华北北缘大青山小井沟岩体年代学、地球化学和Hf同位素特征及其地质意义

为了加深对华北北缘晚古生代构造背景以及古亚洲洋闭合时限的认识, 对华北北缘大青山地区小井沟花岗岩开展了年代学、地球化学和同位素研究。分别利用LA-MC-ICP-MS, ICP-AES和ICP-MS等方法进行锆石U-Pb测年, 主、微量和稀土元素分析以及锆石Hf同位素测试。结果表明, 小井沟岩体侵位于二叠纪, 侵位年龄为275±1 Ma (MSWD=0.93)。该花岗岩具有高硅(SiO₂=70.72%~72.64%)、高钾(K₂O=4.19%~4.23%)的特点, A/CNK均约为1.1, 属于弱过铝质的高钾钙碱性系列, 稀土元素总量(ΣREE)为87.67~101.51 μg/g, 配分曲线呈右倾型, 具有微弱的Eu负异常, 富集大离子亲石元素(LILE, 如Rb, Ba, K, Sr等), 亏损高场强元素(HFSEs, 如Nb, Ta, Y, Yb, Lu等), 应该是由下地壳部分熔融形成。该花岗岩的锆石具有负的ε_Hf(t)值(-9.56~-5.00), 模式年龄(T_DM2)在1.91~1.61 Ga之间, 表明华北陆块古元古代的地壳岩石应该是其主要物源, 可能有幔源物质的参与。结合整个华北北缘二叠纪花岗岩的岩浆演变特征, 小井沟岩体应形成于由俯冲向碰撞-后碰撞转变的过渡时期, 古亚洲洋可能在晚二叠世之前已闭合。     

黑龙江金厂金矿花岗斑岩锆石地球化学、U-Pb年代学及地质意义

为确定与金厂金矿成矿有关的岩浆类型、活动时限和构造背景,采用LA-ICP-MS技术对研究区花岗斑岩开展了锆石U-Pb年龄及原位微区微量元素测定。结果表明:锆石环带发育,wTh/wU值>0.4,具有岩浆锆石特征;锆石年龄分布于220Ma和103～123Ma 2个区间,代表了2期岩浆事件;对2种锆石分别命名为捕获岩浆锆石和新生岩浆锆石;锆石的地球化学和年龄信息显示捕获岩浆锆石的原岩为早三叠世花岗岩;锆石微量元素信息暗示花岗斑岩是早三叠世花岗岩高度熔融结晶分异、侵位于浅部氧化环境而形成,这一过程导致新生岩浆锆石负Eu异常程度降低。新生锆石加权平均年龄为(113.5±3.8)Ma,与成矿年龄一致,据此认为早白垩世的岩浆事件是金厂金矿成矿事件的直接原因,成矿背景为太平洋板块俯冲后的岩石圈伸展。     

Hf isotopes of the 3.8 Ga zircons in eastern Hebei Province, China： Implications for early crustal evolution of the North China Craton

Zircon U-Pb dating indicates that the fuchsite quartzite in eastern Hebei Province was derived from weathering and erosion of the 3.6-3.8 Ga granitic rocks. In-situ zircon Hf analyses show that the Lu-Hf isotopic system remained closed during later thermal disturbances. Zircons with concordant ages have Hf isotopic model ages of about 3.8 Ga, suggesting a recycling of this ancient crust. The -3.8 Ga zircons have similar Hf isotopic compositions to those of chondrite, indicating that their source rocks （granitic rocks） were derived from partial melting of the juvenile crust which originated from a mantle without significant crust-mantle differentiation. Therefore, it is proposed that there was no large-scale crustal growth before -3.8 Ga in eastern Hebei Province. Considering zircon Hf isotopic data from other areas, it is concluded that the most ancient crust in the North China Craton probably formed at about 4.0 Ga, and the possibility to find crust older than 4.0 Ga is very limited.     

辽西万家沟地区钾长花岗岩年代学、岩石地球化学特征及其地质意义

为厘定辽西万家沟地区钾长花岗岩的形成时代及成因,对其进行了年代学和元素地球化学研究。结果表明：钾长花岗岩U-Pb同位素年龄为(155.9±1.8)Ma,岩石具有富Si、Al、Na、K,贫Mg、Ca、Ti等特征,轻稀土元素相对富集,大离子亲石元素Rb、Th、U、Ta等相对亏损,高场强元素Ba、Sr、Zr、Hf等相对富集,具较弱的铕和铈负异常。结合区域构造背景,认为研究区钾长花岗岩形成于晚侏罗世古太平洋板块向华北克拉通持续俯冲的构造环境背景,成因可能为古太平洋板块向欧亚板块俯冲导致幔源岩浆上侵使地壳发生部分熔融有关。     

Hf isotopic composition of zircons from the Huashan-Guposhan intrusive complex and their mafic enclaves in northeastern Guangxi: Implication for petrogenesis

High precision zircon U-Pb dating indicates that main intrusive bodies (Tong'an,Niumiao,Huashan,Lisong),and a mafic microgranular enclave in the Huashan-Guposhan complex were formed at 160-163 Ma.The εHf(t) values of zircons from the Huashan granite vary from -2.8 to +0.3 and those from the Lisong granite vary from -2.3 to +0.3,which are obviously different with those values (+2.6 to +7.4) of the mafic enclaves from the Lisong granite.These Hf isotopic data indicate that the mafic enclaves and host granites...     

甘肃北山桥湾北花岗岩体的年代学、地球化学及其地质意义

为加深对北山晚古生代构造演化的认识, 选取位于甘肃北山南带、敦煌地块北缘的桥湾北花岗岩体进行年代学、地球化学研究。桥湾北角闪石花岗岩体锆石LA-ICP MS年龄为303.7±2.4 Ma, ε_Hf(t)为-1.2~5.8, ε_Nd(t)相对较高(-0.40~-0.06), (⁸⁷Sr/⁸⁶Sr)_i =(0.704524~0.705062)。 桥湾北花岗岩体K₂O含量为4.09%~5.58%, 属高钾钙碱性岩石, A/CNK值在0.92~1.04之间, 为I型花岗岩; 微量元素较富集LILE, 相对亏损HFSE, 有明显的Nb, Ta负异常。岩体具相对富集的微弱分馏的轻稀土元素, 重稀土元素分馏不明显, 基本没有Eu异常。由Hf, Nd同位素及地球化学特征判断, 桥湾北花岗岩体为壳幔混合成因, 其形式可以是底侵的幔源岩浆再分异, 上升过程中遭受地壳物质混染。根据区域构造背景及岩石学、地球化学特征, 可以认为桥湾北岩体是北山南带后碰撞过程岩浆活动的产物, 反映出在晚石炭世北山地区的碰撞拼贴已经完成。     

福建明溪锆石巨晶中的斜锆石、锆石矿物包裹体及其成因启示

福建明溪是我国和火山岩有关的锆石巨晶的典型产地.通过对该地的锆石巨晶晶体形态及其包裹体的研究发现,福建明溪锆石巨晶四方柱明显较双锥发育,表面有熔蚀结构,内部结构均匀;锆石巨晶中发现的斜锆石包裹体和其它产地斜锆石相比具有贫大离子半径元素U,Th,Y,Zr/Hf高的特点.巨晶中的锆石包裹体的拉曼光谱有两种不同类型,其中一类锆石包裹体的拉曼谱和变质及热处理锆石相似,说明锆石巨晶形成时曾经历过变质作用过程.锆石巨晶及其包裹体特征暗示锆石巨晶不是玄武岩结晶产物,和伴生的刚玉巨晶相比其来源可能更深,是壳一幔物质相互作用产物.     

准苏吉花斑岩型钼铜矿床岩体特征及成矿机制研究

通过对辉钼矿的Re-Os及锆石U-Pb同位素年代学研究, 得到辉钼矿的Re-Os等时线年龄为297.2±4.3 Ma, 赋矿花岗斑岩的锆石U-Pb年龄为 301.1±4.0 Ma, 确定准苏吉花斑岩型钼铜(Mo-Cu)矿床的成岩成矿时代为晚石炭世至早二叠世。准苏吉花矿区不含矿花岗闪长岩的锆石U-Pb 年龄为301.2±2.2 Ma, 与花岗斑岩成岩时代一致。发育角闪石和黑云母的矿物学特征以及高Rb, Th和Ba, 低P和Ti的岩石地球化学特征表明, 花岗斑岩和花岗闪长岩同属于I型花岗岩。较低的Re含量、Mg#值、Nb/Ta值和Zr/Hf值以及低ISr值和正ε_Nd(t)值的全岩Sr-Nd同位素特征表明, 准苏吉花花岗岩的源区为新生下地壳的部分熔融, 岩浆演化过程中, 花岗斑岩和花岗闪长岩均经历较强烈的分离结晶作用, 有利于Mo进一步在残余熔体中富集。通过锆石的Ce⁴⁺/Ce³⁺值计算获得岩浆结晶分异时的氧逸度, 发现花岗斑岩岩浆的氧逸度相对较高(ΔFMQ平均值为+4.8), 花岗闪长岩岩浆氧逸度相对较低(ΔFMQ平均值为+2.2), 表明高氧逸度的岩浆更有利于Mo和Cu富集成矿。     

Typomorphism of the {211}-type zircon

A natural {211}-type zircon has been first discovered in the Pingtan gabbro in the Fujian Province, which is a new type of morphology reported in magmatic zircons. Investigation of morphology and trace-element chemistry shows two distinct growth stages, Stage 1 and Stage 2. The {211}-type zircon is the earliest crystallized phase of Stage 1 zircon. The low Hf contents and extreme depletion in U, Th and Y indicate that Stage 1 zircon was approximately in equilibration with melt during the crystallization, so that they are regarded as crystallized phase in deeper magmatic chamber. The Zr-saturated gabbroic magma is demonstrated to be derived from mantle magmas by differentiation. The magma differentiation and zircon crystallization at the lower crust level indicate the existence of underplating of mantle magma beneath this area, which results in strong magmatic activities in the late Mesozoic.     

Oxygen isotope evidence for two-stage water-rock interactions of the Nianzishan A-type granite in NE China

The oxygen isotope ratios of whole-rock, common rock-forming minerals and zircon from Mesozoic A-type granitic pluton at Nianzishan in northeastern China were analyzed by the conventional BrF5 method and the laser-probe technique, respectively. Both whole-rock and rock-forming minerals show large δ18O variations up to 5.5%. with significant oxygen isotope disequilibrium between zircon and the other minerals, whereas the δ18O values of zircon are tightly clustered between 3.12%. and 4.19%. and thus lower than the normal-mantle δ18O values. These results indicate that the Nianzishan A-type granite experienced two-stage water-rock interactions subsequentially. The remarkably low zircon δ18O values are genetically due to sea-water exchange with granite protolith in the first stage, and the oxygen isotope disequilibrium fractionations between zircon and rock-forming minerals are caused by meteoric-hydro thermal alteration in the second stage. It is inferred that the 18O-depleted A-type granitic magma was derived from partial melting of subducted lower oceanic crust which was isotopically exchanged with seawater at high temperatures. In the process of granite emplacement into the upper crust, meteoric-hydrothermal circulation was triggered to overprint crystallizing granite under subsolidus conditions.     

甘肃北山中带早泥盆世的构造-岩浆作用:来自公婆泉花岗岩体年代学和地球化学证据

为加深对北山地区古生代构造演化的认识,选取甘肃省北山公婆泉花岗岩体进行年代学和地球化学研究。公婆泉花岗岩体位于北山中带,明水–小黄山蛇绿岩带与红柳河–牛圈子–洗肠井蛇绿岩带之间。根据花岗岩矿物组成及地球化学特征将其分为两种不同类型:1)淡色花岗岩含石榴石、原生白云母和电气石等特征矿物,轻、重稀土元素分异不明显,(La/Yb)N=1.38~1.93,有明显的Eu负异常(δEu=0.19~0.28),同时富集Cs,Rb,Th,U,K,P和Hf,亏损Ba,Nb,Nd和Ti;2)二长花岗岩具有过铝质S型花岗岩的特征,A/CNK值平均大于1.1,轻、重稀土元素分异不明显,(La/Yb)N=2.74~22.78,具轻微的Eu异常(δEu=0.83~1.07),同时富集Cs,Rb,Ba,Th,U和K,亏损Nb,Ta,P和Ti。公婆泉两类花岗岩的矿物学和地球化学特征表明其形成于后碰撞环境。公婆泉花岗岩体的锆石LA-ICP MS U-Pb年龄为402.2±3.0 Ma,代表其岩浆结晶年龄,εHf(t)值介于0.1~2.9之间,暗示幔源物质的加入。公婆泉后碰撞花岗岩体与北山造山带西段相关花岗岩类的地球化学和年代学特征证实,北山中带的俯冲过程于早泥盆世之前在西段结束,并且此时在区域上处于后碰撞环境。     

伊春花岗质片麻岩锆石 U-Pb 年代学特征

通过1∶5万朱拉比拉河等图幅区域地质调查工作,在伊春地区发现了新元古代花岗质片麻岩。应用锆石 U-Pb 测龄方法对花岗质片麻岩进行了年代学研究,23个点的206 Pb／238 U表面谐和年龄均在850 Ma 左右,加权平均为850．2±2．1 Ma。该年龄代表花岗质片麻岩体的侵位时间为新元古代;而较新的499 Ma 年龄可能是后期构造事件改造的新生锆石,反映后期构造热事件的年龄;1518 Ma 的年龄应是本次岩浆作用过程中捕获的早期锆石的年龄,该锆石年龄可能代表本区花岗岩的继承性锆石年龄。伊春花岗质片麻岩所在的地区存在前寒武纪的古老微陆块。     

Zircon SHRIMP dating of granite from Qaidamshan, NW China

Zircon SHRIMP dating from Qaidamshan granite shows that the granite age is 446 Ma, similar to that of eclogite in the UHP belt. We think that both granite and eclogite may be formed at different stages during tectonic evolution of this area. Together with other studies we suggested that the collision of ocean and continent plates may occur at the early Caledonian in this area, forming the eclogite, and the collision of continent and continent plates at the late Caledonian, forming the Qaidamshan granite with the zircon SHRIMP age of 446 Ma.     

Episodic crustal anatexis and the formation of Paiku composite leucogranitic pluton in the Malashan Gneiss Dome, Southern Tibet

The Paiku composite leucogranitic pluton in the Malashan gneiss dome within the Tethyan Himalaya consists of tourmaline leucogranite, two-mica granite and garnet-bearing leucogranite. Zircon U-Pb dating yields that (1) tourmaline leucogranite formed at 28.2±0.5 Ma and its source rock experienced simultaneous metamorphism and anatexis at 33.6±0.6 Ma; (2) two-mica granite formed at 19.8±0.5 Ma; (3) both types of leucogranite contain inherited zircon grains with an age peak at ～480 Ma. These leucogranites show distinct geochemistry in major and trace elements as well as in Sr-Nd-Hf isotope compositions. As compared to the two-mica granites, the tourmaline ones have higher initial Sr and zircon Hf isotope compositions, indicating that they were derived from different source rocks combined with different melting reactions. Combined with available literature data, it is suggested that anatexis at ～35 Ma along the Himalayan orogenic belt might have triggered the initial movement of the Southern Tibetan Detachment System (STDS), and led to the tectonic transition from compressive shortening to extension. Such a tectonic transition could be a dominant factor that initiates large scale decompressional melting of fertile high-grade metapelites along the Himalayan orogenic belt. Crustal anatexis at ～28 Ma and ～20 Ma represent large-scale melting reactions associated with the movement of the STDS.     

打古寨——罗布里两种类型花岗岩的同位素地质研究

本文研究的打古寨和罗布里岩体是桃山复式花岗岩体内部两种不同成因类型的岩体,成岩时代均为燕山早期。根据Sr,Pb,O和S同位素资料,结合地质特征,笔者认为打古寨岩体是由上部地壳重熔改造形成,属陆壳改造型花岗岩;罗布里岩体是由壳幔混合岩浆形成,属同熔型花岗岩。     

应用斜长石粒度体视学研究花岗质岩浆演化

新疆诺尔特地区花岗岩分布广泛 ,主要为加里东晚期 ,华力西中、晚期及燕山期花岗岩。文章选取各期花岗岩中的主要岩体 ,应用斜长石粒度体视学方法研究了花岗质岩浆的演化。结果表明 ,区内花岗质岩浆的演化受对流分异作用控制。研究结果同时表明了斜长石粒度体视学在花岗质岩浆演化研究中的可行性     

广西大宁岩体和初洞岩体锆石U-Pb年代学 及Hf同位素研究

利用锆石LA-ICP-MS U-Pb定年,获得大宁岩体形成年龄为441.1±3.0 Ma,其包体年龄为439.5±3.6 Ma,初洞岩体形成年龄为423.5~434.2 Ma。结果表明,大宁岩体及其包体几乎同期形成,而初洞岩体是后期的侵入体。Lu-Hf同位素研究结果表明,大宁岩体及其暗色包体与初洞岩体具有相似的Hf同位素组成,在εHf(t)-年龄图上落在1440~1960 Ma的老地壳区间,表明三者的岩浆源区均是早元古代到中元古代地壳,暗色包体是岩浆早期分离结晶的产物,而初洞岩体是岩浆结晶分异晚期的产物。     

U-Pb zircon age of the foliated garnet-bearing granites in western Dabie Mountains, Central China

U-Pb zircon dating on two foliated garnet-bearing granite samples in the western Dabie ultra-high-pressure (UHP) metamorphic unit yields concordant ages of (234±4) Ma and (227±5) Ma, respectively. These ages, following the UHP peak metamorphism, represent the magma emplacement ages for the foliated garnet-bearing granites. This, for the first time, shows that there are the Triassic granites in the Dabie Mountains. The foliated garnet-bearing granites resemble A-type granite in geochemical characteristics, indicating that they were formed in extensional geodynamic setting. The magma formation reflects a reheating event in the Dabie orogenic belt and it enhances the transfer of tectonic regime from collision into extension and promotes the rapid exhumation into lower crust for the UHP metamorphic rocks.     

Th-rich zircon from peralka line A-type granite： Mineralogical features and petrological implications

Zircon is one of the most common accessory minerals in igneous rocks, especially in granites. It exists through the crystallization and evolution of grantic magma be-cause of stable physicochemical properties. Trace element abundances (e.g. Hf, Th, U, Y) in zircon are very sensitive to the crystallization environments[1]. Therefore, zircon plays an important role in the granite research. However, although A-type granites are given more and more regards by the geologists owing to their charac…     

粤北贵东岩体内基性岩脉的成因研究

对贵东花岗岩体内的基性岩脉研究结果表明,岩脉的锶、氧、硫同位素组成均一致显示幔源衍生物特征;岩脉常量元素、微量元素以及稀土元素分析表明、岩脉经历了较完全的结晶分异作用,并遭受到一定程度的陆壳混染作用。由此得出结论,认为形成岩脉的岩浆来自起源于地幔深处并侵位于地壳浅部(<15Km)的二次岩浆房,岩浆在这里发生结晶分异作用,在其随后的上升侵位过程中经受了陆壳的混染作用。