藏北羌塘盆地基底高级变质岩LA—ICP—MS锆石U—Pb年龄及其地质意义

俄久卖高级变质岩位于藏北羌塘盆地中央隆起带北缘的玛依岗日地区,是目前羌塘盆地基底高级变质岩石的唯一代表。该高级变质岩由正片麻岩和副片麻岩组成,本文以正片麻岩为研究重点。正片麻岩锆石CL图像显示出明显的核一幔一边结构。根据LA—ICP—MS锆石u—Pb测年结果,锆石核部年龄范围为242~2490Ma,记录了岩浆岩源岩的年代信息：锆石幔部具有典型的岩浆振荡环带结构,年龄为207Ma±2Ma,相应的Th／u值介于0．02-0．30之间,代表正片麻岩原岩的岩浆结晶时代．该年龄与羌塘中部地区晚三叠世高压变质作用和岛弧岩浆作用在时空上相对应。锆石增生边的年龄为161～197Ma．对应的Th／u值介于0．02-0．15之间,代表片麻岩发生主变质作用的时代,可能是班公湖一怒江洋盆向北的俯冲消减作用在羌塘中部地区的响应。地球化学资料显示,正片麻岩具有类似岛弧型火山岩的地球化学特征。综合区域地质资料,俄久卖高级变质岩原岩的形成与区域上广泛存在的晚三叠世构造、岩浆及角度不整合事件相对应,可能指示羌塘盆地统一基底的形成时代为晚三叠世。这对深入认识羌塘盆地基底的时代、性质及含油气盆地资源远景评价等具有重要意义。     

内蒙古巴林左旗二叠系碎屑锆石LA ICP MS U Pb年龄及构造意义

内蒙古巴林左旗碧流台地区分布一套碎屑岩,以砂板岩组合为特征,时代原为早二叠世。对该套地层所取砂岩样品进行LA-ICP-MS U-Pb年代学研究,对取自长石岩屑砂岩样品中100颗碎屑锆石进行测试分析,结果显示,100颗锆石的谐和年龄主要集中分布在4个年龄区间,分别为253~298Ma,309~362Ma,428~595Ma,851~1526Ma,最年轻的一组锆石年龄为253~298Ma,存在266Ma的锆石峰值年龄。266Ma的年龄限定了地层的沉积时限,即地层应在晚二叠世沉积,且砂岩的碎屑锆石年龄特征和地层岩性组合与区域上发育的林西组可以对比,故将该套地层划为林西组。年龄数据普值特征反映出,该地层具有多物源供给的特点,但物源主体来自东北各地块,未见华北板块的物源区年龄信息,表明华北板块与西伯利亚板块在晚二叠世并未完全碰撞闭合,253~298Ma的锆石年龄与西拉木伦缝合带附近与板块俯冲、碰撞相关的岩浆事件相吻合,表明两大板块的最终碰撞拼合时间应晚于266Ma。     

滇西莲花山富碱斑岩体LA ICP MS锆石U Pb年代学、地球化学特征及其地质意义

莲花山富碱斑岩体位于兰坪盆地东缘,是滇西富碱斑岩带的重要组成部分。该岩体主要由石英二长斑岩和角闪石英二长斑岩组成,对不同岩性中的锆石进行了LA ICP MS U Pb同位素测年,获得其形成年龄为(356±05)~(357±05) Ma,表明该岩体的形成时代为始新世。其K2O/Na2O比值为097~142、K2O+Na2O含量为886%~959%,显示高钾富碱的特征,属于钾玄岩系列岩石;岩体具有轻稀土元素富集、重稀土元素亏损的特征,显示弱的Eu负异常。利用锆石Ti温度计,获得岩体中岩浆锆石样品的结晶温度较低,介于594~788 ℃,说明该岩体岩浆源区的形成与俯冲－碰撞作用有关。样品中存在有少量年龄为1 177~68 Ma的继承锆石,Nb/U比值为131~473,表明莲花山岩体的源区是由洋壳俯冲作用形成的交代富集地幔,在其上侵过程中受到壳源物质不同程度的混染;岩体侵位于印度板块—欧亚板块陆－陆碰撞的挤压环境向后碰撞伸展环境转换的构造背景下。     

陕西秦岭梁岩体LA ICP MS锆石U Pb年龄及其地质意义

三叠纪花岗岩在秦岭造山带广泛发育,直接记录了秦岭造山带演化过程中的重要信息。秦岭梁岩体位于秦岭造山带商丹缝合带北侧,岩性主要为石英二长斑岩,斑晶为钾长石,部分斑晶具有白色斜长石环边;基质主要由石英(25%)、斜长石(45%)、钾长石(10%)、黑云母(15%)、角闪石(5%)组成。秦岭梁岩体中的锆石自形程度好,具环带结构,显示岩浆锆石的特征,两个样品的LA ICP MS锆石U Pb同位素年龄分别为(2165±21) Ma和(2194±26) Ma,二者在误差范围内一致,表明该岩体的岩浆结晶年龄为晚三叠纪。秦岭梁岩体的岩浆结晶年龄与勉略洋壳向北俯冲的时间(248~200 Ma)一致,表明岩体的形成与勉略洋壳向北俯冲有密切联系;同时秦岭梁石英二长斑岩的地球化学特征显示其是洋壳俯冲时弧岩浆活动的产物。因此北秦岭地区勉略洋闭合时间和陆陆碰撞拼合时间应不早于216~219 Ma。     

新疆阿尔泰凯勒克赛依铁矿地质特征、锆石LA MC ICP MS U Pb年龄及其意义

凯勒克赛依铁矿床是新疆阿尔泰唯一的小型镜铁矿床,赋存于一套变质火山-沉积岩系中,近矿围岩为白云母石英片岩,矿体呈层状,与地层产状一致,矿石具有块状、条带状、条纹状构造,矿石中金属矿物主要为镜铁矿(TFeO=87349%~88988%,TiO2=0~1042%,Al2O3=0036%~0256%),矿化具有沉积特征。近矿围岩镜铁矿白云母石英片岩锆石LA MC ICP MS U Pb谐和年龄为(3756 ± 06)Ma,限定成矿时代在376 Ma左右,即中泥盆世成矿,是阿尔泰为数不多的中泥盆世成矿作用的产物。同时也厘定含矿的变质火山-沉积岩系属中—晚泥盆世阿勒泰镇组,不是前人认为的早泥盆世康布铁堡组。     

西天山乌孙山地区大哈拉军山组火山岩LA—ICP—MS锆石U—Pb年龄及其构造环境

西天山乌孙山地区大哈拉军山组由玄武岩、安山岩、英安岩、流纹岩及相应的火山碎屑岩组成,安山岩和流纹岩分布最广。LA—IcP—Ms锆石U-Pb定年结果表明,火山活动喷发的安山岩与安山质晶屑凝灰熔岩分别形成于353．9Ma_6．5Ma和3563Ma±4．4Ma．属于早石炭世早期。通过区域对比,西天山大哈拉军山组的火山岩浆作用显示从伊犁中天山板块南北缘向伊犁盆地内部逐渐变年轻的特点,且火山岩喷发时代差别不大（约40Ma）。岩石地球化学研究表明,火山岩属钙碱性系列,富集轻稀土元素,相对亏损重稀土元素。中性火山岩富集大离子亲石元素（如Cs、Rb、Th、U）,而相对亏损高场强元素,具有明显的Nb、Ta、Ti负异常,显示出岛弧火山岩的特征;酸性火山岩相对富集Rb、Th、u、Ta等元素,具有明显的Ba、sr、P、Eu、Ti等元素的负异常。综合伊犁一中天山板块南缘的构造演化特征,认为大哈拉军山组形成于活动大陆边缘环境,产在板块俯冲一碰撞的最后阶段。     

桑干地区早元古代花岗岩长石Pb同位素组成和锆石U-Pb年龄：变质与地壳熔融作用及构造-热事件演化

桑干地区大同－集宁一带孔兹岩系分布区有大量早元古代花岗岩发育。依据矿物组成和地质特征,这些花岗岩可分为两种类型：大规模的石榴石花岗岩和小规模的淡色花岗岩。花岗岩中长石Ｐｂ同位素组成显示,石榴石花岗岩是孔兹岩系部分熔融的直接产物,而淡色花岗岩不是孔兹岩系简单的部分熔融产物。石榴石花岗岩中锆石Ｕ－Ｐｂ一致年龄为１８３６±１８Ｍａ,代表石榴石花岗岩的形成年代。淡色花岗岩的锆石Ｕ－Ｐｂ一致线上交点年龄为１９１２±９８Ｍａ,形成略早。孔兹岩系２０７Ｐｂ／２０４Ｐｂ值整体上高于华北麻粒岩下地壳,具有上地壳的Ｐｂ同位素组成特征,其沉积原岩应该来自华北晚太古代形成的大陆地壳。根据深成岩浆作用和变质作用的年代学资料,可以确定桑干地区早元古代如下构造－热事件序列：小基性岩体侵入（２．２～２．３Ｇａ）、早期淡色花岗岩生成（２．１～１．９Ｇａ）、麻粒岩相变质作用、剪切作用和大规模石榴石花岗岩发育（１．８４Ｇａ）、伟晶岩的形成（１．８０Ｇａ）和基性岩墙群的出现（１．７７Ｇａ）。大规模石榴石花岗岩形成于构造－热事件峰期     

鄂尔多斯盆地西南部上古生界碎屑锆石U-Pb年龄及其构造意义

通过对鄂尔多斯盆地西南部晚古生代山西组1段和下石盒子组8段碎屑锆石进行LA-ICP-MS U-Pb测年分析,结合周缘地层年龄结构和地质历史事件,进而追寻盆地沉积物物源,推断盆地与造山带的盆山耦合过程。研究表明105个岩浆成因的碎屑锆石可分为4个年龄组段:(1)260~340 Ma,占总数的21.9%,推断物源主要来自北秦岭和西秦岭构造带;(2)370~470 Ma,占总数的24.8%,反映物源主要来自北秦岭、西秦岭构造带和北祁连造山带;(3)1600~2000 Ma,占总数的32.4%,指示物源来自北秦岭造山带、北祁造山带和华北板块;(4)2300~2600 Ma,占总数的15.2%,物源分别来自华北板块基底结晶岩系、北祁连构造带、北秦岭构造带和西秦岭构造带。研究区总体上具有来自北秦岭造山带、西秦岭造山带、北祁连造山带、兴蒙造山带及华北板块基底五个物源区,其中兴蒙造山带、北秦岭造山带和北祁连造山带为主要物源区。古生代碎屑锆石年龄证实了鄂尔多斯盆地西南部奥陶纪被动大陆边缘形成,志留纪—泥盆纪转化为陆-陆碰撞造山带,石炭纪—二叠纪逐渐由造山带转化为沉积盆地。     

连山关-高家沟花岗岩体LA—ICP—MS锆石U—Pb年龄及其地质意义

笔者采用颗粒锆石LA—ICP—MS法获得了连山关-高家沟花岗岩体U—Pb不一致线上交点年龄为2198±31．5Ma和2162±31Ma。通过与辽东地区U—Pb法、Sm—Nd法和颗粒锆石年龄比对,揭示了辽东地区广泛发育的一期古元古代花岗岩浆事件的精确年代。根据岩石地球化学特征和岩体与地层接触关系,笔者认为该期花岗岩是在拉伸状态下早期基底岩石重熔的产物。年代学资料表明,连山关-高家沟岩体为壳源岩浆多次重熔侵位的花岗杂岩体。     

Late Jurassic–Early Cretaceous Plutonism in the Northern Part of the Precambrian North China Craton: SHRIMP Zircon U–Pb Dating of Diorites and Granites from the Yunmengshan Geopark, Beijing

The Yunmengshan Geopark in northern Beijing is located within the Yanshan range. It contains the Yunmengshan batholith, which is dominated by two plutons: the Yunmengshan gneissic granite and the Shicheng gneissic diorite. Four samples of the Yunmengshan gneissic granite give SHRIMP zircon U–Pb ages from 145 to 141 Ma, whereas four samples of the Shicheng gneissic diorite have ages from 159 Ma to 151 Ma. Dikes that cut the Yunmengshan diorite record SHRIMP zircon U–Pb age of 162±2 and 156±4 Ma. The cumulative plots of zircons from the diorites show a peak age of 155 Ma, without inherited zircon cores, and the peak age of 142 Ma for granite is interpreted as the emplacement age of the Yunmengshan granitic pluton, whose igneous zircons contain inherited zircon cores. The data presented here show that there were two pulses of magmatism: early diorites, followed c13 Ma later by true granites, which incorporated material from an older continental crust.     

Oldest zircon xenocryst (4.17 Ga) from the North China Craton

Prior to this work, the existence of crustal materials older than 4.0 Ga has not been reported from the North China Craton (NCC) – one of the few global terrains where crustal rocks from ～3.8 Ga have been identified. Here we report the first occurrence of a xenocrystic zircon with a ²⁰⁷Pb/²⁰⁶Pb age of 4174 ± 48 Ma, from the Anshan–Benxi Archaean supracrustal greenstone belt, based on laser ablation–inductively coupled plasma–mass spectrometry. The 4.17 Ga zircon xenocryst is hosted within ～2523 ± 12 Ma massive fine-grained amphibolites which were subsequently metamorphosed at ～2481 ± 19 Ma. The xenocryst age is ca. 350 million years, older than the oldest zircon previously identified in the NCC, and is consistent with prior zircon Lu–Hf isotopic studies. Documentation of 4.17 Ga xenocrystal zircon not only provided a geochronological record of the oldest known crustal materials in the NCC, but also identified the geologic environment for further search for the rocks that formed during Earth’s earliest recorded evolution.     

Laser Ablation ICP-MS U-Pb Zircon Geochronology of Granitoids and Quartz Veins in the Shihu Gold Mine, Taihang Orogen, North China: Timing of Gold-mineralization and Tectonic Implications

The Shihu gold deposit, situated in the Taihang Mesozoic orogen of the North China Craton (NCC), is hosted by ductile-brittle faults within Archean metamorphic core complex. The deposit is characterized by gold-bearing quartz-polymetallic sulfides veins. The Mapeng granitoids stock and intermediate-basic dikes intruded the metamorphic basement rocks, and are spatially related to gold mineralization. Detailed laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) U-Pb zircon ages of the granitic rocks, dykes and mineralized quartz veins in the studied area reveal its magmatic and mineralized history. The mineralized quartz veins contain inherited zircons with ages of about 2.55 Ga and 1.84 Ga, probably coming from the basement. These two Precambrian events are coeval with those in other parts of the NCC. The Mapeng granitoid stock, the largest intrusion in the area, was emplaced at ca. 130 Ma, and is coeval with magmatic zircon populations from diorites and quartz diorite pophyrites in the same region. The ca. 130 Ma magmatism and gold mineralization were most likely related to an underplating event that took place in the Taihang orogen at Late Mesozoic. The timing of gold mineralization with respect to felsic magmatism in the area is similar to those observed in other major gold-producing provinces in the NCC. This episode is simultaneous with those in the eastern margin of NCC, indicative of a widespread late Yanshanian metallogenic event that was a response to the Early Cretaceous lithosphere in the eastern NCC, in which the mesothermal gold deposits were formed from similar tectono-magmatic environments.     

Zircon U–Pb and pyrite Re–Os age constraints on pyrite mineralization in the Yinjiagou deposit,China

We report new zircon U–Pb and pyrite Re–Os geochronological studies of the Yinjiagou poly-metallic deposit, sited along the southern margin of the North China Craton (SMNCC). In this deposit, pyrite, the most important economic mineral, is intergrown/associated with Mo, Cu, Au, Pb, Zn, and Ag. Prior to our new work, the age of chalcopyrite–pyrite mineralization was known only from its spatial relationship with molybdenite mineralization and with intrusions of known ages. The U–Pb and Re–Os isotope systems provide an excellent means of dating the mineralization itself and additionally place constraints on the ore genesis and metal source. Zircons separated from the quartz–chalcopyrite–pyrite veins include both detrital and magmatic groups. The magmatic zircons confine the maximum age of chalcopyrite–pyrite mineralization to 142.0 ± 1.5 Ma. The Re–Os results yield an age of 141.1 ± 1.1 Ma, which represents the age of the chalcopyrite–pyrite mineralization quite well. The common Os contents are notably low (0.5–20.1 ppt) in all samples. In contrast, the Re contents vary considerably (3.0–199.2 ppb), most likely depending on intensive boiling, which resulted in an increase of Re within the pyrite. This study demonstrates that the main chalcopyrite–pyrite mineralization occurred late in the magmatic history and was linked to a deeper intrusion involving dominant mantle-derived materials. This mineralization event might be related to the Early Cretaceous lithospheric destruction and thinning of the SMNCC.     

Geochronology of the Duguer range metamorphic rocks,Central Tibet: implications for the changing tectonic setting of the South Qiangtang subterrane

The Duguer area represents one of the few occurrences of high-grade metamorphic rocks in the ‘Central Uplift’ zone of the Qiangtang terrane, central Tibet. The metamorphic rocks consist mainly of orthogneiss, paragneiss, and schist. To better understand the formation of these rocks, seven samples of gneiss and schist from the Duguer area were selected for in situ zircon U–Pb analysis and Ar–Ar dating of metamorphic minerals. The results suggest two distinct metamorphic stages, during the Late Triassic (229–227 Ma) and Late Jurassic (150–149 Ma). These stages correspond to the closure of the Palaeo-Tethys Ocean and northward subduction of the Bangong–Nujiang Neo-Tethys oceanic crust, respectively. We suggest that the Late Triassic metamorphic rocks of the Duguer area in the central South Qiangtang subterrane provide evidence of continental collision between the North and South Qiangtang subterranes, following the subduction of oceanic crust. It is likely that deep subduction of oceanic crust occurred along the Longmu Co–Shuanghu–Lancangjiang suture zone (LSLSZ), which would have hindered exhumation owing to the high density of oceanic crust. Subsequent break-off and delamination of the subducted oceanic slab at ~220 Ma may have resulted in exhumation of high-pressure and high-grade metamorphic rocks in the South Qiangtang subterrane. The Late Jurassic ages of metamorphism and deformation obtained in this study indicate the occurrence of an Andean-type orogenic event within the South Qiangtang subterrane. This hypothesis is further supported by an apparent age gap in magmatic activity (150–130 Ma) along the magmatic arc, and the absence of Late Jurassic sediments.     

庐山“星子变质核杂岩”中海会花岗岩的锆石UPb年龄及大地构造意义

单颗锆石ＵＰｂ 年龄测定结果显示,星子变质核杂岩中的海会花岗岩岩体的形成年龄为１２２ Ｍａ～１３０ Ｍａ,表明星子变质核杂岩形成于中生代,这与当时华南岩石圈的伸展构造环境一致。花岗岩中１ ７２３±３４８ Ｍａ的残留锆石年代信息显示,海会花岗岩是由古元古代地壳重熔形成的。扬子古陆南缘确实存在古元古代的基底     

Decoding Provenance and Tectonothermal Events by Detrital Zircon Fission-Track and U-Pb Double Dating: A Case of the Southern Ordos Basin

Multi-dating on the same detrital grains allows for determining multiple different geo-thermochronological ages simultaneously and thus could provide more details about regional tectonics. In this paper, we carried out detrital zircon fission-track and U-Pb double dating on the Permian-Middle Triassic sediments from the southern Ordos Basin to decipher the tectonic information archived in the sediments of intracratonic basins. The detrital zircon U-Pb ages and fission-track ages, together with lag time analyses, indicate that the Permian-Middle Triassic sediments in the southern Ordos Basin are characterized by multiple provenances. The crystalline basement of the North China Craton (NCC) and recycled materials from pre-Permian sediments that were ultimately sourced from the basement of the NCC are the primary provenance, while the Permian magmatites in the northern margin of NCC and Early Paleozoic crystalline rocks in Qinling Orogenic Collage act as minor provenance. In addition, the detrital zircon fission-track age peaks reveal four major tectonothermal events, including the Late Triassic-Early Jurassic post-depositional tectonothermal event and three other tectonothermal events associated with source terrains. The Late Triassic-Early Jurassic (225–179 Ma) tectonothermal event was closely related to the upwelling of deep material and energy beneath the southwestern Ordos Basin due to the coeval northward subduction of the Yangze Block and the following collision of the Yangze Block and the NCC. The Mid-Late Permian (275–263 Ma) tectonothermal event was associated with coeval denudation in the northern part of the NCC and North Qinling terrane, resulting from the subduction of the Paleo-Asian Ocean and Tethys Ocean toward the NCC. The Late Devonian-early Late Carboniferous (348±33 Ma) tectonothermal event corresponded the long-term denudation in the hinterland and periphery of the NCC because of the arc-continent collisions in the northern and southern margins of the NCC. The Late Neoproterozoic (813–565 Ma) tectonothermal event was associated with formation of the Great Unconformity within the NCC and may be causally related to the Rodinia supercontinent breakup driven by a large-scale mantle upwelling.