冀北单塔子杂岩的地质学和锆石U-Pb年代学

冀北单塔子杂岩位于华北克拉通北缘中段隆化-赤城断裂以南,主要由高角闪岩相-麻粒岩相变质的英云闪长质、奥长花岗质、花岗闪长质片麻岩(TTG岩套)和铁镁质二辉麻粒岩、辉石斜长角闪岩、石榴石斜长变粒岩、石榴石夕线石黑云母片岩及少量的大理岩等表壳岩残留体组成。详细的SHRIMP和LA-ICPMS锆石U-Pb同位素地质年代学研究表明,冀北单塔子杂岩的形成主要经历了7个演化阶段:(1)2600～2530Ma之间形成了单塔子杂岩中英云闪长质和奥长花岗质片麻岩的围岩,其中一部分铁镁质表壳岩,即铁镁质二辉麻粒岩,可能为英云闪长质—奥长花岗质岩浆的源岩;(2)2517～2505Ma主要为英云闪长质岩浆的侵位结晶阶段;(3)2494～2473Ma,奥长花岗质片麻岩侵位结晶;(4)2427～2404Ma,发生了早期麻粒岩相变形变质作用和深溶混合岩化作用,使这一杂岩经历了强烈的改造;(5)～1859Ma,铁镁质岩浆侵位,形成一些铁镁质岩株和岩脉;(6)～1834Ma到1793Ma,单塔子杂岩经历了最强烈的变质作用改造,形成了晚期麻粒岩相变质作用;(7)～1730Ma经历了强烈的流体活动,使得一些锆石发生了流体交代和改造。但是总体上单塔子杂岩没有经历～300Ma的古生代的变质作用。     

Geochemical evolution of the Mangalwar Complex,Aravalli Craton,NW India:Insights from elemental and Nd-isotope geochemistry of the basement gneisses

The Banded Gneissic Complex(BGC) of the Aravalli Craton is divided into BGC-I and BGC-Ⅱ; the BGC-Ⅱ(central Rajasthan) is comprised of the Sandmata Complex and the Mangalwar Complex. We report elemental and Nd-isotope geochemistry of basement gneisses of the Mangalwar Complex and constrain its origin and evolution. Geochemically, the basement gneisses have been classified as low-SiO_2 gneisses(LSG) and high-SiO_2 gneisses(HSG). Both the LSG and HSG are potassic, calc-alkaline and peraluminous in nature. The LSG are enriched in incompatible(K, Sr, Ba, large ion lithophile elements) and compatible elements(MgO, Cr, and Ni). They display fractionated rare earth element patterns(avg.La_N/Yb_N=12.1)with small Eu-anomaly(δEu=0.9), and exhibit negative anomalies of Nb and Ti in primitive mantlenormalized multi-element diagram. In terms of Nd-isotope geochemistry, the LSG are characterized by_(εNd)(t)=4.2 and depleted mantle model age of 3.3 Ga. To account for these geochemical characteristics we propose a three-stage petrogenetic model for the LSG:(1) fluids released from dehydration of subducting slab metasomatised the mantle-wedge;(2) the subducting slab underwent slab-breakoff causing upwelling and decompression melting of the asthenosphere during waning stage of subduction; and(3)upwelling asthenosphere provided the requisite heat for partial melting of the metasomatised mantlewedge leading to generation of the LSG parental magma. Asthenospheric upwelling also contributed in the LSG petrogenesis which is evident from its high Mg#(avg. 0.53). The LSG formed in this way are contemporary and chemically akin to sanukitoids of the BGC-I and Archean sanukitoids reported elsewhere. This provides a basis to consider the LSG as a part of the BGC-I. Contrary to the LSG, the HSG are depleted in compatible elements(MgO=avg. 1.1 wt.%; Cr=avg. 8 ppm; Ni=avg. 6 ppm) but enriched in incompatible elements(Sr=avg. 239 ppm, Ba=avg. 469 ppm). Its_(εNd)(t) values vary from-9.5 to-5.4.These chemical features of the HSG are akin to potassic granitoids found elsewhere. In this backdrop, we propose that the HSG suite of the Mangalwar Complex was derived from re-melting(partial) of an older crust(TTG?) occurring within the BGC-Ⅱ.     

煤矸石菱镁矿合成堇青石熟料研究

本文研究了以煤矸石和菱镁矿为主要原料合成堇青石熟料的可行性。实验表明选择高岭石含量高和碱金属氧化物及氧化铁含量低的煤矸石及高纯菱镁矿可以合成出高质量的堇青石熟料,其堇青石的含量可达９５％ 以上,最佳煅烧温度为１ ４００℃,保温１０ｈ,然后在降温过程中在１ ３５０℃再保温１０ ｈ。     

Determining sedimentation ages of high-grade metamorphic gneisses by their palynological record: a case study in the northern Schwarzwald (Variscan Belt, Germany)

 In the northern Schwarzwald (Moldanubian Zone of the Variscan belt), paragneisses drilled from approximately 2500 m depth at the Bühl-1 drilling site have experienced amphibolite-facies metamorphism and were later partly transformed to mylonites under greenschist-facies conditions. Completely graphitised acritarchs and chitinozoans were recovered from these paragneisses and mylonitic gneisses. Whereas both groups of palynomorphs give evidence of a marine depositional environment, only the chitinozoans are stratigraphically useful in this case and indicate a Paleozoic age. Previous findings of acritarchs in a different high-grade metamorphic gneiss unit within the Schwarzwald indicate an Upper Proterozoic marine sedimentation age. The spectrum of palynomorphs found thus far in the Schwarzwald in combination with petrological data suggests that within the Moldanubian domain of the Armorican microplate, rocks with different sedimentary records later experienced diverging prograde metamorphic histories. In general, the results reveal that palynological studies in combination with petrological data are a promising tool for reconstructing pre-orogenic settings in collisional belts. Received: 17 July 1998 / Accepted: 19 January 1999     

东昆仑东段巴隆花岗质片麻岩年代学、地球化学特征及地质意义

东昆仑东段巴隆南侧古元古界白沙河岩组中可识别出一套花岗质片麻岩。采用LA-MC-ICP-MS方法进行锆石UPb年代学分析,获得加权平均年龄为870.6±7.7Ma(MSWD=2.6)和875.3±6.3Ma(MSWD=0.92),显示该岩体形成于新元古代早期,并经历了早古生代变质作用。地球化学特征显示,岩石具有高Si O2、Al2O3、K2O,低Ti O2、Na2O的特征,A/CNK平均为1.14,A/NK平均为1.47,矿物组合中出现石榴子石和白云母等富铝矿物,显示S型过铝质花岗岩特征;球粒陨石标准化配分曲线显示轻稀土元素富集((La/Yb)N=6.22~25.37),有明显的铕负异常(δEu=0.16~0.44)。原始地幔标准化蛛网图显示,岩石富集Rb、Th,明显亏损Ba、Ta、Nb、Sr、P和Ti元素。综合分析,岩浆来源于陆壳部分熔融,可能为古元古界白沙河岩组长英质岩石发生脱水熔融的产物,形成于同碰撞环境,是东昆仑地区响应Rodinia超大陆汇聚事件的物质记录。     

祁连造山带东端张家川地区长宁驿中元古代花岗质片麻岩LA-ICP-MS锆石U-Pb年龄及其构造意义

在祁连造山带东端古元古界陇山岩群中新识别出中元古代长宁驿花岗质片麻岩。为了精确确定长宁驿花岗质片麻岩的形成时代,对其进行了激光等离子体质谱LA-ICP-MS单颗粒锆石微区U-Pb同位素测年,花岗质片麻岩中锆石Th/U比值介于0.50～2.09,阴极发光图像显示锆石内部发育典型的岩浆生长震荡环带结构,属于岩浆结晶的产物。测得花岗质片麻岩的原岩岩浆结晶年龄为1765Ma±57Ma,表明该花岗质片麻岩原岩的形成时代为中元古代早期,可能形成于华北板块经吕梁运动后的后造山环境中,同时还从捕获的继承性岩浆锆石中得到了时代为1.95Ga、2.20Ga左右的继承年龄。结合区域地质资料,综合分析研究后认为,在中元古代祁连地块仍为华北板块西南缘的一部分,两者之间具有相同的结晶基底和构造属性,它们共同经历了吕梁运动后的后造山和陆内伸展裂解阶段,很可能与Columbia超大陆的聚合和裂解相关。     

吉南板石沟地区英云闪长质片麻岩LA-ICP-MS锆石U-Pb定年、地球化学特征及地质意义

以吉南板石沟地区的英云闪长质片麻岩为例,探讨其成因及演化机制。研究表明该片麻岩具有富硅(SiO_2=66.61%~72.35%)高铝(Al_2O_3=13.56%~16.18%)富钠(Na_2O=3.65%~4.69%)贫钾(K_2O=1.13%~2.25%)的特点,稀土元素配分曲线右倾,富集Ba、Rb等大离子元素,亏损Ni、Ta等高场强元素,属于高铝TTG片麻岩,其与高硅埃达克岩特征相似。对该片麻岩锆石进行了LA-ICP-MS U-Pb年龄测试,其形成年龄为(2 619±28)Ma,属于新太古代时期产物。锆石Lu-Hf同位素分析结果显示岩浆在形成演化过程中较少受到地幔物质混染,两阶段模式年龄T_(DM2)为2 976~2675 Ma。板石沟地区英云闪长质片麻岩可能是在俯冲环境下含玄武质洋壳部分熔融产生,认为华北克拉通东北部地区在新太古代时期可能存在一个由洋壳俯冲到弧陆碰撞造山带即吉—辽—冀弧陆碰撞增生造山带。     

藏南亚东地区早古生代花岗质片麻岩的成因与构造意义

早古生代的花岗质片麻岩产出在青藏高原南部亚东地区的高喜马拉雅结晶岩系中。岩石学研究表明,这些岩石主要由斜长石、钾长石、石英和黑云母组成,普遍发育片麻状构造。它们的SiO_2含量为68.04%~76.55%,铝饱和指数(A/CNK)为1.04~1.14,属钙性-钙碱性弱过铝-过铝质岩石。它们的稀土总量较高,轻稀土富集,Eu负异常,富集大离子亲石元素Rb,相对亏损Nb、Ta、P和Ti等高场强元素。三个样品获得的岩浆锆石U-Pb年龄分别为491Ma、512Ma和501Ma,代表花岗质片麻岩的原岩年龄。它们的锆石εHf(t)值分别为-8.1~+0.1、-5.9~-2.1和-4.8~+6.7,对应的二阶段模式年龄分别为1440~1959Ma、1585~1826Ma和1028~1762Ma。本研究表明,亚东地区的早古生代花岗岩包括I型和S型花岗岩。I型花岗岩以K_2O/Na_2O1、弱过铝质和刚玉标准矿物分子数1为特征,具较高且正的锆石εHf(t)值,或相对集中的负的εHf(t)值。I型花岗岩显示岩浆弧花岗岩的特征。S型花岗岩以K_2O/Na_2O1、过铝质和刚玉标准矿物分子数1为特征,锆石εHf(t)值均为低的负值,且变化范围较大。本文和现有的研究成果表明,I型和S型花岗岩在喜马拉雅地体广泛存在,这为冈瓦纳超大陆北缘经历了早古生代的安第斯型造山作用提供了重要证据。     

西藏门巴地区德宗花岗质片麻岩SHRIMP锆石U-Pb年龄及地质意义

德宗花岗质片麻岩体侵位于蒙拉组地层之中。阴极发光分析表明,样品的锆石具岩浆成因特点,离子探针(SHRIMP)U-Pb同位素年龄为(209±6.9)Ma,时代为晚三叠世。同时,对该样品中的黑云母进行40Ar-39Ar测年,其结果为(105.2±1.7)Ma。这些精确的年龄数据不仅进一步厘定了冈底斯地区的确存在晚印支期岩浆活动,也为该区蒙拉组变质变形的确定提供了科学依据。     

东昆仑祁漫塔格花岗片麻岩记录的岩浆和变质事件

东昆仑青海祁漫塔格尕林格一带原定为金水口群的眼球状花岗片麻岩,实际为新元古代早期形成的花岗岩.采用SHRIMP和LA-MC-ICP-MS两种方法对其中的锆石进行了测试,获得的年龄分别为938±5Ma和938±2Ma,代表了花岗岩的形成时代.花岗岩地球化学特征显示为S-型,属于钙碱性系列的弱过铝-过铝质花岗岩,εNd(0)为-9.4～-11.7,εNd(t=938Ma)为-0.6～-3.2,显示低的负值,tDM为1.6 ～2.1Ga,推测其源岩与白沙河岩组类似.东昆仑东段、柴北缘以及阿尔金均有1000～900Ma的花岗岩形成,表明这次岩浆活动比较广泛,可能与我国西部不同陆块间的汇聚有关,是我国西部新元古代克拉通基底形成的反映,同时也响应于全球Rodinia超大陆的形成.花岗片麻岩中1粒锆石边部获得了416±11Ma的年龄,与区域上志留-泥盆纪花岗岩形成时代一致,代表了新元古代花岗岩发生变质作用的时代,其中白云母40Ar/39Ar的坪年龄和等时线年龄为406±2Ma,代表了变质花岗岩的冷却年龄,这些年龄表明新元古代花岗岩卷入了古生代中期与祁漫塔格洋/海盆关闭有关的造山事件.