inferred that the Xinghuadukou and Zhalantun Groups represent Cambrian and/or Neoproterozoic vol- cano-sedimentary sequences formed in an active continental margin setting.     

Metallogenetic epoch and mechanism of the tungsten ore in Yushan,Beishan orogenic belt,Gansu

Using SHRIMP zircon U-Pb dating methods and cathodoluminescence （CL） technique, we have precisely dated ore samples from tungsten ore deposit, ore-bearing volcanic-sedimentary strata and granite in the Yushan region within the east Tianshan-Beishan W-bearing zone. This study reveals Yushan tungsten ore deposit zone formed in 250 ± 9 Ma, and it is closely associated with the Permian granites and felsic volcanic rocks. This new type of tungsten ore deposit in the east Tianshan-Beishang multiple-metallogenetic province may probably be associated with the regionally right-literal shearing after the main Carboniferous collisional orogeny, and the formation of pull-apart basin and the felsic magmatism.     

Developing the plate tectonics from oceanic subduction to continental collision

The studies of continental deep subduction and ultrahigh-pressure metamorphism have not only promoted the development of solid earth science in China, but also provided an excellent opportunity to advance the plate tectonics theory. In view of the nature of subducted crust, two types of subduction and collision have been respectively recognized in nature. On one hand, the crustal subduction occurs due to underflow of either oceanic crust （Pacific type） or continental crust （Alpine type）. On the other hand, the continental collision proceeds by arc-continent collision （Himalaya-Tibet type） or continent-continent collision （Dabie-Sulu type）. The key issues in the future study of continental dynamics are the chemical changes and differential exhumation in continental deep subduction zones, and the temporal-spatial transition from oceanic subduction to continental subduction.     

SHRIMP U-Pb zircon age of the Jinbaoshan ultramafic intrusion, Yunnan Province, SW China

SHRIMP U-Pb zircon dating gives ages of 260.6± 3.5 Ma and 260.7± 5.6 Ma for serpentinised wehrlite and plag-hornblendite in the Jinbaoshan ultramafic intrusion, respectively. The results indicate that the Jinbaoshan intrusion was emplaced at ca.260 Ma and contemporaneous with the Emeishan continental flood basalts (ECFB), similar to other mafic-ultramafic intrusions of the Emeishan large igneous province (LIP). The new ages provide a geochronological constraint on the origin of the Jinbaoshan ultramafic intrusion. It confirms that the Jinbaoshan ultramafic intrusion belongs to the Emeishan LIP that formed at ca. 260 Ma. Supported by Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KZCX2-YW-111), National Natural Science Foundation of China (Grant Nos. 40573020 and 40773033) and National Basic Research Program of China (Grant No. 2007CB411408)     

Zircon U-Pb age and Hf isotope of Quanyishang A-type granite in Yichang: signification for the Yangtze continental cratonization in Paleoproterozoic

Zircon U-Pb age and Hf isotope, and major and trace element compositions were reported for granite at Quanyishang, which intruded into the Kongling complex in Yichang, Hubei Province. The results show that the Quanyishang granite is rich in silicon and alkalis but poor in calcium and magnesium, and displays enrichment in Ga, Y, Zr, Nb but depletion in Sr and Ba, exhibiting the post-orogenic A-type affinity. 90% zircons from the granite are concordant, and give a middle Paleoproterozoic magmatic crystallization age (mean 1854 Ma). Initial Hf isotope ratios (176Hf/177Hf)i of the middle Paleoproterozoic zircons range from 0.280863 to 0.281134 and they have negative eHf(t) values with a minimum of -26.3. These zircons give the depleted mantle model ages (TDM) of 2.9―3.3 Ga (mean 3.0 Ga), and the average crustal model ages (Tcrust) of 3.6―4.2 Ga (mean 3.8 Ga). A Mesoarchean grain with 207Pb/206Pb age of 2859 Ma has a slightly high TDM (3.4 Ga) but similar Tcrust (3.8 Ga) to the Paleoproterozoic zircons. All these data suggest that the source materials of the Quanyishang A-type granite are unusually old, at least ≥2.9 Ga (even Eoarchean). The event of crustal remelting, which resulted in the formation of the Quanyishang granite in the middle Paleoproterozoic, recorded the cratonization of the Yangtze conti-nent. The process may have relation to the extension and collapse of the deep crust with Archean ages, in response to the transition stage of the assembly and breakup of the Columbia supercontinent.     

Zircon isotopic ages from magnetite quartzites of the Jianping metamorphic complex, western Liaoning Province

Using the zircon Pb evaporation method and high precise ion microprobe analyses, zircon isotopic ages in supracrustal magnetite quartzites from Jianping metamorphic complex have been obtained. The results show that the deposition of the supracrustal rocks occurred around 2 552–2 520 Ma and the peak of granulite facies metamorphism reached at about 2 487 Ma; about (1 806±16) Ma, i.e. Mid-Proterozoic, the other possible metamorphic event with relatively low intensity happened in the study area. It is speculated that a likely tectonic setting where the Jianping metamorphic complex formed is an active continental margin that is involved in continental collision and crustal thickening shortly after its formation.     

Deep sea pollen record during 12－1.6 Ma from the southern South China Sea and its response to envi-ronmental change

Based on deep-sea pollen results (512-76 m) from ODP Site 1143 in the southern South China Sea (SCS),the climate and vegetation evolution sequence on the surrounding islands and the exposed continental shelf are discussed. The pollen records show that the pollen influx was quite low before 8.15 Ma and increased dramatically afterwards. The influx changes can be ascribed,on one side,to tectonics deformations around the southern SCS resulting in rapid uplift of islands and subsequent increase of the sediment rates and pollen influx and on the other side to climate cooling and monsoon enhancement. Around 2.63 Ma was another obvious boundary,the increasing of pollen and spores influx since this time was mainly related to global climate cooling. Spectrum analysis of pollen influx values shows that 2 Ma,0.67 Ma,and 0.19-0.17 Ma cycles existed during 12-3.0 Ma,while 0.1 Ma and 46.9 ka cycles existed during 3.0-2.0 Ma.     

He-Ar isotopic system of fluid inclusions in pyrite from the molybdenum deposits in south margin of North China Block and its trace to metallogenetic and geodynamic background

The helium and argon isotopic compositions of the ore-forming fluids from the molybdenum deposits such as Jinduicheng, Donggou, Shijiawan, and Sandaozhuang, which are located in the East Qinling molybdenum belt in south margin of North China Block （SMCNB）, are reported in this paper. The origin and the evolution of the ore-forming fluids and their coupled-relationships with the intra-continental collision and orogenic process of Qinling Orogen in Mesozoic-Cenozoic have been discussed. The 3He/4He and 40Ar/36Ar values （3He/4He=1.38-3.64 Ra, and 40Ar/36Ar=295.68-346.39, respectively） of the fluid inclusions in pyrite from the molybdenum deposits in East Qinling suggest that, the ore-forming fluid system is mixed by two end members. One is the high temperature deep-derived fluid congenetic with the porphyries generated by crust-mantle mixing, and the other is the low-temperature meteoric water which is rich in crustal radiogenic He with the component of atmospheric Ar. From the Pb isotopic composition, and ore-bearing potential of the porphyry and the regional stratum, we can conclude that the ore-forming materials of the deposits in the East Qinling molybdenum belt are derived from the deep source by the mixing of lower crust and upper mantle. Therefore, the formation of the molybdenum deposits in SMNCB can be related to the crust-mantle interaction, which is accompanied by the intra-continental orogenic and extension process in the post-collision period of Qinling Orogen. The granitic porphyries which are related to Mo mineralization are not simple crust-remelting type granites or S type granites, but belong to syntexis-type or mantle-derived granites, hence their formation has a profound and regional geodynamic background.     

Geochronolosic constraints on masmatic intrusions and mineralization of the Zhunuo porphyry copper deposit in Gansdese, Tibet

In situ zircon U-Pb ages for the recently discovered Zhunuo porphyry copper deposit in the western part of the Gangdese metallogenic belt in Tibet were determined by sensitive high-resolution ion microprobe （SHRIMP）. The ages can be divided into two separate groups, reflecting more than four major tectono-magmatic events in the area. The 62.5±2.5 Ma age of inherited zircons may be related to the volcanic eruption of the Linzizong Group formed shortly after the India-Asia continental collision. The 50.1±3.6 Ma age most likely corresponds to the time of underplating of mantle-derived mafic magma in Gangdese. The 15.6±0.6 Ma age obtained from magmatic zircons is interpreted as the age of crystallization of the Zhunuo ore-forming porphyry. Finally, a molybdenite Re-Os isochron age of 13.72±0.62 Ma is consistent with another zircon U-Pb age of 13.3±0.2 Ma, representing the time of copper mineralization. These ages, in combination with available literature data, indicate that magmatic crystallization and copper mineralization in the Gangdese metallogenic belt became gradually younger westward, and further suggest that the Zhunuo porphyry copper deposit was formed in the same tectonic stage as other porphyry copper deposits in the eastern and central Gangdese belt. This conclusion provides critical information for future exploration of porphyry copper deposits in western Gangdese.     

Zircon U-Pb ages of granites at Changba and Huangzhuguan in western Qinling and implications for source nature

Mesozoic granitoids are widespread in the Qinling-Dabie-Sulu orogenic belt. Precise U-Pb dating on these granitoids can reveal the evolution of the continental collision orogen and thus provide information on the nature of magma sources. This study presents zircon LA-ICP-MS U-Pb dating and whole-rock geochemical analyses for two intrusions at Changba and Huangzhuguan in western Qinling. Zircon U-Pb ages for central and marginal phases of the Huangzhuguang intrusion are 214±1 Ma and 213±3 Ma, respectively. Zircons from the Changba intrusion yield a dominant cluster with an U-Pb age of 213±2 Ma. Collectively, these ages are younger than ages of 220 to 240 Ma for ultrahigh-pressure metamorphism due to the continental collision between the South China Block and the North China Block, corresponding to syn-exhumation magmatism. Some inherited zircons occur in the Changba intrusion, yielding a weighted mean of ²⁰⁶Pb/²³⁸U ages at 757±14 Ma. This indicates that the Changba intrusion has the crustal source of mid-Neoproterozoic ages and a tectonic affinity to the South China Block. Geochemically, the two intrusuons are both rich in LILE and LREE but depleted in HFSE and HREE, similar to arc-type igneous rocks. The Huangzhuguang intrusion exhibits linear correlations between SiO₂ and the other major oxides, implying chemical evolution from a cognate magma source. It contains mafic enclaves, suggesting possible mixing of felsic-mafic magmas. The Changba granite is rich in Si and K but poor in Fe and Mg as well as has a high value of Fe*, suggesting strong differentiation of granitic magma. Therefore, the two intrusions were derived from the Late Triassic anatexis of the continental crust of different compositions in the northern margin of South China Block. This process may be coupled with exhumation of the subducted continental crust in the stage of late collision.     

Porphyry Cu-Mo deposits in the eastern Xing’an-Mongolian Orogenic Belt: Mineralization ages and their geodynamic implications

The eastern Xing’an-Mongolian (Xing-Meng) Orogenic Belt (XMOB) is one of the important areas of porphyry copper (Cu)-molybdenum (Mo) deposits in China. However, studies on the exact ages of mineralization and their geodynamic significance are very limited. In this study, granodioritic rocks from the Duobaoshan Cu deposit and Daheishan Mo deposit were selected to make zircon SHRIMP U-Pb analyses in order to constrain their mineralization ages. Geochronological data indicate that two episodes of mineralization took place in the Duobaoshan Cu deposits. The granodiorite related to the Duobaoshan porphyry Cu deposit was formed in the Early Paleozoic with zircon U-Pb age of 485±8 Ma, whereas the granodiorites related to the Sankuanggou skarn-type Cu deposit were emplaced in the Jurassic with zircon U-Pb ages of 176±3 and 177±3 Ma. In the Daheishan area of Jilin Province, the emplacement age of the granodiorite porphyry related to the porphyry Mo deposit was dated at 170±3 Ma, and the unmineralized monzogranite at 178±3 Ma. Therefore, two episodes of Cu-Mo mineralization were developed in the eastern XMOB, at ~485 Ma and ~175 Ma, respectively. Based on the geological history and spatial-temporal distribution of the granitoids in northeastern (NE) China, it is proposed that the Duobaoshan Cu deposit was related to the collision of the Xing’an and Erguna blocks in the Early Paleozoic, and the Sankuanggou Cu and Daheishan Mo deposits were related to subduction of the Paleo-Pacific plate during the Jurassic.     

U-Pb zircon dating constraints on formation time of Qilian high-grade metamorphic rock and its tectonic implications

In order to constrain the formation time of high-grade metamorphic rocks in the Qilian Mountains, U-Pb zircon dating was carried out by using LA-ICPMS technique for a paragneiss of the Hualong Group in the Qilian Mountains basement series and a weakly foliated granite that intruds into the Hualong Group. Zircons from the paragneiss consist dominantly of detrital magma zircons with round or sub-round shape. They have 207Pb/206Pb ages mostly ranging from 880 to 900 Ma, with a weighted mean age of 891 ±9 Ma, which is interpreted as the magma crystallization age of its igneous provenance and can be taken as a lower age limit for the Hualong Group. Magma crystallization age for the weak-foliated granite is 875±8 Ma, which can be taken as an upper age limit for the Hualong Group. Accordingly, the formation time of the Hualong Group is constrained at sometime between 875 and 891 Ma. A few zir- cons from both paragneiss and weak-foliated granite display old inherited ages of 1000 to 1700 Ma and young metamorphic ages of Early Paleozoic. The zircon age distribution pattern confirms that the Qilian Mountains and the northern margin of Qaidam Basin had a united basement, with geotectonic affinity to the Yangtze Block. The results also reveal that sediments of the Hualong Group formed by rapid accumulation due to rapid crustal uplift-erosion. This process may result from intensive Neoproterozoic orogenesis due to assembly of the suppercontinent Rodinia.