内蒙古西部狼山地区晚古生代变质事件的厘定及其地质意义——来自乌拉山岩群LA-ICP-MS锆石U-Pb定年的新证据

乌拉山岩群是狼山地区最重要的前寒武纪变质基底之一,准确测定其原岩成岩与变质时代,对于进一步探讨狼山地区前寒武纪地质演化具有重要的意义。对狼山地区乌拉山岩群角闪黑云斜长片麻岩及其伴生的花岗质浅色脉体进行了岩石学和锆石U-Pb年代学研究。碎屑锆石U-Pb定年和野外地质调查表明,狼山地区乌拉山岩群角闪黑云斜长片麻岩碎屑锆石年龄介于2591～1800Ma之间,其中最小一组碎屑锆石年龄为1873Ma,结合其约270Ma的变质年龄,初步限定乌拉山岩群角闪黑云斜长片麻岩的原岩沉积年龄为1873～270Ma。综合新的研究资料,认为狼山地区乌拉山岩群除存在新太古代—古元古代变质岩外,可能还存在中元古代—晚古生代变沉积岩。锆石阴极发光图像与U-Pb定年结果综合表明,角闪黑云斜长片麻岩中发育大量变质锆石,获得的206Pb/238U年龄加权平均值为269±4Ma,代表狼山地区乌拉山岩群遭受晚古生代末期角闪岩相变质作用的时代,可能与华北板块与西伯利亚板块晚古生代末期碰撞造山作用有关。此外,采用预剥蚀方法,在乌拉山岩群高硅花岗质浅色脉体高U锆石中,获得的~(206)Pb/~(238)U年龄加权平均值为264±3Ma,被解释为乌拉山岩群花岗质浅色脉体的形成时代,代表本区晚古生代造山作用由同碰撞挤压向碰撞后伸展转换的时限。     

北京密云沙厂BIF型铁矿地质地球化学特征与成矿时代

北京密云沙厂铁矿为产于太古宙密云岩群变质岩系中的鞍山式铁矿,具有条带状铁建造(BIF)特征。矿石主要呈条带状构造,致密块状结构;矿石类型以磁铁石英岩型铁矿石为主,w(Fe)=25%~30%。围岩主要有黑云母(角闪)斜长片麻岩、辉石变粒岩和斜长角闪岩,局部见英云闪长质片麻岩。对选自黑云斜长片麻岩中的锆石进行U-Pb定年,具有核边结构、Th/U0.4的锆石其核部2组年龄为2 619Ma±25Ma和2 544Ma±16Ma。锆石的2 619Ma±25Ma年龄代表原岩为基性火山岩的黑云斜长片麻岩的形成年龄,同时也代表火山喷发和沙厂铁矿BIF沉积年龄;2 544 Ma±16 Ma锆石年龄可能代表后期TTG特征的花岗质片麻岩的侵位、叠加改造年龄。岩石的主、微量元素特征表明黑云斜长片麻岩富集Rb、Ba、Sr,亏损Th、U,表明沙厂BIF沉积时可能存在新太古代(约2 550 Ma)地壳增生事件。认为沙厂铁矿属Algoma型BIF,是新太古代末华北陆块初步克拉通化和大规模岩浆热事件的代表之一。     

辽北清原杂岩锆石年代学与地球化学测试数据集

辽北清原杂岩锆石年代学与地球化学测试数据库（集）基于亚洲系列地质图件编制及相关重大地质问题研究项目“亚洲晚太古代地层全球性对比”资助,进行岩石测试分析整理而得。锆石年代学数据可以准确地反映原岩的形成时代与形成后所经历的构造岩浆事件,岩石地球化学数据可以反映岩石本身的化学特征,二者相结合可以为研究辽北地区地壳演化历史提供科学依据与基础数据支持。本文采集了辽北地区清原杂岩中高级变质表壳岩与英云闪长质—奥长花岗质—花岗闪长质（TTG）片麻岩单元中的岩石样品,岩石类型包括片麻状花岗闪长岩、云英闪长质片麻岩、奥长花岗质片麻岩、花岗闪长质混合片麻岩、透辉角闪变粒岩、角闪黑云斜长片麻岩、片麻状紫苏花岗岩、石榴紫苏黑云斜长片麻岩、花岗质片麻岩、角闪透辉黑云斜长片麻岩、变质中性火山岩、黑云斜长片麻岩、斜长角闪岩以及矽线石榴片麻岩等。本数据集为Excel表格型数据,包括2个.xls类型文件（Zircon_U-Pb dating data_QY.xls,Geochemistry data_QY.xls）分别记录了样品的锆石U-Pb测年数据与地球化学数据。本数据集样品测试工作均在国家地质实验测试中心完成,数据质量可靠。     

山东即墨马连山地区新元古代花岗质片麻岩捕掳体的发现及其地质意义

在苏鲁造山带中段即墨马连山地区新发现的花岗质片麻岩组合,为亚铝质-弱过铝质高钾钙碱性花岗岩类。呈捕掳体形体赋存于中生代崂山花岗岩中,普遍含有大理岩、变粒岩、片岩类等表壳岩包体。采用LA-MC-ICPMS技术,对岩石中的锆石进行了U-Pb同位素测定,测得黑云二长花岗质片麻岩样品(D04)的206Pb/238U年龄,其中15个数据的加权平均值为770.2±2.4 Ma(n=15,MSWD=0.14),该年龄解释为原岩的形成时代。含石榴二长花岗质片麻岩样品(D0501)的206Pb/238U年龄为784.3±6.6 Ma和930±5 Ma,其中8个数据的加权平均值为784.3±6.6 Ma(n=8,MSWD=0.39),该年龄解释为原岩的形成时代;12个数据的加权平均值为930±5 Ma(n=12,MSWD=0.65),该年龄解释为早期残留锆石年龄。该套岩石形成于新元古代中期,其岩石特征、岩石化学特征与胶南月季山片麻岩及胶北高钾I型花岗质片麻岩相似,属苏鲁造山带基底岩系。该套岩石组合类型的新发现为造山带北段西边界的研究提供了新的依据。     

藏东类乌齐地区花岗质片麻岩锆石Cameca U-Pb定年及其地质意义

西藏东部类乌齐一带吉塘岩群中新识别出一套花岗质片麻岩。花岗质片麻岩中锆石Th/U比值较高(Th/U0.49),阴极发光图像显示锆石内部韵律环带明显,具典型岩浆锆石成因的特征。Cameca锆石U-Pb同位素加权平均年龄为282.1Ma±0.9Ma,表明花岗岩岩体形成于早二叠世,暗示澜沧江结合带在早二叠世存在岩浆增生事件。     

滇西哀牢山岩群变沉积岩碎屑锆石LA-ICP-MS U-Pb年龄及其地质意义

出露于扬子板块与印支板块之间的哀牢山岩群中包含大量副片麻岩、大理岩、云母片岩和石英岩等变沉积岩,这些岩石对研究扬子板块西南缘构造演化具有重要意义。但有关哀牢山岩群的沉积时代、物质来源以及形成的构造背景等一系列问题长期以来存在着较大争议。本文选取哀牢山岩群内石榴黑云斜长片麻岩、石榴夕线黑云斜长/二长片麻岩和大理岩4件变沉积岩进行了锆石阴极发光和LA-ICP-MS U-Pb年代学研究。哀牢山岩群碎屑锆石具有明显或弱的振荡环带和较高的Th/U比值,指示其岩浆成因。年代学分析结果表明石榴黑云斜长片麻岩记录始新世末-渐新世(36.3~25.0Ma)哀牢山岩群部分熔融-岩浆活动,其余3件变沉积岩碎屑锆石均得到550~450Ma与900~700Ma两组峰值,其中片麻岩与大理岩中最年轻的碎屑锆石峰值分别为452Ma、461Ma和458Ma,指示哀牢山岩群沉积时代应不早于中奥陶世,而非原先认为的古元古代。碎屑锆石年龄分布特征表明哀牢山岩群物源区包含扬子板块西缘新元古代花岗岩和印支板块内泛非期花岗岩,支持前人提出的哀牢山岩群沉积于古特提斯分支洋盆——金沙江-哀牢山-马江洋拉张过程中沉积的观点。     

浙西北平水铜矿细碧角斑岩成岩年龄及其地质意义

文章对浙西北平水铜矿赋矿围岩双溪坞群平水组细碧角斑岩中锆石进行了LA-ICP-MS U-Pb同位素年龄测定。结果显示锆石Th/U比值为0.42~2.28,明显高于Th/U比值小于0.1的变质成因的锆石,为典型岩浆成因锆石。几乎所有的锆石颗粒样品都投影在谐和曲线上及其附近,细碧岩年龄加权平均值为(952±5)Ma(n=18,MSWD=0.19),角斑岩年龄加权平均值为(954±8)Ma(n=15,MSWD=0.51);结合锆石自形、发育岩浆环带等特点,该年龄是平水组细碧角斑岩的形成年龄。结合前人研究及地质事实,本次研究确定双溪坞群平水组细碧角斑岩的成岩年龄为新元古代(950 Ma左右);认为江南造山运动发生的上、下限虽还没有最终限定,但它东端的造山运动很可能介于1.0~0.9 Ga;钦杭成矿带北东段地区有寻找同类矿床的潜力。     

柴北缘锡铁山早古生代HP／UHP变质作用后的构造热事件——花岗岩和片麻岩的同位素与岩石地球化学证据

对柴达木盆地北缘锡铁山地区花岗岩中的锆石采用单颗粒锆石U-Pb法(TIMS)测年,谐合线上数据点给出的结果为428±1Ma,代表了花岗质岩浆的形成(上侵)时代.在锆石岩相学研究的基础上对围岩片麻岩中的锆石采用SHRIMP进行测年,10个点的Th/U比变化范围在0.01～0.08,表明锆石为变质成因,10个数据点给出的206Pb/238U表面年龄的加权平均值为435±7Ma,代表变质岩的变质年龄.两类岩石的锆石年龄在误差范围内一致,岩浆作用与变质作用时差很小.花岗岩地球化学特征表明岩体为Ⅰ型花岗岩,它的主量元素、稀土元素和微量元素特征与围岩片麻岩十分类似,表明花岗岩的源岩在成分上与围岩片麻岩相似.花岗岩的Nd模式年龄TDM=1.4～1.5Ga,围岩片麻岩的TDM=1.9～2.2Ga,暗示岩浆形成时有地幔物质的添加或混染.推测是由于地幔上隆使深部地壳岩石发生了部分熔融作用,形成花岗质岩浆时添加了地幔物质.锡铁山花岗岩是早古生代碰撞后伸展环境的产物.     

西秦岭天水地区新阳新元古代花岗质片麻岩的锆石LA-ICP-MS定年及其地质意义

西秦岭造山带北缘古元古界秦岭岩群中,新识别出新阳新元古代花岗质片麻岩,位于天水市西北部新阳镇东侧.花岗质片麻岩中锆石Th/U比值较高,阴极发光图像显示锆石内部振荡环带清晰,具岩浆成因特征.LA-ICP-MS锆石U-Pb同位素加权平均年龄为(938±4)Ma～(981±5)Ma,表明花岗质片麻岩形成于新元古代,反映了晋宁期西秦岭北缘存在一次构造热(俯冲碰撞)事件,可能是Rodinia超大陆的汇聚事件在西秦岭的响应.     

江山—绍兴断裂带陈蔡岩群片麻岩SHRIMP锆石U-Pb年龄及其地质意义

浙江诸暨陈蔡地区发育一套副变质岩层,由中深度变质的片麻岩、片岩和大理岩组成,与双溪坞群为断层接触,其地质时代多被视为与江山—绍兴断裂带南东侧的华夏地块古老变质基底相同。陈蔡岩群地处江绍断裂带北东端,变质和变形较深,前人将其划归为华夏古陆的小块体,地层时代曾与福建古元古代麻源岩群或浙江八都岩群和江西中元古代铁砂街组对比。在陈蔡岩群下吴宅组含榴夕线黑云斜长片麻岩及含榴黑云斜长片麻岩中获得大量的锆石,具有典型的核、幔、边结构,核部年龄集中在新元古代,206Pb/238U年龄加权平均值为848±10Ma和845±9Ma,锆石后期增生边部年龄多为加里东期的年龄,206Pb/238U年龄加权平均值为431.4±7.8Ma。陈蔡岩群斜长片麻岩获得的锆石核部较单一的U-Pb年龄,反映出陈蔡岩群下吴宅组夕线石所代表的泥质成分所含的锆石单一,其锆石核部年龄应代表陈蔡岩群下吴宅组的原岩年龄;同时,认为陈蔡岩群既不能与福建古元古代的麻源岩群对比,也不能与江西地区中元古代铁砂街组对比。其原岩为稍晚于双溪坞群(905~877Ma)的新元古代地层(848~845Ma),从年代上倾向于江南造山带范畴。该年龄对江山—绍兴断裂带变质地层的区域对比和构造演化有重要意义。     

阿尔金江尕勒萨依榴辉岩和围岩锆石LA-ICP-MS微区原位定年及其地质意义

阿尔金江尕勒萨依榴辉岩及其直接围岩——石榴子石黑云母片麻岩锆石的阴极发光图像、微区原位LA-ICP-MS微量元素分析研究表明,榴辉岩锆石内部结构比较均匀,少数颗粒保留斑杂状残核;位于锆石斑杂状残核测点的重稀土相对富集,Th/U比值多大于0.4,为岩浆锆石的特征;位于锆石边部与内部结构均匀颗粒上的测点显示HREE近平坦型或弱亏损型的稀土配分模式,显示了与石榴石平衡共生的变质锆石特征;而石榴子石黑云母片麻岩的锆石具有核-幔-边结构,核部为碎屑锆石,幔部则为与石榴石平衡共生的变质锆石。LA-ICP-MS微区定年获得榴辉岩的变质年龄为(493±4.3)Ma,其原岩形成年龄为(754±9)Ma;石榴子石黑云母片麻岩的变质年龄为(499±27)Ma。榴辉岩的变质年龄滞后于其原岩的形成年龄约250Ma,并且榴辉岩与其直接围岩副片麻岩的变质年龄几乎完全一致,充分表明该超高压榴辉岩的形成是陆壳深俯冲作用的产物。     

SHRIMP Dating and Recrystallization of Metamorphic Zircons from a Granitic Gneiss in the Sulu UHP Terrane

An unusual zircon SHRIMP dating result of a granitic gneiss from the Qinglongshan eclogite-gneiss roadcut section is presented in this paper. The very peculiar and complicated internal structures, as well as the very low Th/U ratios (0.01-0.08) of the zircons indicate that they were formed by metamorphic recrystallization. Strongly in contrast with previously published zircon U-Pb ages of the Dabie-Sulu UHP metamorphic rocks where protolith ages of 600-800 Ma are commonly recorded, only metamorphic age of 218±5 Ma, defined by 18 analytical spots either in rim or in core of zircons, are recorded in this granitic gneiss. This age represents the time of the complete metamorphic recrystallization overprint on primary magmatic zircons. The recrystallization was derived by the UHP metamorphism, and was strengthened by the early stage of retrograde metamorphic fluid activity.     

Post-peak,fluid-mediated modification of granulite facies zircon and monazite in the Trivandrum Block,southern India

The quarry at Kottavattom in the Trivandrum Block of southern India contains spectacular examples of fluid-assisted alteration of high-grade metamorphic rocks. Garnet-biotite gneiss has undergone a change in mineral assemblage to form submetre scale orthopyroxene-bearing patches, later retrogressed to form an amphibole-bearing lithology. These patches, often referred to as arrested or incipient charnockite, crosscut the original metamorphic foliation and are typically attributed to passage of a low aH₂O fluid through the rock. Whilst this conversion is recognised as a late stage process, little detailed chronological work exists to link it temporally to metamorphism in the region. Zircon and monazite analysed from Kottavattom not only record metamorphism in the Trivandrum Block but also show internal, lobate textures crosscutting the original zoning, consistent with fluid-aided coupled dissolution-reprecipitation during formation of the orthopyroxene-bearing patches. High-grade metamorphism at the quarry occurred between the formation of metamorphic monazite at ~585 Ma and the growth of metamorphic zircon at ~523 Ma. The fluid-assisted alteration of the garnet-biotite gneiss is poorly recorded by altered zircon with only minimal resetting of the U–Pb system, whereas monazite has in some cases undergone complete U–Pb resetting and records an age for fluid infiltration at ~495 Ma. The fluid event therefore places the formation of the altered patches at least 25 Myr after the zircon crystallisation in the garnet-biotite gneiss. The most likely fluid composition causing the modification and U–Pb resetting of zircon and monazite is locally derived hypersaline brine.     

柴北缘大陆深俯冲板片折返过程中的深熔作用研究

柴北缘锡铁山地区长英质（花岗）片麻岩普遍经历了不同程度的部分熔融作用,常见新生的花岗质浅色体呈层状、脉状或网络状分布于长英质片麻岩中,并显示出混合岩化的特征。岩相学观察结果显示长英质片麻岩保留了关键的深熔作用显微结构证据:（1）石榴石内部发育有钾长石、石英和斜长石组成的矿物包裹体;（2）长石颗粒边界出现由石英+钾长石±斜长石±白云母组成的楔形矿物集合体;（3）云母颗粒边界发育尖锐的、不规则的微斜长石,而且云母边界溶蚀明显,形成锯齿状不规则的边界;（4）石英、斜长石或钾长石颗粒边界发育圆珠状（stringofbeads）结构,而且颗粒边界或三联点中尖锐状微斜长石与周围矿物的形成较小的二面角。阴极发光图像和锆石U-Pb定年结果表明花岗质浅色体中的锆石具有明显的核、幔、边三层结构,而且具有明显不同的年龄结果。发光较强的继承性锆石岩浆核部的²⁰⁶Pb/²³⁸U年龄约为~910Ma,而且具有高的Th/U比值;弱发光的变质锆石幔的²⁰⁶Pb/²³⁸U年龄结果约为~450Ma。新生的锆石增生边中等程度发光,并发育震荡环带和较低的Th/U比值,与世界典型地区混合岩中深熔锆石的特征十分相似,其²⁰⁶Pb/²³⁸U年龄结果为432±3Ma。野外关系、显微结构特征和年代学的研究结果显示柴北缘锡铁山地区花岗质浅色体可能是其寄主岩石长英质片麻岩在折返到高压麻粒岩相条件下深熔作用的产物,而且白云母的脱水熔融是引发岩石发生深熔作用的主要机制。柴北缘地区已有的资料综合研究表明,大陆深俯冲板片在俯冲/碰撞和折返过程中可能经历了多重深熔作用。     

Zircon Dating and Geological Implications of Granitic Gneiss in the Metamorphic Zone of Gaoligong Mountains in Western Yunnan, China

The general classification of intermediate-acid intrusive rocks in the metamorphic zone of Gaoligong Mountains as one of the metamorphic terranes of Proterozoic Gaoligong Mountains is problematic regarding the intrusion stage and age, as well as the subsequent metamorphism and deformation. In this study, we investigated granitic gneiss in the metamorphic zone of Gaoligong Mountains based on the 1:50,000 regional geological survey of Qushi Street (2011-2013) and SHRIMP U-Pb zircon geochronology. Results showed that the SHRIMP U-Pb zircon dating of granitic gneiss ranged from 163.5±5.7 Ma to 74.0±2.0 Ma. Thus, the granitic gneiss was grouped into orthometamorphic rocks (metamorphic intrusions). The dating data of granite rocks associated with intense metamorphism and deformation were divided into three groups, 163.5±5.7 to 162.3±3.1 Ma, 132.2-101.0 Ma and 99.4±3.5-74.0±2.0 Ma, which respectively represented three independent geologic events including an important magma intrusion with superimposed metamorphic effects in the late Middle Jurassic, regional dynamic metamorphism and superimposed reformation of fluid action in the early Cretaceous, and dynamic metamorphism dominated by ductile shear and metamorphism starting from the late Cretaceous.     

滇西哀牢山变质岩系锆石U-Pb定年及其地质意义

哀牢山-红河构造带是滇西地区最著名的带状变质带之一,其主体由哀牢山深变质岩系(哀牢山岩群)组成,一直被认为是扬子陆块古元古代结晶基底.本文选取哀牢山深变质岩系内的花岗片麻岩(11 ALl7-1和11AL09-1)和石英岩(11AL08-1),以及邻区的花岗岩(11ALl2-1)进行LA-ICP-MS锆石U-Pb定年.结果显示,花岗片麻岩11 ALl7-1有岩浆和变质两类锆石,两者的206Pb/238U年龄加权平均值分别为700±6Ma(MSWD=1.4,n=14)和27.4±1.2Ma(MSWD=1.9,n=3),代表原岩形成时代和变质年龄.花岗片麻岩llAL09-1岩浆锆石206 pb/238U年龄为220±3Ma(MSWD=3.1,n=14),变质锆石年龄为31.2±2.3Ma(MSWD =6,n=5),分别代表原岩结晶时代和后期变质年龄.石英岩11AL08-1中所有锆石具有核-边结构,92颗锆石核部年龄集中分布在6组,分别为493～528Ma(n=42)、635 ～ 640Ma(n=2)、701～784Ma(n=44)、976 ～980Ma(n=2)、1839Ma(n=1)和2487Ma(n=1).92个核部分析点具有高的Th/U比值(＞0.23),指示岩浆来源.最年轻一组的42个核部年龄加权平均值为509Ma,代表石英岩原岩的最大沉积时代.7颗锆石变质边年龄为26～ 75 Ma内,代表变质年龄.花岗岩11 ALl2-1锆石206pb/238U年龄加权平均值为750±4Ma(MSWD =0.6),代表岩石形成时代.这些年龄表明哀牢山变质岩系是一个原岩复杂的变质杂岩带,它的原始物质至少包含新元古代～ 700Ma岩浆岩、～509 Ma沉积地层及220 ～ 240Ma的岩浆岩和地层,而不是以往认为的古元古代结晶基底.现今所见的哀牢山岩群“古老”岩石面貌主要是由地质历史上的浅变质或未变质的地层和岩浆岩在新生代26～31Ma发生变质变形作用改造的结果.哀牢山变质带的源区物质特征和主要岩浆事件与扬子陆块西缘十分相似,具有亲扬子的构造属性.     

A new understanding of Demala Group complex in Chayu Area,southeastern Qinghai-Tibet Plateau: Evidence from zircon U-Pb and mica 40Ar/39Ar dating

The Chayu area is located at the southeastern margin of the Qinghai-Tibet Plateau. This region was considered to be in the southeastward extension of the Lhasa Block, bounded by Nujiang suture zone in the north and Yarlung Zangbo suture zone in the south. The Demala Group complex, a set of high-grade metamorphic gneisses widely distributed in the Chayu area, is known as the Precambrian metamorphic basement of the Lhasa Block in the area. According to field-based investigations and microstructure analysis, the Demala Group complex is considered to mainly consist of banded biotite plagiogneisses, biotite quartzofeldspathic gneiss, granitic gneiss, amphibolite, mica schist, and quartz schist, with many leucogranite veins. The zircon U-Pb ages of two granitic gneiss samples are 205 ± 1 Ma and 218 ± 1 Ma, respectively, representing the ages of their protoliths. The zircons from two biotite plagiogneisses samples show core-rim structures. The U-Pb ages of the cores are mainly 644 –446 Ma, 1213 –865 Ma, and 1780 –1400 Ma, reflecting the age characteristics of clastic zircons during sedimentation of the original rocks. The U-Pb ages of the rims are from 203 ± 2 Ma to 190 ± 1 Ma, which represent the age of metamorphism. The zircon U-Pb ages of one sample taken from the leucogranite veins that cut through granitic gneiss foliation range from 24 Ma to 22 Ma, interpreted as the age of the anatexis in the Demala Group complex. Biotite and muscovite separates were selected from the granitic gneiss, banded gneiss, and leucogranite veins for ⁴⁰Ar/³⁹Ar dating. The plateau ages of three muscovite samples are 16.56 ± 0.21 Ma, 16.90 ± 0.21 Ma, and 23.40 ± 0.31 Ma, and the plateau ages of four biotite samples are 16.70 ± 0.24 Ma, 16.14 ± 0.19 Ma, 15.88 ± 0.20 Ma, and 14.39 ± 0.20 Ma. The mica Ar-Ar ages can reveal the exhumation and cooling history of the Demala Group complex. Combined with the previous research results of the Demala Group complex, the authors refer that the Demala Group complex should be a set of metamorphic complex. The complex includes not only Precambrian basement metamorphic rock series, but also Paleozoic sedimentary rock and Mesozoic granitic rock. Based on the deformation characteristics, the authors concluded that two stages of the metamorphism and deformation can be revealed in the Demala Group complex since the Mesozoic, namely Late Triassic-Early Jurassic (203 –190 Ma) and Oligocene –Miocene (24 –14 Ma). The early stage of metamorphism (ranging from 203 –190 Ma) was related to the Late Triassic tectono-magmatism in the area. The anatexis and uplifting-exhumation of the later stage (24 –14 Ma) were related to the shearing of the Jiali strike-slip fault zone. The Miocene structures are response to the large-scale southeastward escape of crustal materials and block rotation in Southeast Tibet after India-Eurasia collision.©2021 China Geology Editorial Office.     

Single zircon evaporation ages: Evidence for the Mesoproterozoic crust in the southeastern Nigerian basement complex

The Ukwortung area of the Obudu Plateau exposes high-grade metamorphic rocks of upper amphibolite to granulite facies. Zircon grains from three locations of Southwest Obudu Plateau were dated by the Pb-Pb dating method. This single zircon radiometric dating method confirms the occurrence of Mesoproterozoic crust in the southeastern Nigeria basement complex. The pyroxene gneiss (granulite facies) sample from Biereberi near Ukpe yielded a mean age of 2061.4±0.4 Ma, suggesting the presence of Paleoproterozoic crust. The combined 207Pb/206Pb ratios from the garnet-biotite gneiss in the area gave a mean age of 1794.4±0.4 Ma. This further confirms the pres-ence of Paleoproterozoic crustal components within the Obudu Plateau. The Mesoproterozoic (1548.8±0.5 Ma) and Neoproterozoic (619.8±0.9 Ma) ages were obtained from two populations of zircon grains from the leucogranite in Okordem, Southwest Obudu. These "signatures" collaborate the occurrence of Mesoproterozoic ages in the Afar region and Cameroon basements along with the southeastern Nigerian basement complex, constituting the major portion of the West Central African mobile belt. The zircon age of 1794.4±0.4 Ma may probably be the maximum depositional age of the meta-sediment (garnet-biotite gneiss) and the age of 1548.8±0.5 Ma may be interpreted as the time of emplacement of the granitoid. The Neoproterozoic age (619.8±0.9 Ma) probably records the metamorphic event that was prevalent during the Pan-African period and thus affected the area.     

北天山温泉群的地质特征、时代和构造意义

北天山温泉群长期以来被认为是古元古代的变质岩。最新的野外调查和锆石SHRIMP U-Pb 测年结果表明,温泉群可 以划分为三个岩石构造单元：（1）前早新元古代变质火山岩和变质沉积岩,主要包括斜长角闪岩、云母片岩、石英片岩、 黑云母片麻岩、大理岩等;（2）早新元古代混合岩和正片麻岩;（3）早古生代未变质变形的辉长岩和闪长岩。上述三种岩 石组合类型均被后期二云母花岗岩（脉）所侵入。温泉县以南的混合岩和花岗片麻岩中锆石的SHRIMP U-Pb 年龄分别为 926±12 Ma 和907±11 Ma,与天山地区出露的新元古代花岗岩类的时代基本一致。结合前人对花岗片麻岩Nd 同位素组成 的研究,花岗片麻岩应为古老地壳物质部分熔融的产物,而同期的混合岩化作用则是新元古代地壳加厚和部分熔融的直接 地质证据。辉长岩和闪长岩侵入到温泉群花岗片麻岩和斜长角闪岩中,其中闪长岩的锆石SHRIMP U-Pb 年龄为452±7 Ma, 并含有1.1 Ga 和1.4 Ga 的继承锆石。根据前人的研究成果,本区早古生代辉长岩和闪长岩具有岛弧岩浆岩的地球化学特征, 可能与准噶尔-巴尔喀什洋的俯冲作用有关,这一俯冲增生作用最终导致伊犁北部与哈萨克斯坦陆块在志留纪拼贴造山, 并使温泉群前寒武纪变质岩与侵入岩发生变质变形作用。