Compositions & Melt Evolution of Upper Mantle Peridotites in the Tethyan Ophiolites

正The Jurassic–Cretaceous ophiolites in the Alpine–Himalayan orogenic belt represent fragments of oceanic lithosphere,developed in different seaways separated by Gondwana–derived ribbon continents within a broad     

Isotopic compositions and geological significance of sediments from the Heihe River,North Qilian Orogen,NW China

Strontium (Sr), neodymium (Nd), and hafnium (Hf) isotopic analyses of different size-fractions of sediments collected from the Heihe River in the North Qilian Orogen (NQO) were carried out to trace the sediment sources and to determine the relationships between the Sr–Nd–Hf isotopic behaviors, and the grain-size and hydraulic sorting effects during fluvial transport. Our results demonstrate that the sand and suspended load samples collected from the same site have different Nd isotopic compositions, while their ⁸⁷Sr/⁸⁶Sr ratios are only slightly different. These features indicate that contributions to the sediments from different sources with different grain sizes vary greatly. Coarse-grained sand may more intuitively reflect the variations in local sources than the suspended load. The suspended load samples are distributed along the Clay Array on the Ԑ_Hf vs. Ԑ_Nd diagram due to zircon sorting. A similar NdHf decoupling phenomenon is widespread in the global oceans and large river systems, indicating that the NdHf isotopic behavior depends on the hydrodynamic sorting of minerals during fluvial transport. Moreover, the crustal accretion of the NQO from the depleted mantle occurred in the Proterozoic based on the constraints provided by the T_DM values of the river sediments. The Nd isotopic compositional characteristics of the river sediments indicate that the NQO has an affinity with the Yangtze Craton. The determination of the basement tectonic affinity of the NQO significantly contributes to our understanding of the Neoproterozoic evolution of the Gondwana continental margin.     

Archean-Paleoproterozoic Lithospheric Mantle at the Northern Margin of the North China Craton Represented by Tectonically Exhumed Peridotites

Tectonically emplaced peridotites from North Hebei Province, North China Craton, have retained an original harzburgite mineral assemblage of olivine(54%–58%) + orthopyroxene(40%–46%)+minor clinopyroxene(1%)+spinel. Samples with boninite-like chemical compositions also coexist with these peridotites. The spinels within the peridotites have high-Al end-members with Al_2O_3 content of 30 wt % –50 wt %, typical of mantle spinels. When compared with experimentally determined melt extraction trajectories, the harzburgites display a high degree of melting and enrichment of SiO_2, which is typical of cratonic mantle peridotites. The peridotites display variably enriched light rare earth elements(REEs), relatively depleted middle REEs and weakly fractionated heavy REEs, which suggest a melt extraction of over 25% in the spinel stability field. The occurrence of arc-and SSZ-type chromian spinels in the peridotites suggests that melt extraction and metasomatism occurred mostly in a subduction-related setting. This is also supported by the geochemical data of the coexisting boninite-like samples. The peridotites have ~(187)Os/~(188)Os ratios ranging from 0.113–0.122, which is typical of cratonic lithospheric mantle. These ~(187)Os/~(188)Os ratios yield model melt extraction ages(TRD) ranging from 981 Ma to 2054 Ma, which may represent the minimum estimation of the melt extraction age. The Al_2O_3-~(187)Os/~(188)Os-proxy isochron ages of 2.4 Ga–2.7 Ga suggest a mantle melt depletion age between the Late Achaean and Early Paleoproterozoic. Both the peridotites and boninite-like rocks are therefore interpreted as tectonically exhumed continental lithospheric mantle of the North China Craton, which has experienced mantle melt depletion and subduction-related mantle metasomatism during the Neoarchean-Paleoproterozoic.     

罗布莎蛇绿岩地幔橄榄岩同位素特征及其成因

罗布莎蛇绿岩岩石单元出露齐全,以地幔橄榄岩为主,构造变形强烈;在堆积杂岩中上部产出有仰冲型斜长花岗岩小岩块;辉绿岩以脉状形式产出于地幔橄榄岩的方辉橄榄岩中;壳层岩石相对很薄且比较复杂.Sr、Nd和Pb同位素组成研究表明:罗布莎地幔橄榄岩绝大多数样品具有高ISr值(0.705489～0.714625);低INd值(0.5...     

祁连山玉石沟橄榄岩岩浆作用的记录和铬铁矿的成因

祁连山玉石沟蛇绿岩型铬铁矿是典型的高铬型铬铁矿床.通过对玉石沟铬铁矿和橄榄岩围岩进行原位定距离采样研究表明:全岩高的MgO和低的SiO2含量指示这些蛇纹石化橄榄岩不是简单的部分熔融残余,而是经历了后期的熔体再富集作用.玉石沟近矿橄榄岩尖晶石(Cr#>65)和铬铁矿尖晶石的矿物包裹体化学特征反映矿体和围岩形成于俯冲带上覆岩石圈地幔.距离矿体由近及远,赋矿围岩尖晶石化学成分(Cr#=43.9~77.2,TiO2=0.06%~0.34%)无规律变化.这说明高铬型铬铁矿体成矿所需铬并非来源于围岩,而是与玻安质熔体渗滤有关.尖晶石含水矿物包裹体仅赋存在玉石沟铬铁矿尖晶石与硅酸盐矿物接触部位附近可能揭示了玉石沟铬铁矿体的成因:成矿铬尖晶石在上侵的含水熔体与周围方辉橄榄岩的反应过程中形成聚集.     

北山造山带白云山蛇绿岩地幔橄榄岩成因及形成环境

白云山蛇绿岩是北山造山带保存最好的蛇绿岩之一,其起源存在较大的争议。该蛇绿岩主要由橄榄岩、辉长岩以及基性火山岩等组成。橄榄岩主体为方辉橄榄岩以及少量二辉橄榄岩。这些岩石均发生不同程度的蛇纹石化以及强烈的变形变质作用。研究选取9件橄榄岩样品进行了全岩主量元素分析及单矿物电子探针分析,结果显示岩石均来自地幔。岩石中的尖晶石和辉石组分用来限定白云山蛇绿岩的成因及形成环境。样品包含了高Cr#（46.7~68.1）的尖晶石以及低Al2O3（0.78%~2.94%）含量的单斜辉石。矿物化学特征显示,地幔橄榄岩熔融程度为23%~36%,表明白云山地幔橄榄岩可能经历了多个阶段。矿物学及全岩地球化学特征表明,白云山蛇绿岩形成于洋中脊（MOR）环境,后受到俯冲带（SSZ）环境的改造。     

雅鲁藏布江蛇绿岩的氦、氩同位素组成及其地质意义

笔者测定了雅鲁藏布江地区蛇绿岩的氦、氩同位素组成.白朗的玄武岩He同位素组成比较均一,3He/4He平均值为5.359 Ra;与之伴生的变橄岩的3He/4He比值范围为1.104~3.384 Ra,平均为2.383 Ra.从东部的仁布和大竹到西部的下鲁和白朗,辉绿岩的3He/4He由低变高.吉定的辉绿岩样品3He/4He比值平均高达31.57 Ra,该值与夏威夷发现的高比值样品接近.分步加热法结果显示,该样品高比值的He是在低温区段释放的,这种高3He/4He比值He的捕获表明雅鲁藏布江蛇绿岩的成因存在地幔柱型富集地幔岩浆作用.     

广西龙胜蛇绿岩的元素和同位素地球化学研究

广西北部新元古代早期龙胜蛇绿岩中火山岩的主量和微量元素地球化学特征非常类似于现代岛弧火山岩，并具有一致的但较低的εNd(t)值（—+1.0），表明其源区不可能是高度亏损型地幔源，而很可能来源于亏损程度较低的地幔源。龙胜蛇绿岩很可能形成于大陆弧靠大陆一侧的弧后盆地扩张阶段，或在大陆弧伸展-减薄过程中形成。     

Rheological Properties of Mantle Peridotites at Yushigou in the North Qilian Mountains and Their Implicationsfor Plate Dynamics

This paper discusses in detail the deformation textures, glide system, petrofabrics and olivine dislocation microstructures of mantle peridotites at Yushigou in the North Qilian Mountains, northwestern China. The peridotites have undergone high-pressure, high-temperature and low-strain rate plastic flow deformation. According to the dynamic recrystallized-grain size of olivine and the average spacing between the dislocation walls as well as the chemical composition of enstatite, the authors calculated the rheological parameters of the ancient upper mantle in the study area as follows: temperatures 1025-1093℃; pressures 3043-4278 MPa; depths 95-132 km; deviatoric stress 28-32 MPa; strain rates 0.2×10-14-2.13×10-14s-1 and equivalent viscosities 0.45×1020-4.65×1020 Pa · s. These parameters suggest that the position where plastic flow took place was correspondent to the low-velocity zone beneath the oceanic lithosphere and that oceanization characterized by middle-velocity (1-3 cm/a) sea-floor spreadi     

西藏西南部达巴-休古嘎布蛇绿岩带中地幔橄榄岩的成因

本文报道了西藏西南部达巴-休古嘎布蛇绿岩带中橄榄岩的矿物化学资料.橄榄岩中主要造岩矿物化学成分的分析研究表明,该区橄榄岩为残余地幔成因,但它们不是地幔简单熔化的残余物.尖晶石中Cr#及辉石中Ti的广泛变化表明它们具有复杂的熔融历史及地幔交代作用的印记,其形成过程可能经历了两种构造环境的转变.早期在MORB环境下形成低Cr#(尖晶石)橄榄岩;其后由于洋内俯冲作用,早先形成的低熔橄榄岩被消减到岛弧之下再度发生熔融形成高Cr#(尖晶石)橄榄岩.从而,在古大洋消失之后形成的碰撞带上同时保存了MORB型和SSZ型两类蛇绿岩.     

上地幔Os同位素组成的演化

Re-Os同位素体系为研究地幔的成分-结构-演化提供了新的地球化学示踪和定年的工具.上地幔Os同位素组成演化的球粒陨石模型是Re-Os体系用于地幔物质定年的基础, 尤其在采用Re亏损模式年龄和Os同位素代理等时线年龄时.综合了铁陨石和各类球粒陨石、地幔橄榄岩包体和蛇绿岩豆荚状铬铁矿的Re-Os同位素体系研究的近期成果, 为认识对流上地幔Os同位素组成的演化提供了制约.对河北遵化蛇绿岩豆荚状铬铁矿岩的研究, 获得新太古代(2.5 Ga)时形成豆荚状铬铁矿的对流上地幔的187Os/188Os=0.110 2, 与球粒陨石型模式的一致.文献中常用的球粒陨石模式的参数如下: 地球形成时(4.558 Ga)初始值187Os/186Os为0.095 31, 现代值分别采用碳质球粒陨石的187Os/186Os比值为0.127 0和原始上地幔(PUM)的187Os/186Os比值为0.129 6, PUM与普通球粒陨石和顽火球粒陨石的187Os/186Os比值接近.      

秦岭蟒岭高Sr花岗岩的锆石Lu-Hf同位素特征及其成因

蟒岭花岗岩体位于商丹构造带北侧的北秦岭构造带上,为一呈近东西走向的中生代花岗岩基。蟒岭花岗岩属高钾钙碱性—钾玄岩系列、过铝质I型花岗岩,其w(SiO2)=67.3%～73.7%,w(Al2O3)=14.0%～16.3%,w(Na2O)=3.17%～3.93%、w(K2O)=3.9%～6.3%,具高Sr、Ba、LREE、Sr/Y、La/Y,低HREE、Y、Mg#(50)、Rb/Sr,亏损Nb、Ta、Ti和P,无明显负Eu异常的特征,与中国东部的高Sr、Ba低HREE花岗岩的地球化学特征相似。锆石εHf(t)值为-9.4～-3.1,二阶段模式年龄(tDM2)集中于1.4～1.8 Ga,暗示蟒岭高Sr花岗岩的原岩主要为中新元古代地壳物质,并混入少量幔源物质;源区残留石榴石,而角闪石、斜长石为主要熔融相。蟒岭高Sr花岗岩形成于陆内造山阶段,由增厚的下地壳物质发生减压部分熔融形成,而底侵的镁铁质岩浆可能为部分熔融作用提供了热量。     

藏北商旭金矿床成因研究：流体包裹体及氢-氧同位素证据

商旭金矿床处于班公湖-怒江缝合带中段南侧,位于藏北双湖县境内,是班公湖-怒江缝合带已发现的可能是造山型的金矿床。矿体赋存于中-下侏罗统木嘎岗日群浅变质复理石中,受北西西向断裂构造控制,金矿化与石英脉密切相关。含矿石英脉中见较多黄铁矿、黄铜矿、方铅矿、闪锌矿等硫化物,自然金沿石英裂隙分布,少量分布在黄铁矿裂隙中。岩相学特征和激光拉曼测定结果显示,商旭金矿床中存在两类流体包裹体:1富液相包裹体(L型);2含CO2包裹体(C型),此类包裹体主要为CO2三相包裹体,见CO2两相包裹体。显微测温结果表明:1L型包裹体的均一温度为89.6~409.8℃,盐度ω(NaCleq)为0.35%~9.34%,流体密度为0.55~0.98g/cm3;2C型包裹体的均一温度为264.3~395.2℃,盐度ω(NaCleq)为4.62%~9.74%,流体密度为0.66~0.81g/cm3。商旭金矿床成矿流体具有富CO2、中低温度、低盐度、低密度的特征,与典型造山型金矿成矿流体特征相似。同时,商旭金矿床成矿流体的氢氧同位素组成分别为δD=-108‰~-89‰、δ18 O=-0.8‰~5.8‰,表明成矿流体主要来自变质水与建造水的混合。综合流体包裹体和氢氧同位素证据,进一步论证商旭金矿床为造山型金矿。     

西藏雅鲁藏布江蛇绿岩带罗布莎地幔橄榄岩的成因

广泛分布在雅鲁藏布江蛇绿岩带的地幔橄榄岩是没有蛇纹石化的新鲜岩石,因而通过显微镜下观察和全岩化学分布探索其成因和地幔活动,是十分有利和简便的。最近几年来,利用岩石化学资料揭示远洋橄榄岩,认为它不是原始地幔岩经分或分离熔化作用萃取出玄武岩熔体后的直接残余物。罗布落蛇绿岩橄的镜下观察和岩石化学资料也证明,它不是原始地幔岩经局部熔化后的残余物,而且由来自更深层的地幔过渡带的硅酸盐超高压矿物底辟上升经熔取     

Chemical Variation of Chromian Spinel Compositions in a Serpentinized Peridotites: Implications for Evolution of the Neoproterozoic Ophiolites

正Tectonic setting of the Neoproterozoic ophiolites is poorly understood.Because of extensive serpentinization/metamorphism in the mantle section,accessory chromian spinel has been used as an important geotectonic indicator.     

Mineralogy, Petrology and Geochemistry of Peridotites from the Northern Ophiolititc Sub-belt, Western Yarlung Zangbo Suture Zone, Tibet

<正>1 Introduction Ophiolites,mostly found in orogenic belts,represent fragments of palaeo–oceanic lithosphere emplaced on the continent through tectonic processes(Bizimis et al.,2000;Dilek and Furnes,2011,2014;Pearce,2014).These remnants of ancient oceanic lithosphere have recorded important information for the magmatic,tectonic and     

Late Cambrian SSZ-type Ophiolites in Acite Zone, East Kunlun Orogen of Northern Tibet Plateau: Insights from Zircon U-Pb Isotopes and Geochemistry of Oceanic Crust Rocks

<正>1 Introduction East Kunlun orogen(EKO)stretching more than 1000km in E-W extension is located in the western segment of Central Orogen Belt(COB),China(Xu et al.,2006,Li et al.,2014).There outcropped Cambrian ophiolites