内蒙古商都地区白云鄂博群尖山组碎屑锆石U-Pb年代学和Hf同位素研究:对华北克拉通西部陆块北缘前寒武纪地壳演化的制约

华北克拉通前寒武纪地壳生长过程是地质学的研究热点。本文对华北克拉通西部陆块北缘的白云鄂博群尖山组进行了碎屑锆石U-Pb定年和Hf同位素分析,并在此基础上对尖山组的沉积时限、物源及西部陆块北缘的前寒武纪地壳生长过程进行了探讨。样品TM33中碎屑锆石年龄主要集中在1872～2100Ma (n=64),并有～1960Ma的主年龄峰值和～2000Ma的次年龄峰值,Hf同位素分析结果显示锆石的εHf(t)值在-2.6～7.9之间(除1个测点为-17.6),Hf同位素二阶段模式年龄(tDM2)变化范围在2240～4386Ma之间。样品Z1724中碎屑锆石年龄分布于1784～2200Ma(n=44),并有～1965Ma的主年龄峰值和～2080Ma次年龄峰值。谐和线上最年轻一组锆石的加权平均年龄为1794±73Ma(n=2),εHf(t)在-5.4～3.2之间,Hf同位素二阶段模式年龄(tDM2)变化范围在2584～3203Ma之间。基于以上数据,结合侵入白云鄂博群底部1670Ma的辉长岩以及白云鄂博群底部全岩207Pb-206Pb等时线年龄为1649Ma的沉积碳酸盐岩,尖山组的沉积时间被限定在1800～1650Ma。尖山组的物源主要来自固阳、西乌兰不浪地区新太古代基底以及大青山和卓资地区古元古代晚期孔兹岩带。综合前人对研究区及邻区中新元古代沉积地层中碎屑锆石的Hf同位素研究结果,认为华北克拉通西部陆块北缘可能存在2600～2750Ma、～2500Ma和2000～2100Ma三期主要的地壳生长事件,且最后一次规模可能小于前两次。     

青藏高原东北缘寺口子剖面碎屑锆石示踪及其构造意义

位于青藏高原东北缘的寺口子剖面发育巨厚的新生代沉积地层,分析这些沉积物的碎屑来源能够为高原东北缘构造变形过程提供重要证据。本文在寺口子剖面磁性地层年代的约束下,对该剖面27～4 Ma的砂岩样品进行了碎屑锆石示踪研究。研究结果显示27~12 Ma的砂岩样品中锆石U-Pb年龄主要分布在200～470 Ma（以230 Ma、440 Ma为峰值年龄）1600-1890 Ma、2100-2450 Ma,与>12 Ma的砂岩样品相比,7 Ma的砂岩样品中新增了锆石U-Pb年龄为720-980 Ma的年龄峰值;4 Ma的砂岩样品中锆石U-Pb年龄谱主要为200～490 Ma。这些样品中,1600-1890 Ma与2100-2450 Ma的锆石εHf(t)值偏负（-31.1～5.1）,720-980 Ma的锆石具有负的εHf(t)值为-15.1～-1.7, 200～470 Ma的锆石εHf(t)值范围较宽-11.2～12.5。通过与周围构造单元对比,发现1600-1890 Ma与2100-2450 Ma的锆石可能源于与鄂尔多斯地块西缘,720-980 Ma的锆石与宁夏中部的南华山、西华山岩石具有亲缘性,U-Pb年龄为200～470 Ma的锆石则与六盘山南部岩浆岩的锆石U-Pb年龄一致。寺口子剖面碎屑锆石示踪与半定量估算表明：六盘山南部可能在27 Ma已隆升、变形,成为宁夏南部盆地的物源区,而宁夏南部盆地晚中新世的物源变迁可能反映了海原-六盘山断裂的强烈构造变形。     

长江现代沉积物碎屑锆石U-Pb年龄及Hf同位素组成与物源示踪研究

为研究长江沉积物物质来源,对长江流域24个现代沉积物样品进行碎屑锆石U-Pb年龄测试,并对10个样品做Hf同位素分析,结果表明:长江河流沉积物的碎屑锆石多为岩浆成因.碎屑锆石U-Pb年龄主要有6组峰:2.4 ～2.6Ga,1.8 ～2.0Ga,700 ～ 1000Ma,400 ～ 550Ma,200～300Ma和＜65Ma,其中200～300Ma和700 ～1000Ma为主要的两组峰.锆石U-Pb年龄谱系可区分出长江4个区段,即金沙江段、川江段、长江中游段和下游段,与地理分段相吻合.结合Hf同位素研究,可认为长江流域沉积物中生代-新生代的锆石主要来自松潘-甘孜褶皱带和秦岭造山带;古生代的锆石多来自秦岭造山带;元古代-太古代的锆石则多来自扬子克拉通、华夏地块和大别造山带.     

内蒙古武川西乌兰不浪地区早前寒武纪变质基底锆石SHRIMP定年及Hf同位素组成

本文报道了华北克拉通西部武川西乌兰不浪地区太古宙变质基底的锆石SHRIMP年龄和Hf同位素组成。一个片麻状奥长花岗岩样品的锆石具核边结构,核部岩浆锆石和边部变质锆石的207Pb/206Pb加权平均年龄分别为2692±17Ma和2528±16Ma。对9个样品进行了锆石Hf同位素分析。新太古代早期(2692～2697Ma)片麻状奥长花岗岩(2个样品)的岩浆锆石的εHf(t)、tDM1(Hf)和tDM2(Hf)分别为4.78～8.83、2646～2780Ma和2632～2845Ma;新太古代二辉麻粒岩(2个样品)中的捕获锆石的εHf(t)、tDM1(Hf)和tDM2(Hf)分别为-2.30～8.62、2543～2954Ma和2529～3189Ma;新太古代变质深成岩(4个样品)的岩浆锆石的εHf(t)、tDM1(Hf)和tDM2(Hf)分别为-2.60～8.09、2529～2880Ma和2538～3089Ma;古元古代蓝晶石榴长英质片麻岩(1个样品)的碎屑锆石的εHf(t)、tDM1(Hf)和tDM2(Hf)分别为1.52～6.59、2432～2774Ma和2498～2925Ma。结合前人研究结果,可得出如下结论和认识:1)该区存在新太古代早期片麻状奥长花岗岩,太古宙岩石在新太古代晚期普遍遭受高级变质作用影响;2)新太古代早期为该区地壳形成主要时期,新太古代晚期则主要表现为陆壳物质再循环;3)作为阴山地块的典型代表,固阳-武川地区与华北克拉通东部太古宙基底十分类似,可能表明华北克拉通在新太古代晚期已成为统一的整体。     

青藏高原东北缘柴达木盆地红沟剖面物源分析及其构造意义

青藏高原东北缘柴达木盆地红沟剖面发育巨厚的新生代沉积地层,通过分析其沉积物质来源,可以揭示柴达木盆地潜在物源区隆升、剥蚀历史,为高原东北缘新生代构造变形过程提供证据。本文以红沟剖面磁性地层年代框架为约束,对剖面晚渐新世-上新世的碎屑砂岩样品进行了碎屑锆石物源示踪分析。研究结果表明,23.7～12.5 Ma样品的锆石U-Pb年龄主要分布在220～290 Ma,锆石ε_(Hf)(t)集中在—13.53～9.27,Hf同位素t_(DM)范围524～1456Ma;大于300Ma的锆石,ε_(Hf)(t)介于—31.77～13.44,其中76.3%为负值,Hf同位素t_(DM)介于484～3727 Ma。采集自12.5～7.6 Ma样品的锆石U-Pb年龄主要集中在400～500 Ma(峰值～440 Ma),ε_(Hf)(t)值(—27.75～10.75)的90%为负值,Hf同位素t_(DM)介于615～2115 Ma之间。6.8～5.5 Ma样品的锆石U-Pb年龄主要分布在400～500 Ma,ε_(Hf)(t)值(—26.8～8.97),t_(DM(Hf)))介于668～2093 Ma,但220～290 Ma的锆石显著增加,其ε_(Hf)(t)值(—11.86～9.42),Hf一阶段模式年龄范围549～1399Ma。碎屑锆石Hf同位素组成对比分析显示红沟剖面220～290 Ma的锆石与东昆仑山锆石Hf同位素特征相似,而400～500 Ma的锆石则与南祁连山锆石Hf同位素组成相似,揭示东昆仑山在24 Ma开始抬升成为柴达木盆地的源区,～12 Ma南祁连山开始隆升,为柴达盆地提供碎屑物质,成为青藏高原东北缘的构造与地貌边界。     

河北阜平杂岩中阜平岩群浅粒岩锆石U-Pb-Hf同位素特征及其地质意义

华北克拉通中部阜平地区的阜平岩群是该区分布较广的太古宙地层,经历了角闪岩相-麻粒岩相变质,其时代限定对研究阜平杂岩的早期演化过程具有重要意义。本文利用LA-MC-ICP-MS(多接收激光剥蚀电感耦合等离子体质谱仪)对阜平岩群元坊岩组中的浅粒岩进行了锆石U-Pb-Hf同位素原位分析,获得两期变质锆石年龄分别为2 531±15 Ma和1 943±16 Ma,并根据碎屑锆石内部结构特征和年龄结果,认为核部年龄在2 549～2 500 Ma的碎屑锆石中最大207Pb/206Pb年龄2 549±4 Ma可以代表原岩的最大沉积年龄,初步限定元坊岩组浅粒岩原岩沉积时代为2 550～2 530 Ma。锆石Lu-Hf同位素结果中,176Lu/177Hf值为0. 000 289～0. 004 262,176Hf/177Hf值为0. 281 255～0. 281 791。176Hf/177Hfi值为0. 281 230～0. 281 623,εHf(t)值为-5. 86～13. 62,变化范围较大。单阶段和两阶段模式年龄分别为2 748～2 242 Ma和2 810～2 272 Ma。根据锆石U-Pb年龄和Hf同位素结果,提出元坊岩组浅粒岩物源区主要来自新太古代TTG质片麻岩,2. 8～2. 6 Ga为阜平地区强烈的地壳生长阶段,阜平地区2. 5 Ga和1. 95 Ga变质信息分别代表华北初步克拉通化和最终克拉通化过程中的变质作用。     

内蒙古大青山地区早前寒武纪变质岩的锆石Hf同位素组成和稀土模式

本文报道了内蒙古大青山地区早前寒武纪变质岩石的锆石Hf同位素和稀土组成。两个古元古代晚期（1.9～2.1 Ga）变质碎屑沉积岩样品中碎屑锆石的(n(176Hf)/n(177Hf))c、tDM1(Hf)和tDM2(Hf)分别为0.281079～0.281502、2548～3000 Ma、2612～3153 Ma和0.280916～0.281451、2533～2717 Ma、2600～3404 Ma; 一个古元古代早期（2.37 Ga）变质辉长岩样品中岩浆锆石的εHf(t)和tDM1(Hf)分别为1.50～6.68和2449～2647 Ma,表明大青山及邻区在新太古代晚期—古元古代早期存在强烈的构造岩浆热事件,既有地幔添加又有壳内再循环作用。三个样品的边部变质锆石εHf(t)、tDM1(Hf)和tDM2(Hf)分别为-9.49～3.91、2201～2686 Ma、2285～2887 Ma;-7.29～-2.42、2350～2540 Ma、2499～2740 Ma和-5.46～-0.53、2319～2507 Ma、2443～2687 Ma,Th/U比值普遍小于01。与核部锆石相比,边部变质锆石tDM2(Hf)变小,Th/U比值和稀土含量降低,但稀土模式十分类似。研究表明,变质锆石增生边的形成及其Hf同位素、稀土和U—Th组成受核部锆石和变质作用的双重制约。变质增生边的形成至少部分与核部锆石溶解以后的再结晶有关,变质流体起了重要作用。     

班韦乌卢地块中部变质表壳岩碎屑锆石U-Pb年代学、Hf同位素研究及其构造意义

通过LA-MC-ICP-MS碎屑锆石微区U-Pb定年和Hf同位素分析,对班韦乌卢地块中部地区基底变质表壳岩进行了同位素定年及物源示踪研究.结果显示,两件样品的碎屑锆石峰值年龄分布表现出较好的一致性,总体呈现出2490～2059 Ma、2044～1954 Ma、1942～1887 Ma、1865～1817 Ma以及164...     

重庆南川铝土矿物源分析：碎屑锆石U- Pb定年、Hf同位素和锆石微量元素示踪

重庆南川沉积型铝土矿主产于中下志留统韩家店组与中二叠统梁山组的侵蚀面上,即赋矿层位为梁山组中下部(大竹园组),是黔北-渝南铝土成矿带的重要组成部分。铝土矿中碎屑锆石丰富,具有典型振荡环带特征,应用LA-(MC)-ICP-MS方法分别对碎屑状和土状铝土矿样品进行了全岩地球化学、锆石U-Pb定年、Hf同位素和锆石微量元素分析。结果显示两件样品为陆壳的、与造山/弧作用相关的花岗岩类锆石,其U-Pb年龄分布从晚泥盆世到太古宙(406～3054Ma)均有出现,主要显示出4个年龄区间:410～630Ma、732～1251Ma、1405～1757Ma和2409～2553Ma,但高峰在732～1251 Ma,峰值年龄出现在800 Ma和1000 Ma,且锆石εHf(t)值主要分布在-15～10间。铝土矿和韩家店组、黄龙组一致的碎屑锆石年龄分布和沉积古地理环境暗示后两者是铝土矿最直接的母岩。通过对比华南板块不同地层的地质特征、锆石U-Pb年龄和Hf同位素,发现来自江南造山带西段和华夏陆块南岭-云开地块的中新元古代长英质岩石是碎屑锆石最主要的初始物源区,而扬子西缘和北缘贡献较少。     

内蒙赤峰楼子店拆离断层带下盘变形花岗质岩石的时代、成因及其地质意义——锆石U-Pb年龄和Hf同位素证据

对内蒙赤峰楼子店拆离断层带下盘前人划为前寒武纪岩石的糜棱状花岗质岩石中锆石进行了U-Pb年龄测定和Hf同位素测试,结果显示其时代为晚古生代至中生代。楼子店扎兰营子片麻状花岗岩的锆石206Pb/238U年龄为253.6±1.2Ma,锆石εHf(t)值为-8.6～-14.6,锆石Hf同位素地壳模式年龄为1.8～2.2Ga;朝阳沟糜棱岩化片麻状花岗岩的锆石206Pb/238U年龄为150.43±0.79Ma,锆石εHf(t)值为-5.6～-14.9,锆石Hf同位素地壳模式年龄为1.6～2.1Ga;莫里海沟片麻状闪长岩的锆石206Pb/238U年龄为127.6±3.1Ma,锆石εHf(t)值为-5.1～-13.9,锆石Hf同位素地壳模式年龄为1.5～2.1Ga。不同岩性、不同形成年龄的3个样品的εHf(t)值主要为负值,说明这些岩石主要来自地壳岩石的部分熔融。2.2～1.5Ga的锆石Hf同位素两阶段模式年龄表明它们可能主要来源于华北克拉通下地壳物质的部分熔融。结合该区已经获得的锆石U-Pb年龄,将该区古生代至中生代花岗质岩浆作用划分为4个时期:早石炭世(327Ma)、二叠纪(285～252Ma)、中三叠世—早侏罗世(241～184Ma)、中侏罗世—早白垩世(163～125Ma)。早石炭世喇嘛洞混合花岗岩的产出对应于古亚洲洋古生代向南俯冲于华北板块的时期,二叠纪花岗岩是古亚洲洋最后闭合、蒙古弧与华北陆块北缘拼合与伸展有关的岩浆活动的产物,大面积的中三叠世—早侏罗世的花岗岩是西伯利亚与华北陆块碰撞后地壳伸展的记录,中侏罗世—早白垩世(163～125Ma)岩浆活动则发育在伸展构造背景中,与岩石圈减薄存在密切的成因联系。这些新年龄资料将为华北陆块北缘古生代—中生代的地质构造演化提供重要的年代学制约。     

鄂尔多斯盆地周缘地壳形成与演化历史:来自锆石U-Pb年龄与Hf同位素组成的证据

锆石U-Pb定年及Hf同位素测定结果表明,鄂尔多斯盆地周缘的华北板块北缘、兴蒙造山系及扬子板块-秦岭-大别-苏鲁造山带等构造单元系统具有明显不同的形成与演化历史。华北板块北缘锆石年龄平均值为1 837 Ma,最强烈的岩浆活动出现于2 200~1 800 Ma,该期锆石约占全部的40%;次为强烈的岩浆活动在2 800~2 200 Ma,其众数在全部锆石中约占30%;1 500~1 200 Ma、500~100 Ma这两个阶段形成的锆石在全区所占比例各约为15%。华北板块北缘最突出的特征是基本不含1 000~700 Ma期间形成的锆石,>3 000 Ma的锆石在全区分布极为有限。锆石Hf同位素亏损地幔模式年龄表明华北板块北缘平均值为2.55 Ga,较U-Pb平均年龄老,说明2 200~1 800 Ma期间形成的锆石含有较多的古老地壳再循环组分。Hf亏损地幔模式年龄最强峰值约为2.8 Ga,与全岩Nd亏损地幔模式年龄的峰值相一致,Hf模式年龄为3.0~2.25 Ga的颗粒占全部的近95%,证明华北板块北缘的地壳增生主要在太古宙至古元古代期间。Hf同位素亏损地幔模式年龄>3.0 Ga的锆石颗粒所占比例不到0.1%,另外近5%锆石的模式年龄分布于中元古代。晚古生代-中生代所形成的锆石均是先存地壳组分,尤其是中元古代增生地壳的熔融作用形成。兴蒙造山系中锆石U-Pb年龄平均值为497 Ma,最强峰分布于石炭纪(约320 Ma),石炭纪-二叠纪末(350~250 Ma)形成的锆石所占比例达30%以上。新元古代至早古生代(600~440 Ma)形成的锆石占全部锆石的55%以上,而中元古代末-新元古代期间(1 200~600 Ma)形成的锆石在全区仅占4%。中元古代以前形成的锆石非常有限,说明该区最早形成的地壳组分在兴蒙造山系的形成过程中较充分地参与到后期的岩浆作用过程中。兴蒙造山系中锆石相应的Hf同位素亏损地幔模式年龄平均为1.13 Ga,明显较相应的U-Pb年龄老,最强峰值出现于约0.6 Ga。Hf亏损地幔模式年龄为0.7~0.28 Ga的颗粒在兴蒙造山系所占比例达57%,证明该区最强烈的地壳增生发生于新元古代至古生代期间。Hf同位素亏损地幔模式年龄分布于1.5~0.7 Ga的锆石在全区约占38%,说明此期间也是该区地壳较强烈的增生期。Hf亏损地幔模式年龄大于1.5 Ga的锆石所占比例不到5%,古生代以后兴蒙造山系也基本没有明显的地壳增生。扬子与秦岭-大别-苏鲁造山带构造单元中的锆石U-Pb年龄平均为799 Ma,年龄为1 300~750 Ma的锆石在全部锆石中约占70%。晚古生代-燕山期形成的锆石约占20%。年龄在3 500~2 650 Ma、2 118~1 680 Ma的锆石在该区各约占5%。结合扬子与秦岭-大别-苏鲁造山带平均为1.56 Ga的Nd亏损地幔模式年龄特征,说明1 300~750 Ma期间该区较强烈的岩浆作用事件中有较多的古老地壳组分加入其中。锆石U-Pb年龄及Hf同位素组成均说明鄂尔多斯盆地周缘各构造单元具有不同的形成演化历史。地壳是幕式增长的,但各构造单元每幕发生的时间、强度存在明显差别。因此,由盆地中不同时代地层中碎屑锆石U-Pb年龄及Hf同位素组成及全岩Nd同位素特征的系统研究可反演盆地物源供给与周围构造单元之间的关系。     

Autochthonous inheritance of zircon through Cretaceous partial melting of Carboniferous plutons: the Arthur River Complex,Fiordland, New Zealand

TIMS and SHRIMP U–Pb analyses of zircons from Milford Orthogneiss metadiorite (P = 1–1.4 GPa; T ≥ 750°C) of the Arthur River Complex of northern Fiordland reveal a bimodal age pattern. Zircons are predominantly either Paleozoic (357.0 ± 4.2 Ma) and prismatic with oscillatory zoning, or Cretaceous (133.9 ± 1.8 Ma) and ovoid with sector or patchy zoning. The younger age component is not observed overgrowing older grains. Most grains of both ages are overgrown by younger Cretaceous (~120 Ma) metamorphic zircon with very low U and Th/U (0.01). We interpret the bimodal ages as indicating initial igneous emplacement and crystallisation of a dioritic protolith pluton at ~357 Ma, followed by Early Cretaceous granulite-facies metamorphism at ~134 Ma, during which a significant fraction (~60%) of the zircon grains dissolved, and subsequently reprecipitated, effectively in situ, in partial melt pockets. The remaining ~40% of original Paleozoic grains were apparently not in contact with the partial melt, remained intact, and show only slight degrees of Pb loss. Sector zoning of the Cretaceous grains discounts their origin by solid state recrystallisation of Paleozoic grains. The alternative explanation—that the Paleozoic component represents a 40% inherited component in an Early Cretaceous transgressive dioritic magma—is considered less likely given the relatively high solubility of zircon in magma of this composition, the absence of 134 Ma overgrowths, the single discrete age of the older component, equivalent time-integrated ¹⁷⁷Hf/¹⁷⁶Hf compositions of both age groups, and the absence of the Cambrian-Proterozoic detrital zircon that dominates regional Cambro-Ordovician metasedimentary populations. Similar bimodal Carboniferous-Early Cretaceous age distributions are characteristic of the wider Arthur River Complex; 8 of 12 previously dated dioritic samples have a Paleozoic component averaging 51%. Furthermore, the age and chemical suite affinity of these and several more felsic rocks can be matched with those of the relatively unmetamorphosed Carboniferous plutonic terrane along the strike of the Mesozoic margin in southern Fiordland, also supporting the in situ derivation of the Carboniferous “inherited” component.     

U–Pb–Hf zircon study of two mylonitic granite complexes in the Talas-Fergana fault zone,Kyrgyzstan, and Ar–Ar age of deformations along the fault

A 2000 km long dextral Talas-Fergana strike–slip fault separates eastern terranes in the Kyrgyz Tien Shan from western terranes. The aim of this study was to constrain an age of dextral shearing in the central part of the fault utilizing Ar–Ar dating of micas. We also carried out a U–Pb–Hf zircon study of two different deformed granitoid complexes in the fault zone from which the micas for Ar dating were separated. Two samples of the oldest deformed Neoproterozoic granitoids in the area of study yielded U–Pb zircon SHRIMP ages 728 ± 11 Ma and 778 ± 11 Ma, characteristic for the Cryogenian Bolshoi Naryn Formation, and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and their calculated tHfc ages varied from 2.42 to 2.71 Ga. Thus varying Cryogenian ages and noticeable heterogeneity of Meso- to Paleoproterozoic crustal sources was established for mylonitic granites of the Bolshoi Naryn Formation. Two samples of mylonitized pegmatoidal granites of the Kyrgysh Complex yielded identical ²⁰⁶Pb/²³⁸U ages of 279 ± 5 Ma corresponding to the main peak of Late-Paleozoic post-collisional magmatism in the Tien Shan (Seltmann et al., 2011), and zircon grains analyzed for their Lu–Hf isotopic compositions yielded εHf(t) values from −11.43 to −16.73, and calculated tHfc ages from 2.42 to 2.71 Ga indicating derivation from a Paleoproterozoic crustal source. Microstructural studies showed that ductile/brittle deformation of pegmatoidal granites of the Kyrgysh Complex occurred at temperatures of 300–400 °C and caused resetting of the K–Ar isotope system of primary muscovite. Deformation of mylonitized granites of the Bolshoi Naryn Formation occurred under high temperature conditions and resulted in protracted growth and recrystallization of micas. The oldest Ar–Ar muscovite age of 241 Ma with a well defined plateau from a pegmatoidal granite of the Kyrgysh Complex is considered as a “minimum” age of dextral motions along this section of the fault in the Triassic while younger ages varying from 227 Ma to 199 Ma with typical staircase patterns indicate protracted growth and recrystallization of micas during ductile deformations which continued until the end of the Triassic.     

Attribution of the Langshan Tectonic Belt: Evidence from zircon U-Pb ages and Hf isotope compositions

This study describes a previously unidentified Neoproterozoic mafic dyke emplaced in the northern flank of the Langshan Tectonic Belt. This dyke intruded into the micaquartz schist of the Zhaertaishan Group, and yielded an age of 908 ± 8 Ma. The youngest U-Pb ages of micaquartz schist from the Zhaertaishan Group in the Langshan area were 1118 ± 33 Ma,1187 ± 3 Ma and 1189 ± 39 Ma,suggesting that the depositional age of the protolith of the schist was between 908 ± 8 Ma and 1118 ± 33 Ma. In addition, 436 U-Pb age data and 155 Lu-Hf isotopic data from six samples in the Langshan Tectonic Belt and one Permian greywacke from the Wuhai area show distinct differences between the northern and southern flanks of the Main Langshan area. The U-Pb ages of the northern flank are primarily Meso-Neoproterozoic; similar ages have not been identified in the southern flank to date.Moreover, two-stage Hf model ages of the northern flank feature three age peaks at ～900 Ma,～1700 Ma and ～2600 Ma; this differs from Hf model ages of the southern flank, which feature one strong age peak at ～2700 Ma. These results suggest that the northern and southern flanks of the Main Langshan area have different geochronologic characteristics and should be divided further. Based on the U-Pb ages and Hf model ages, the northern and southern flanks of the Main Langshan area are named the North and South Langshan Tectonic Belts. Comparison of the U-Pb age and two-stage Hf model age distributions from the North Langshan Tectonic Belt, South Langshan Tectonic Belt, Alxa Block and the North China Craton(NCC) reveal that the North Langshan Tectonic Belt is similar to the Alxa Block and that the South Langshan Tectonic Belt is similar to the NCC. In addition, the zircon U-Pb age of 860 ±7 Ma commonly observed in the Alxa Block was detected in the Permian greywacke from the Wuhai area of the NCC, which suggests that the amalgamation of the North and South Langshan Tectonic belts(i.e.,the amalgamation of the Alxa Block and the NCC), occurred between Devonian and late Permian.     

Origin and evolution of topaz-bearing granites from the Nanling Range, South China: a geochemical and Sr–Nd–Hf isotopic study

Summary The F-rich Hongshan pluton in the eastern Nanling Range, southern China, is a topaz-bearing albite leucogranite. It is distinctive from other topaz-bearing felsic rocks in South China with respect to age, size, geochemical evolution and topaz mode and morphology. The Hongshan granites are highly peraluminous and characterized by high K₂O/Na₂O, Si, Rb, Cs, Nb, Ta and F, and low Ca, Ba, Sr, Zr, Hf, P, K/Rb, Zr/Hf and Eu/Eu^*. The granites show significant trace-element variations with magma evolution, with increasing Rb, Cs, Nb, Ta, Sn, W and decreasing Sr, Ba, Zr, Hf, Y, REE, Pb, Th, K/Rb, Zr/Hf, Th/U and Eu/Eu^*. These changes dominantly reflect fractional crystallization of plagioclase, biotite and accessory minerals such as zircon and monazite. The granites also exhibit a decrease in ɛNd(t = 225 Ma) from −7.9 to −11.7 with magma evolution. Modeling shows that the Nd isotopic variation could result from assimilation of the Taoxi Group wall rocks during fractional crystallization. The Hongshan pluton also shows spatial geochemical variations; the most evolved parts are located in the southeastern part of the pluton, which would be the most likely target area for rare-metal mineralization commonly associated with other topaz-bearing granites. Zircon grains from two rock types in the Hongshan body were analyzed in situ for U–Pb ages and Hf isotopic values. The concordant zircon grains mostly range from 218 to 230 Ma with an average of 224.6 ± 2.3 Ma (Indosinian). Some zircons with different internal structures and Hf isotope compositions, as well as monazite fragments, yield U–Pb ages of ca. 280 to 240 Ma, suggesting older thermal events in the studied area. The ɛHf(t) of these older zircons is strongly negative (−12.3), implying a crustal source with a Paleoproterozoic model age, similar to that for the Proterozoic Zhoutan Group. The main (∼225 Ma) zircon population exhibits less negative ɛHf(t) (−3.0 to −7.6) and Mesoproterozoic model ages, suggesting that the original magma of the Hongshan granite was generated from deeper Mesoproterozoic crust.     

Mantle and crustal sources of Archean anorthosite: a combined in situ isotopic study of Pb–Pb in plagioclase and Lu–Hf in zircon

Isotopic analyses of ancient mantle-derived magmatic rocks are used to trace the geochemical evolution of the Earth’s mantle, but it is often difficult to determine their primary, initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration. We present in situ analyses by LA-MC-ICPMS for the Pb isotopic compositions of igneous plagioclase (An_75–89) megacrysts and the Hf isotopic compositions of BSE-imaged domains of zircon grains from two mantle-derived anorthosite complexes from south West Greenland, Fiskenæsset and Nunataarsuk, which represent two of the best-preserved Archean anorthosites in the world. In situ LA-ICPMS U–Pb geochronology of the zircon grains suggests that the minimum crystallization age of the Fiskenæsset complex is 2,936 ± 13 Ma (2σ, MSWD = 1.5) and the Nunataarsuk complex is 2,914 ± 6.9 Ma (2σ, MSWD = 2.0). Initial Hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ε_Hf units. In terms of Pb isotopic compositions of plagioclase, both anorthosite complexes share a depleted mantle end member yet their Pb isotopic compositions diverge in opposite directions from this point: Fiskenæsset toward a high-μ, more radiogenic Pb, crustal composition and Nunataarsuk toward low-μ, less radiogenic Pb, crustal composition. By using Hf isotopes in zircon in conjunction with Pb isotopes in plagioclase, we are able to constrain both the timing of mantle extraction of the crustal end member and its composition. At Fiskenæsset, the depleted mantle melt interacted with an Eoarchean (~3,700 Ma) mafic crust with a maximum ¹⁷⁶Lu/¹⁷⁷Hf ~0.028. At Nunataarsuk, the depleted mantle melt interacted with a Hadean (~4,200 Ma) mafic crust with a maximum ¹⁷⁶Lu/¹⁷⁷Hf ~0.0315. Evidence from both anorthosite complexes provides support for the long-term survival of ancient mafic crusts that, although unidentified at the surface to date, could still be present within the Fiskenæsset and Nunataarsuk regions.     

北苏鲁变辉长岩锆石U-Pb年龄和Lu-Hf同位素组成及其对源区的指示

在北苏鲁经历超高压变质的花岗片麻岩中零星分布着一些变质表壳岩包体或者残片,通常称之为荆山群。北苏鲁荆山群的年代学特征以及构造亲缘性仍不清楚。本文报道了威海市郊和海阳所附近的荆山群露头,有脉状变质辉长岩侵入到荆山群岩系中。对上述两处变质辉长岩中锆石的阴极发光照片、稀土元素配分模式和Th/U比值的综合分析表明,它们均为岩浆锆石,部分岩浆锆石具有弱发光效应和岩浆韵律环带的核部、以及被改造的强发光效应的边部,但未见新生的变质锆石。未被改造的岩浆锆石微区的LA-ICP-MS U-Pb定年结果表明,威海市郊变辉长岩(S4-2)的~(207)Pb/~(206)Pb年龄值变化范围是1831~1966 Ma,加权平均年龄为1870±34 Ma(MSWD=0.6);海阳所变辉长岩(H3)中绝大部分锆石的~(207)Pb/~(206)Pb年龄值变化范围是1769~1887 Ma,加权平均年龄为1839±37 Ma(MSWD=0.5),两件样品共同记录了一致的ca.1.85 Ga岩浆事件。另外,海阳所变辉长岩中3颗捕获的岩浆锆石分别测得ca.2.9 Ga、ca.2.3 Ga和ca.2.1 Ga等三组~(207)Pb/~(206)Pb年龄。两件样品ca.1.85 Ga岩浆锆石具有相似的Hf同位素组成,其ε_(Hf)(t)=-6.1~-10.7,t_(DM2)=2.74~2.98 Ga,捕获ca.2.1 Ga和ca.2.9 Ga锆石的ε_(Hf)(t)值和t_(DM2)年龄分别是-1.9和2.7 Ga、-4.8和3.5 Ga。锆石U-Pb年龄、Lu-Hf同位素特征表明,北苏鲁两处变辉长岩记录的岩浆事件与胶北前寒武纪陆壳的多期演化历史一致,变辉长岩及其围岩荆山群可能是源自胶北的外来地质体。     

西藏冈底斯东部然乌地区早白垩世岩浆混合作用:锆石SHRIM PU-Pb年龄和Hf同位素证据

目前对西藏冈底斯带早白垩世大规模岩浆作用的岩石成因以及冈底斯带不同构造单元的东延仍存在不同看法。为探讨这些问题,文中对冈底斯带东部地区然乌岩体中的闪长岩脉进行了锆石SHRIM PU-Pb定年和锆石Hf同位素分析。结果表明:然乌岩体中闪长岩脉的锆石SHRIM PU-Pb年龄为(114.2±0.9)Ma,与二长花岗岩为同期侵位。然乌闪长岩脉具有不均一的锆石Hf同位素组成,其εHf(t)值介于-4.2～+4.9,对应的Hf同位素地壳模式年龄为0.85～1.44Ga。闪长岩脉的全岩εNd(t)值为-4.7,Nd同位素两阶段模式年龄(TDM2)为1.29Ga,与锆石Hf同位素模式年龄一致。然乌地区同期发生的闪长质岩浆和花岗质岩浆侵位以及不均一的锆石Hf同位素组成,很可能表明然乌地区大约在115Ma发生了重要的岩浆混合作用。结合锆石Hf同位素地壳模式年龄的区域性对比,我们认为,与北冈底斯带相比,然乌地区同中冈底斯带之间具有更好的可对比性。     

闽西南清流地区加里东期花岗岩锆石U-Pb年代学及Hf同位素组成研究

通过对玮埔岩体两个代表性样品进行锆石的LA-ICP-MSU-Pb测年获得的206Pb/238U加权平均年龄分别为(429.9±3.0)Ma、(446.3±4.0)Ma,证明该岩体确实不是最早认为的印支期花岗岩,而是加里东期岩体。同时对宁化岩体一个样品的锆石LA-ICP-MSU-Pb测年结果得到206Pb/238U加权平均年龄为(448.2±2.5)Ma,同样属于加里东期岩体。进一步对闽西南清流地区玮埔和宁化岩体的锆石Lu-Hf同位素组成分析表明,εHf(t)值主要集中在-0.8～-10.4之间,且Hf二阶段模式年龄(TDM2)集中在1.72～2.34Ga之间,表明其岩浆源区来自华夏地块元古代地壳物质。并结合其他己发表的相关年代学和地球化学资料,可以推断包括玮埔和宁化岩体在内的华南内陆地区加里东期花岗岩很可能是元古宙基底深熔而成的产物;其形成与陆内地壳物质叠置加厚所引起的再造作用关系紧密。     

松潘-甘孜褶皱带南部上三叠统物源及构造抬升:碎屑锆石年代学和Hf同位素证据

碎屑锆石年代学不但能够限定地层沉积开始的最大时限,还能为示踪沉积物源区提供关键信息。中国西南部的松潘-甘孜褶皱带广泛出露一套巨厚的三叠纪复理石沉积,其物源区和可能存在的同期抬升与剥蚀历史并未得到很好约束。本文获得的松潘-甘孜褶皱带南部雅江地区上三叠统四套地层（由老至新分别为侏倭组、新都桥组、两河口组和雅江组）5件砂岩样品的碎屑锆石U-Pb年龄和锆石Hf同位素数据表明,最年轻锆石年龄指示侏倭组从~229Ma后开始沉积,新都桥组则从~223Ma后开始沉积。碎屑锆石年龄频谱图显示四套地层都具有中奥陶世-早泥盆世（465~398Ma）和中二叠世-晚三叠世（271~225Ma）的年龄峰。除两河口组外的其他三套地层还具有较强的古元古代（1.90~1.86Ga）和新元古代（872~712Ma）的年龄峰。锆石Hf同位素显示松潘-甘孜褶皱带南部上三叠统小于300Ma的锆石颗粒主要来自峨眉山大火成岩省和义敦岩浆弧。本文物源区示踪结果表明,华南板块和义敦地体可能为松潘-甘孜褶皱带南部地层的主要物源区。晚三叠世由于周缘地体的强烈汇聚,松潘-甘孜褶皱带在小于~18Myr的时间内经历了快速的隆升和剥蚀作用,剥蚀产生的碎屑物质被搬运至四川盆地的西缘再沉积。