Extensive Resetting of Feldspar Pb Isotopic Composition in Archean–Paleoproterozoic Granitic Rocks from the Kongling Terrane,South China:Implications for Tracing Thermal Evolutionary History

Feldspar Pb isotopes have been widely used to trace magmatic formation and evolution processes.However,it remains unclear whether post-magmatic thermal events can affect feldspar Pb isotopic ratios.Here,the in situ Pb isotopic composition of feldspar hosted in granitic rocks(thirteen Archean and one Paleoproterozoic)from the northern Kongling terrane,Yangtze Craton,South China,is analyzed.The samples reveal a substantial variation in their Pb isotopic composition,spanning the gap between the 1.9 Ga and present-day geochrons,which indicates extensive resetting by later tectonothermal events.This resetting was interpreted to have likely resulted from Paleoproterozoic and Neoproterozoic tectonothermal events related to the assembly and breakup of the Columbia and Rodinia supercontinents.These results suggest that Pb isotopes should be used cautiously when tracing magma sources and petrogenesis in magmatic rocks that have experienced post-magmatic reworking.However,the in situ Pb isotopic composition of feldspar in ancient granitoids may also potentially be used to reveal later tectonothermal events.The extensive resetting of the Pb isotopic composition in feldspar by regional thermal events may also provide new insights into our understanding of the Pb isotope paradox.

川西九龙白台花岗岩地球化学特征、锆石U-Pb年龄和Hf同位素组成及其稀有金属成矿意义

为揭示川西九龙白台花岗岩成因及其与稀有金属成矿的关系,笔者等对白台花岗岩开展了岩相学、岩石地球化学、LA-ICP-MS锆石U-Pb年代学、锆石微量元素地球化学、Hf同位素地球化学测试。结果显示,白台黑云母二长花岗岩具高SiO_(2)(69.01%~70.55%)、高K_(2)O(3.86%~4.99%)、高碱(K_(2)O+Na_(2)O=7.41%~8.18%),低CaO(1.49%~2.2%)、低MgO(0.44%~0.57%)等特征,里特曼指数σ=2.03~2.43,A/CNK=1.07~1.14,为弱过铝质的高钾钙碱性系列岩石;微量元素特征显示相对富集Rb、K等大离子亲石元素,相对亏损Nb、P、Hf等高场强元素;球粒陨石标准化的稀土元素配分模式为明显的轻稀土富集和重稀土亏损的右倾型。白台黑云母二长花岗岩锆石U-Pb年龄为(213~212)Ma,ε_(Hf)(t)值为-7.4~-1.1,Hf同位素二阶模式年龄T_(DM2)为(1.6~1.2)Ga。研究结果表明,白台花岗岩形成于后碰撞伸展背景,是在松潘—甘孜地块遭受挤压碰撞后,增厚的岩石圈地幔拆沉,软流圈地幔物质上涌后发生减压熔融形成的晚三叠世I型花岗岩。同时,笔者等认为白台伟晶岩型铍矿床与白台花岗岩密切相关,是该花岗岩体残余岩浆热液分异的产物。

太岳山17.9亿年英安斑岩的发现及其对华北克拉通南部裂谷系成因的约束

熊耳群火山岩系主要分布于华北克拉通南部地区,主体喷发时限是1.75~1.80Ga。作为华北克拉通早期板块构造调整阶段的首次大规模岩浆活动,该火山-侵入岩系的形成时限、时空分布规律和地球动力学背景尚有争议,制约了对华北克拉通中元古代构造演化的进一步认识。本次研究在华北中部太岳山地区识别出中元古代英安斑岩,与古元古代花岗质片麻岩围岩呈侵入关系,无明显变质变形。锆石U-Pb年龄分析结果表明,该英安斑岩形成于1.79Ga,属于熊耳期岩浆活动的产物。样品中约90%的锆石显示出~1.92Ga和~2.16Ga两期年龄峰值,推测其为围岩的捕获锆石。英安斑岩SiO_(2)含量为65.12%~66.11%,A/CNK=0.91~1.05,全碱(K_(2)O+Na_(2)O)含量为6.85%~7.28%,Na_(2)O/K_(2)O=0.48~0.54,属于钾玄岩系列。样品稀土元素含量较高,亏损Nb、Ta、Zr、Hf等高场强元素,富集Rb、Ba、La等大离子亲石元素,与华北克拉通南部地区熊耳期英安-流纹斑岩的地球化学特征完全一致,指示板内伸展环境。太岳山熊耳期英安斑岩的发现,贯通了中条山地区的熊耳群火山岩系、吕梁地区汉高山群和小两岭火山岩系,完善了熊耳三叉裂谷系北支的空间分布范围,是华北克拉通中部地区开启中元古代裂陷作用演化的重要岩浆记录。

锡霍特—阿林造山带那丹哈达地体四平山金矿床成因与构造背景:锆石U-Pb年代学、岩石和流体地球化学制约

黑龙江省虎林市四平山金矿床位于那丹哈达地体东部,为该地体中规模最大的金矿床,已探明金资源量5.8 t,矿体赋存于四平山组硅质岩、硅质角砾岩以及大塔山林场组流纹岩中。为了进一步厘定成岩成矿时代,阐释成矿过程,查明成岩成矿地球动力学背景,本文在四平山金矿床进行详细的野外调查基础上,以四平山组硅质岩、硅质角砾岩和大塔山林场组流纹岩为研究对象,开展赋矿围岩LA-ICP-MS锆石U-Pb定年和全岩主微量组成测试,以及含金石英脉流体包裹体热力学分析。结果表明,大塔山林场组流纹岩(SPS-H-002、SPS-H-009)的成岩年龄分别为(134±1)Ma和(123±1)Ma,金矿化时代略晚于流纹岩的成岩时代,属于早白垩世晚期;硅质岩和硅质角砾岩具有较高的SiO2含量,富集Cu、Ni、Ba元素而贫Co元素,ΣREE含量较低,轻重稀土分异程度不明显,具有铕正异常,形成于热水沉积作用;流纹岩具有高硅、富铝、贫钛锰磷和低Zr含量,为亚碱性过铝质钙碱性-高钾钙碱性系列岩石,轻稀土富集,重稀土亏损,具有明显的铕负异常,无铈异常,属于高分异S型花岗岩,可能为亏损地幔增生的年轻地壳物质部分熔融的产物;含金石英脉中流体包裹体大小介于5~12μm之间,呈椭圆形和不规则形状成群或孤立分布,均一温度为118.7~223.4℃,盐度为0.40%~8.9%,密度为0.84~0.97 g/cm^(3),捕获压力为21.2~51.4 MPa,δD值为-113.8‰~-84.0‰,δ^(18)O_(H2O)值为-3.1‰~2.2‰,成矿流体以岩浆水为主,并显示出岩浆水与大气降水混合的特点。结合区域构造演化,认为四平山金矿床形成于碰撞后构造背景,成矿与古太平洋板块俯冲作用密切相关。

Thorium anomaly on the lunar surface and its indicative meaning

The Moon has been divided into three terranes:Procellarum KREEP Terrane(PKT),Feldspathic Highland Terrane(FHT),and South Pole-Aitken Terrane(SPAT),using globally measured Th and FeO.Many lunar evolu-tion models have predicted that a lunar magma ocean will produce a residual layer enriched in incompatible elements such as K,REE,and P(i.e.,KREEP)in the late age of crys-tallization;and that the distribution of thorium can be used as a proxy for determining the global distribution of KREEP.The thorium distribution in these three terranes is inhomo-geneous.The highest concentration of thorium is in PKT,the medium concentration of thorium is in SPAT,and almost none in FHT.Then what is the specific distribution in each of the terrane and what enlightenment can it tell us?Here we present and describe the detailed thorium distribution in PKT,SPAT,and FHT and provide some information for the origin of asymmetries on the lunar surface.

藏南叶巴组玄武岩锆石捕掳晶的年代学和微量元素特征及其对拉萨地块构造演化的启示

拉萨地块作为青藏高原的重要组成部分,其起源具有很大争议,原因在于对拉萨地块基底特征的认识不足,限制了对拉萨地块演化和成矿潜力的评价。本研究对拉萨地块南部叶巴组玄武岩中的锆石捕掳晶开展了锆石U-Pb年代学和微量元素特征研究。微量元素特征表明,捕获锆石绝大多数具有S型花岗岩结晶锆石的特征,或代表了拉萨地块南部存在变质沉积岩基底。锆石年龄呈现210、552和1 043 Ma三个峰值,分别对应三叠纪拉萨地块南缘的弧岩浆事件、埃迪卡拉纪末期冈瓦纳大陆北缘原特提斯洋俯冲事件以及拉萨地块在中元古代时期与非洲大陆的亲缘性。该成果为拉萨地块的基底特征以及构造演化研究提供了新的约束。

松潘-甘孜地体青海吾和玛高分异Ⅰ型花岗岩的识别与岩石成因

为探讨松潘-甘孜地体内青海吾和玛花岗质岩石的成因类型与岩石成因,对其开展了岩相学、地球化学、锆石U-Pb年代学和Lu-Hf同位素等研究。吾和玛花岗质岩石被确定为高分异的Ⅰ型花岗岩,其结晶年龄为216.6±4.3 Ma,为正长花岗岩,具有弱过铝质高钾钙碱性特征。岩石轻、重稀土元素分馏不明显((La/Yb)_(N)<7),轻稀土元素轻微富集,重稀土元素分布平缓,具有明显的负Eu异常(δEu=0.08~0.43),明显亏损Ba、Sr和高场强元素Nb、Ta、P、Ti,富集大离子亲石元素Rb、Th、U、K,ε_(Hf)(t)值为-2.54~1.32。吾和玛花岗质岩石的母岩浆是岩石圈拆沉作用引起软流圈上涌,诱发中元古代地壳物质(类似于变杂砂岩的物质)部分熔融形成长英质母岩浆,后期又受到斜长石、钾长石、磷灰石等矿物分离结晶作用的控制,经历了高程度的分异形成的。

Ages and Compositions of the Precambrian High-grade Basement of the Qilian Terrane and Its Adjacent Areas

Based on geological, chronological, geochemical and Nd isotopic studies of the high-grade basement of the Qilian terrane, the authors have drawn the following main conclusions: (1) the high-grade basement of the Qilian terrane consists mainly of meta-argillo-arenaceous rocks and granites and its bulk part was formed in the period of 0.8–1.0 Ga (the Jinningian period); (2) most of the meta-argillo-arenaceous rocks and granitic rocks have strong negative Eu and Ba anomalies (Eu/Eu*= 0.47–0.71 and Ba/Ba*=0.16–0.64), with fDM and ENd (1.0 Ga) ranging from 1.87 to 2.26 Ga and from ?8.54 to ?4.06 respectively, showing relatively high maturity; and (3) the Jinningian granitic rocks are a typical product of continent-continent collision, being probably related to the formation of the supercontinent Rodinia. These studies, combined with the study of high-grade basement rocks near the Qilian terrane, suggest that before the Jinningian period, the Qilian-Qaidam northern-margin terrane and Dunhuang-Alxa terrane were separated from each other, belonging to different plate systems of the North China craton and Yangtze platform respectively. The Qilian orogenic belt was the same as or similar to the Qiling orogenic belt in terms of the geological evolution history at least before the Jinningian period.

SHRIMP Age and Geochemistry of the Bikou Volcanic Terrane:Implications for Neoproterozoic Tectonics on the Northern Margin of the Yangtze Craton

The Bikou volcanic terrane is predominated by subalkaline tholeiitic lavas. Rock samples display lower initial ratios of Sr and Nd, 0.701248-0.704413 and 0.511080-0.512341 respectively. 207Pb and 208Pb are significantly enriched in the lavas. Most samples have positive εNd, which indicates that the magma was derived from EM-type mantle source, while a few samples with negative εNd indicate that there was contamination in the magma evolution. Magma differentiation is demonstrated by variations of LREE and LILE from depletion to enrichment. Additionally, normalized REE patterns and trace elements showed that lavas from the Bikou volcanic terrane have similar characteristics to those of basalts in arc settings caused by subduction and collision. Analyses showed that the Bikou volcanic terrane is a volcanic arc. New evidence proved that the Hengdan Group, north of the Bikou arc, is a turbidite terrane filling a forearc basin. Consequently, the Bikou volcanic terrane and the Hengdan turbidite terrane construct an arc-basin system. New SHRIMP ages showed that this arc-basin system developed on the northern margin of the Yangtze craton in the Neoproterozoic (846-776 Ma), and this arc-basin system is in agreement with the tectonic processes of Rodinia in the Neoproterzoic.

Helium Isotope Geochemistry of Ultrahigh-Pressure Metamorphic Eclogites From the Dabie-Sulu Terrane in East China

The 3He/4He ratios of most eclogites from the Dabie-Sulu terrane range from 0.056 to 0.67 Ra; the data points fall into the mixing part of the crust and the mantle in the 3He-4He diagram. The 3He/4He ratios of eclogites are obviously correlated with the types of their surrounding rocks. The helium isotope composition of the eclogites from the Bixiling complex possesses characters of mantle-derived rocks with the 3He/4He ratio being 5.6 Ra. The 4He concentration of the eclogites exhibits visible inverse correlation with the δ18O value of the quartz in the eclogites from the Sulu area. The δ18O values of the eclogites change synchronously with those of the country rocks. Those results suggest that protoliths of the eclogites were basic-ultrabasic rock bodies or veins intruding into the continental crust in the early stage; strong exchange and hybridization between the basic-ultrabasic rocks and continental rocks and the atmospheric water during the intrusion led to abrupt increase of the 3He/4He ratios, δ18O values and Nd(0) values of the intrusive bodies or veins, which show characters of continental rocks. This indicates that the eclogites are autochthonous.

Indosinian Orogenesis in the Lhasa Terrane, Tibet: New Muscovite ^(40)Ar-^(39)Ar Geochronology and Evolutionary Process

Muscovite 40 Ar-39 Ar dating of muscovite-quartz schist, eclogite and retrograde eclogite indicates an Indosinian orogenesis occurred at 220-240 Ma in the Lhasa terrane, which is caused by the closure of Paleo-Tethyan ocean basin and the following collision of the northern Lhasa terrane and southern Gondwana land. This Indosinian orogenesis is further confirmed by the regional sedimentary characteristics, magmatic activity and ophiolite mélange. This evidence suggests that the Indosinian orogenic belt in the Lhasa terrane is widely distributed from the Coqen county in the west, and then extends eastward through the Ningzhong and Sumdo area, finally turning around the eastern Himalayan syntaxis into the Bomi county. Based on the evolutionary process, the geological development of Lhasa terrane from early Paleozoic to early Mesozoic can be divided into seven stages. All of the seven stages make up a whole Wilson circle and reveal a perfect evolutionary process of the Paleo-Tethys ocean between the northern Lhasa terrane and southern Gondwana land. The Indosinian orogenisis is a significant event for the evolution of the Lhasa terrane as well as the Tibetan Plateau.

Geochronology and Geochemistry of the Subduction-related Rocks with High Sr/Y Ratios in the Zedong Area: Implications for the Magmatism in Southern Lhasa Terrane during Late Cretaceous

The southern Lhasa Terrane is famous for its huge magmatic belt which records the magmatism during Mesozoic and Cenozoic. Although the Mesozoic continental-margin setting in the southern Lhasa Terrane has been identified, details of this tectonic setting and the evolution history during the Late Cretaceous remain unclear. To further constrain these issues, we present zircon LA-ICP-MS U-Pb, Hf isotopic and geochemical data of the Gongbari dacites （of the Sangri Group） which intruded by Paleocene granodiorites from the eastern part of the southern Lhasa Terrane, Tibet. New age data indicate that the dacites were generated at -95.4 Ma, which suggests the Sangri Group volcanism may last to Late Cretaceous. The Gongbari dacites are characterized by high Sr （428-758 ppm） contents, low concentration of heavy rare earth elements and Y （e.g. Yb=0.78-1.14 ppm; Y=8.85-11.4 ppm） with high Sr/Y （41.91-67.59） and La/Yb （22.64-30.64） ratios, similar to those of adakite. The rocks are calc-alkaline, metaluminous, enriched in LILEs, depleted in HFSEs, and have positive tar（t） values （＋7.7 to ＋11.6）. The Gongbari dacites were probably produced by partial melting of young and hot subducted Neo- Tethyan oceanic crust under amphibolite to garnet amphibolite-facies conditions. Though the Gangdese Mountains may have formed before Indo-Asian collision, the southern margin of Lhasa Terrane might not go through obviously crustal thickening during the northward subduction of Neo-Tethyan oceanic lithosphere.

In Situ Stress Measurements in the Lhasa Terrane,Tibetan Plateau,China

the India and Eurasia plates. Knowledge of the stress state is critical to evaluate the crustal stability and the design of underground excavations. Because of the limitations imposed by natural conditions, little research has been performed on the present crustal in situ stress in the Tibetan Plateau, and further study is imperative. In this study, hydraulic fracturing measurements were conducted in Nyching County （LZX） and Lang County （LX）, Lhasa terrane to characterize the shallow crustal stress state. The results indicate that the stress state in the LZX borehole is markedly different from that in the LX borehole, in both magnitude and orientation. At the same measurement depths, the magnitudes of horizontal principal stresses in the LX borehole are 1.5–3.0 times larger than those in the LZX borehole. The stress regime in the LX borehole favors reverse faulting characterized by SH〉Sh〉Sv, where SH, Sh, and Sv are maximum horizontal, minimum horizontal, and vertical principal stresses, respectively. The SH and Sh values are approximately three and two times greater than Sv. Fracture impression results reveal that SH in the LX borehole are predominantly N–S, while in the LZX borehole the maximum horizontal principal stress is mainly in the NNE-direction. The heterogeneity of the regional stress state might be a result of the population and distribution of local structures and seismic activities. The stress state in the LX borehole has exceeded the critical state of failure equilibrium, and there is an optimally orientated pre-existing fault near the borehole. It can be concluded that the optimally orientated fault is likely to be active when the stress has built up sufficiently to destroy the frictional equilibrium; it is suggested that research focus should be placed on this in future. The stress states in boreholes LZX and LX indicate uniformity of the regional stress field and diversity of the local stress fields resulting from the interactions among regional dynamic forces, tectonic stress field, and geological structures.

Geochronology and geochemistry of the Donglufang porphyry-skarn Mo-Cu deposit in the southern Yidun Terrane and their geological significances

The newly-discovered Donglufang Moe Cu porphyry-skarn deposit is located in the southern Yidun Terrane, southeast Tibet, with more than 80 million tonnes(Mt) of reserves(grading 0.15 wt.% Mo and0.48 wt.% Cu) hosted in Triassic strata and Late Cretaceous granodiorite porphyry. Ree Os dating of molybdenum ore yielded a weighted mean age of 84.9 ± 1.0 Ma and an isochron age of 85.2 ± 0.6 Ma.LA-ICP-MS Ue Pb dating of zircons from the granodiorite porphyry yielded206 Pb/238 U ages ranging from 87.4 Ma to 84.2 Ma with a weighted mean206 Pb/238 U age of 85.1 ±0.5 Ma, indicating a temporal linkage between granitic magmatism and Moe Cu mineralization. Geochemical analyses show that the granodiorite porphyries are I-type granites with Si O_2 contents of 64.3 -66.7 wt.%. These rocks are typically metaluminous with high K_2 O/Na_2 O ratios, low Mg O(1.32 -1.56 wt.%), Cr(5.6 -12.9 ppm), Ni(3.79 -10.81 ppm), Mg#(43 -52) values, and high Sr(304 -844 ppm), Sr/Y(21.2 -50.8) and La/Yb ratios(37.0 -60.1). They are enriched in light rare-earth elements(LREE) relative to heavy rare-earth elements(HREE), with slightly negative Eu anomalies, and are enriched in Th, U and large ion lithophile elements(LILE, e.g., K and Rb), and depleted in high field strength elements(HFSE, e.g., Nb, Ta, P and Ti). They also show negative zircon εHf(t) values(-6.7 to -2.3) and negative whole rock εNd(t) values(à5.2 to-4.3), as well as old Hfe Nd model ages, indicating the magmas were derived from a thickened ancient lower crust within the garneteamphibolite facies. Considering the tectonic evolution of the Yidun Terrane, geochemical characteristics of granodiorite porphyry, and the ages of mineralization obtained in this study. We suggest that the Donglufang deposit was formed in a post-collisional setting, which has a genetic relationship with the emplacement of the granodiorite porphyry. The present study provide key information for the exploration of the Late Cretaceous metallogeny in the Yidun Terrane.

Early Cretaceous I-type granites in the Tengchong terrane:New constraints on the late Mesozoic tectonic evolution of southwestern China

The Early Cretaceous granitoids that are widespread in the Tengchong terrane of Southwest China play a critical role in understanding the tectonic framework associated with the Tethyan oceans. In this study, we present a detailed description of zircon U-Pb ages, whole-rock geochemistry and Hf isotopes for the Laoxiangkeng pluton in the eastern Tengchong terrane and elucidate their petrogenesis and geodynamic implications. Zircon U-Pb dating of the Laoxiangkeng pluton yields ages of 114 ± 1 Ma and 115 ± 1 Ma,which imply an Early Cretaceous magmatic event. The Laoxiangkeng pluton enriched in Si and Na, is calcalkaline and metaluminous, and has the characteristics of highly fractionated I-type granites. Zircons from the pluton have calculated ε_(Hf)(t) values of-12.7 to-3.7 and two-stage model ages of 1327-1974 Ma,respectively, indicating a mixed source of partial melting of Paleo-Neoproterozoic crust-derived compositions with some inputs of mantle-derived magmas. By integrating all available data for the regional tectonic evolution of the eastern Tethys tectonic domain, we conclude that the Early Cretaceous magmatism in the Tengchong terrane was produced by the northeastward subduction of the Meso-Tethyan Bangong-Nujiang Ocean.

Geochemistry, Geochronology and Lu-Hf Isotopes of Peraluminous Granitic Porphyry from the Walegen Au Deposit, West Qinling Terrane

Walegen Au deposit is closely correlated with granitic intrusions of Triassic age, which are composed of granite and quartz porphyries. Both granite porphyry and quartz porphyry consist of quartz, feldspar and muscovite as primary minerals. WeaMy peraluminous granite porphyry （A/ CNK=1.10-1.15） is enriched in LREE, depleted in HREE with Nb-Ta-Ti anomalies, and displays subduction-related geochemistry. Quartz porphyry is strongly peraluminous （A/CNK=1.64-2.81） with highly evolved components, characterized by lower TiO2, REE contents, Mg#, K/Rb, Nb/Ta, Zr/Hf ratios and higher Rb/Sr ratios than the granite porphyry. REE patterns of quartz porphyry exhibit lanthanide tetrad effect, resulting from mineral fractionation or participation of fluids with enriched F and CI. LA- ICP-MS zircon U-Pb dating indicates quartz porphyry formed at 233±3 Ma. The ages of relict zircons from Triassic magmatic rocks match well with the detrital zircons from regional area. In addition, εHf（t） values of Triassic magmatic zircons from the granite and quartz porphyries are -14.2 to -9.1 （with an exception of ＋4.1） and -10.8 to -8.6 respectively, indicating a crustal-dominant source. Regionally, numerous Middle Triassic granitoids were previously reported to be formed under the consumption of Paleotethyan Ocean. These facts indicate that the granitic porphyries from Walegen Au deposit may have been formed in the processes of the closing of Paleotethyan Ocean, which could correlate with the arc-related magmatism in the Kunlun orogen to the west and the Qinling orogen to the east.

Crustal strain rates of southeastern Tibetan Plateau derived from GPS measurements and implications to lithospheric deformation of the Shan-Thai terrane

The link between the crustal deformation and mantle kinematics in the Tibetan Plateau has been well known thanks to dense GPS measurements and the relatively detailed anisotropy structure of the lithospheric mantle.However, whether the crust deforms coherently with the upper mantle in the Shan-Thai terrane(also known as the Shan-Thai block) remains unclear.In this study, we investigate the deformation patterns through strain rate tensors in the southeastern Tibetan Plateau derived from the latest GPS measurements and find that in the Shan-Thai terrane the upper crust may be coupled with the lower crust and the upper mantle.The GPS-derived strain rate tensors are in agreement with the slipping patterns and rates of major strike-slip faults in the region.The most prominent shear zone, whose shear strain rates are larger than 100×10^(–9) a^(–1), is about 1000-km-long in the west, trending northward along Sagaing fault to the Eastern Himalayan Syntaxis in the north, with maximum rate of compressive strain up to –240×10^(–9) a^(–1).A secondary shear zone along the Anninghe-Xiaojiang Fault in the east shows segmented shear zones near several conjunctions.While the strain rate along RRF is relatively low due to the low slip rate and low seismicity there, in Lijiang and Tengchong several local shear zones are present under an extensional dominated stress regime that is related to normal faulting earthquakes and volcanism, respectively.Furthermore, by comparing GPS-derived strain rate tensors with earthquake focal mechanisms, we find that 75.8%(100 out of 132) of the earthquake T-axes are consistent with the GPS-derived strain rates.Moreover, we find that the Fast Velocity Direction(FVDs) at three depths beneath the Shan-Thai terrane are consistent with extensional strain rate with gradually increasing angular differences, which are likely resulting from the basal shear forces induced by asthenospheric flow associated with the oblique subduction of the India plate beneath the Shan-Thai terrane.Therefore, in this region the upper crust deformation may be coherent with that of the lower crust and the lithospheric mantle.

The Crustal Structure and Assembly of Terranes in the Qaidam-Qilian-Beishan Area, Western China

Abstract: Through a study of the geotransect from Golmud to Ejin Qi published recently, the tectonics of the crust beneath the area from the northern Qinghai-Tibet plateau (Qaidam and the Qilian Mountains) to the border between China and Mongolia and its structure, composition and tectonic evolution have been revealed, and abundant information about the deep structures has been provided. Based on the research into the geotransect, it is suggested that the crust in this area was formed by the assembly of the terranes in different geological stages. Following the formation of the Palaeo-Asian continent, the north part of the corridor of the transect became a part of the huge unifying continent by the end of the Early Permian. In the Mesozoic and Cenozoic, as a result of the compression mainly by the push of the Qinghai-Tibet plateau on the south, the unique crustal structure and geomorphologic features on the northern Qinghai-Tibet plateau were formed. This geotransect together with the Yadong-Golmud geotransect constitutes a long geotransect which runs across the western Chinese continent.

GENESIS OF COPPER MINERALIZATION IN THE WESTERN KOHISTAN ISLAND ARC TERRANE,NW HIMALAYA—HINDUKUSH, N. PAKISTAN

The Kohistan Terrane in N. Pakistan straddles the suture zone between the collided Indian and Eurasian plates in the NW Himalayas of N. Pakistan. Initiated as an intra\|oceanic island\|arc crust in Neotethys, the terrane accreted with the southern margin of the Karakoram Plate in the Late Cretaceous (about 90Ma). Continued subduction at its southern margin resulted in Andean\|type magmatism and tectonics, until collision with the Indian Plat in the Early Eocene.Whereas much of the central and eastern Kohistan expose Cretaceous volcanic and sedimentary rocks of intra\|oceanic island arc setting, the western Kohistan in Swat, Dir, Chitral and upper Ghizer valleys preserves remnants of the Late Paleocene—Early Eocene volcanism and sedimentation of Andean\|type setting. These volcano\|sedimentary lithologies in western Kohistan are geographically divisible into two, separated by the Lowari Pluton; (1) Dir\|Swat Belt, (2) Drosh\|Shamran Belt. Late Paleocene\|Early Eocene siliceous volcanics termed Utror Volcanics and Shamran Volcanics are common in the two belts and so are the plutons intrusive into these volcanics (Late Stage\|II of the Kohistan Batholith). The difference is in the sedimentary successions presented in the two belts. Whereas in the Dir\|Swat belt, the sedimentary succession comprises Baraul Banda Slate Formation, deposited in deep\|water fore\|arc setting and contemporaneous in age with that of the nearby Utror Volcanic Formation (about 55～60Ma), the sediments in the Drosh\|Shamran belt are distinctly red beds of fluvial origin (the Purit Formation) those overlie unconformably on top of the Shamran Volcanics and their equivalents.

Late Neoarchean volcanic rocks in the southern Liaoning Terrane and their tectonic implications for the formation of the eastern North China Craton

The late Neoarchean metamorphosed volcanic rocks in the southern Liaoning Terrane(SLT) of the eastern North China Craton(NCC) are mainly composed of amphibolites and felsic gneisses and can be chemically classified as basalt(Group#1),basaltic andesite(Group#2),dacite(Group#3) and rhyodacite(Group#4).LA-ICP-MS zircon U-Th-Pb dating reveals that they formed at ~2.53-2.51 Ga.Group#1 samples are characterized by approximately flat chondrite-normalized rare earth element(REE) patterns with low(La/Yb)_N ratios and a narrow range of(Hf/Sm)N ratios,and their magmatic precursors were generated by partial melting of a depleted mantle wedge weakly metasomatized by subducted slab fluids.Compared to Group#1 samples,Group#2 samples display strongly fractionated REE patterns with higher(La/Yb)_N ratios and more scattered(Hf/Sm)N ratios,indicative of a depleted mantle wedge that had been intensely metasomatized by slab-derived melts and fluids.Group#3 samples are characterized by high MgO and transition trace element concentrations and fractionated REE patterns,which resemble typical high-Si adakites,and the magmatic precursors were derived from partial melting of a subducted oceanic slab.Group#4 samples have the highest SiO_2 and the lowest MgO and transition trace element contents,and were derived from partial melting of basaltic rocks at lower crust levels.Integrating these tholeiitic to calcalkaline volcanic rocks with the mass of contemporaneous dioritic-tonalitic-trondhjemitic-granodioritic gneisses,the late Neoarchean volcanic rocks in the SLT were most likely produced in an active continental margin.Furthermore,the affinities in lithological assemblages,metamorphism and tectonic regime among SLT,eastern Hebei to western Liaoning Terrane(EH-WLT),northern Liaoning to southern Jilin Terrane(NL-SJT),AnshanBenxi continental nucleus(ABN) and Yishui complex(YSC) collectively indicate that an integral and much larger continental block had been formed in the late Neoarchean and the craton-scale lateral accretion was a dominantly geodynamic mechanism in the eastern NCC.