北冰洋Gakkel洋中脊科学考察的初步认识——AMORE2001简介

AMORE2001首次成功地对地球上扩张速率最慢的Gakkel洋中脊进行了系统的地质和地球物理以及海冰等多学科的综合考察.对所取得资料的初步研究表明Gakkel洋中脊之下的地幔仅经受了较低程度的部分熔融,具有较低的温度.Gakkel洋中脊处的地壳性质和岩浆活动与扩张速率之间不存在任何直接的岩石学相关性,扩张而导致的岩石圈顶部的冷却并不是决定洋脊处地壳性质的最重要因素.地幔温度和断裂带的存在与否及其密度对地壳性质具有重要的影响意义.本次科考活动中还对在现代环境和古环境研究具有重要意义的该地区的海冰分布和环流特征以及深海沉积物特征进行了系统的采样研究.这些研究将对人类更好的理解地球上最重要的地质过程-海底扩张和壳幔相互作用以及全球环境变化起到关键的作用.     

南海深海盆的沉积充填

 依据大洋钻探井及地震剖面资料，定量确定南海同扩张期和后扩张期深海盆沉积充填差异及沉积物来源变化。研究显示，南海深海盆自渐新世（32 Ma）开始形成，随着南海二次扩张海盆范围逐渐扩大，海盆内主要充填火山碎屑角砾岩及火山灰，碳酸盐岩、超微化石软泥，泥质粘土、粉砂质粘土、泥岩及粉细砂岩。深海盆充填主要沉积物为晚中新世（11.6Ma）以来的陆源碎屑沉积，丰富陆源碎屑的供给与南海闭合过程中同期区域构造事件（如青藏高原快速隆升、菲律宾板块俯冲）密切相关，也与晚中新世以来东亚季风增强以及源区强烈的风化剥蚀有关。     

北秦岭榴辉岩的地球化学特征及形成环境

地球化学研究表明：北秦岭榴辉岩原岩属于拉斑玄武岩系列，岩石的Nb／La＝0．68-0．92，Ce／Nb＝2．11—3．62，Th／Yb＝0．42—1．00，Ti／Zr＝75．26—125．84，显示源区具有N—MORB的特征；岩石Nb含量9—13μg／g，Zr／Y值为4．24—6．92，同时明显富Ti和Fe，又显示了OIB的特征，因此岩石源区应是N—MORB和OIB两个端元的混合。结合地质资料综合分析，认为北秦岭榴辉岩原岩可能是洋盆中的海山玄武岩，具有洋壳性质，洋壳被消减俯冲到下地壳或地幔深处，经高压变质形成榴辉岩。     

蛇绿岩套研究现状、科学问题及新思考

蛇绿岩套是运用板块构造理论解释造山带构造演化的关键,合理理解蛇绿岩套的形成、演化有助于构建与板块构造理论吻合的造山带演化模型。经典的蛇绿岩套发育由地幔橄榄岩、堆晶辉长岩、枕状玄武岩-深海沉积岩组成的“三位一体”岩石组合,与现代洋中脊新形成的大洋岩石圈具有相同的岩石组合及岩石空间分布样式。地球化学家们在经典蛇绿岩套定义基础上,根据玄武质岩石地球化学特征的变化,提出了“俯冲带之上(SSZ)蛇绿岩套”概念,并对其划分亚类、构建相应的概念性成因模型,从而引发了“在俯冲汇聚阶段能否发生岩石圈裂解、形成新生洋壳”的“蛇绿岩难题”。针对阿尔卑斯-喜马拉雅造山系内的两个典型SSZ型蛇绿岩套的详细解剖表明,部分SSZ型蛇绿岩套的定义可能存在玄武质岩石“年代误植”、“构造背景误植”问题。前人定义的伊朗克尔曼沙赫弧前蛇绿岩套、巴基斯坦拉斯科赫弧间蛇绿岩套均由两部分组成：下部为形成于洋中脊环境、时代较老的大洋岩石圈残片;其上分别被年轻的弧前盆地和岛弧相关火山-沉积岩组合不整合覆盖。显然,大部分SSZ型蛇绿岩套是经典洋中脊蛇绿岩套残片与形成于其他构造环境(如大陆裂谷、俯冲带)的火山-沉积岩序列的组合体,并不存在...     

EMI型皖东上第三系大陆碱性玄武岩：Nd,Sr同位素证据

本文报导皖东地区上第三系大陆碱性玄武岩的地球化学特征和成因初探，其中８个样品的Ｎｄ、Ｓｒ同位素组成范围为．在大洋玄武岩的Ｎｄ－Ｓｒ同位素平面上，表现为从原始地幔向ＥＭＩ型地幔端元展布趋势．皖东玄武岩具有轻稀土和高度不相容性元素相对富集的微量元素丰度模式，类似于洋岛玄武岩（ＯＩＢ）．皖东玄武岩的源区可能是Ｎｄ、Ｓｒ同位素组成不均一、且富化的大陆岩石圈地幔．     

南秦岭岚皋基性火山岩的地质学 、地球化学及其构造意义

为了研究南秦岭岚皋地区基性火山岩的岩石成因和区域构造背景, 选择岚皋县境内的玄武岩和火山角砾岩进行岩相学、常量元素、微量元素和稀土元素的分析。通过分析发现, 它们同属于碱性玄武岩系列, 轻重稀土分异明显, Rb, K 相对Th, Hf 亏损, 不具 Nb, Ta, Zr, Ti 负异常, 整体表现出与 OIB( 洋岛玄武岩) 高度一致的稀土图谱和微量元素特征。岩石成因分析表明, 它们为同源岩浆演化序列, 其原始岩浆起源于尖晶石-石榴石二辉橄榄岩和石榴石二辉橄榄岩地幔源区的低度部分熔融, 在上升过程中基本未受地壳混染的影响。岩浆演化过程中主要经历了单斜辉石、少量斜长石的分离结晶作用。综合区域地质学和微量元素特征, 该组火山岩最有可能形成于洋盆台地或大洋岛构造环境。     

塔里木大火成岩省瓦吉里塔格层状岩体的钼同位素组成特征及其地质意义

钼同位素地球化学是国际地学研究领域的一个前沿和热点问题。塔里木大火成岩省中的瓦吉里塔格镁铁质-超镁铁质层状岩体的Mo同位素研究结果表明,瓦吉里塔格镁铁质-超镁铁质岩石具有相近的Mo同位素组成,其平均值为-0.18‰±0.04‰(2 s.d.;n=11)。在瓦吉里塔格岩浆体系中,岩浆分异没有导致明显的Mo同位素分馏。瓦吉里塔格岩石中Mo同位素与其(~(87)Sr/~(86)Sr)_i和ε_(Nd)(t)值呈现一定相关性,指示其地幔源区不均一性。此外,这些岩石δ~(98)Mo值低于洋中脊玄武岩δ~(98)Mo值范围(～-0.15‰),指示它们的源区很可能受到早-中古生代南天山洋板片俯冲的熔体交代的影响。结合前人研究成果,塔里木大火成岩省中丰富的岩石系列可归因于不均一的地幔源区以及地幔柱-岩石圈-俯冲洋壳的相互作用。     

海底地形对不同时间尺度岩浆供给变化的响应

利用有限差分数值模拟方法, 恢复洋中脊地形的形成过程, 模型中岩浆供给按一定的时间周期和幅度规律性地变化。结果表明: 只有当岩浆供给变化周期的时间尺度大于在洋中脊同一侧形成两条断层的时间间隔时, 才能影响海底地形的形成过程并被记录。结合数值模拟实验结果和不同类型洋中脊的地形特征, 认为快速扩张洋中脊是唯一可能在地形上记录到米兰科维奇气候周期(偏心率(100 ka)、倾斜度(41 ka)和岁差(23 ka)) 3个时间尺度岩浆变化周期的洋中脊类型, 中速扩张洋中脊和部分岩浆供给充足慢速扩张洋中脊的地形可能与100 ka尺度的岩浆供给变化周期有关, 大部分慢速扩张洋中脊海底地形不受100 ka及以下的岩浆供给变化周期影响。     

滇西二叠系玄武岩及其与东古特提斯演化的关系

目的 研究云南祥云地区二叠系玄武岩和东古特提斯洋在三江地区演化历史之间的关系.方法 运用地球化学及S卜Nd-Pb同位素特征分析.结果 祥云地区二叠系玄武岩属钙碱性系列,TiO2=1.41%-2.05%(平均1.69%),K2O=0.13%～3.04%(平均1.19%),轻稀土中度富集.岩石形成于大陆溢流环境,属于中国南方广泛分布的峨眉山玄武岩中的低钛玄武岩类.该组玄武岩微量及稀土元素地球化学显示出显著的过渡属性,总体表现为板内玄武岩特征,但又具有大洋拉斑玄武岩的部分特征,并在一些特征元素组合方面指示了俯冲物质(或陆壳)的混染.岩石具有富集的同位素地球化学特征,(87Sr/86Sr)i=0.706 350～0.706 635,143Nd/144 Nd=0.512 355～0.512 381,8Nd(t)=-3.99～-3.3,TDM=1.14～1.18 Ga.结论 本区玄武岩可能形成于东古特提斯主洋盆地紧邻的大陆边缘环境,其地幔源区具有大陆富集地幔+大洋亏损地幔的混源特征,并在其源区内或岩浆形成演化过程中受到过俯冲物质(或陆壳)的混染.     

北祁连直河蛇绿岩的地质和地球化学特征

直河蛇绿岩是在北祁连造山带中新发现的,该蛇绿岩由地幔橄榄岩、辉长辉绿岩、玄武岩、硅质岩等组成,以构造岩片为接触关系.蛇绿岩中岩石地球化学特征表明,该蛇绿岩具有大洋拉斑玄武岩的特征,同时具有某些岛弧特征,基性岩类的REE球粒陨石标准化配分模式为平坦型和LREE亏损型,超基性岩为亏损地幔源和富集地幔物质加入的混合成因,稀土分配型式不同于典型洋脊玄武岩,源区为亏损地幔和富集地幔的混合源.北祁连自寒武纪末大陆裂解,完成大陆裂陷向洋盆的转化,至奥陶纪形成沟弧盆体系.直河蛇绿岩属于弧后盆地扩张环境的产物.     

Opening and evolution of the South China Sea constrained by studies on volcanic rocks: Preliminary results and a research design

The South China Sea (SCS) is characterized by abundant seamounts, which provide important information about the evolution of the SCS and related deep processes. Cenozoic volcanism in the SCS and its surroundings comprises three stages relative to the spreading of the SCS:prespreading (>32 Ma), syn-spreading (32-16 Ma), and post-spreading (<16 Ma). The pre-spreading magmatism predominantly occurs on the northern margin of the SCS and in South China coastal areas and shows a bi-modal affinity. The syn-spreading magmatic activity was very limited on the periphery of the SCS, but may be concentrated in the SCS. However, seafloor samples of this stage are not available yet because of overlying thick sedimentary deposits. Post-spreading magmatism is widespread in the central and southwest sub-basins of the SCS, Hainan Island, Leizhou Peninsula, Thailand, and Vietnam. These are mainly alkali basalts with subordinate tholeiites, and display OIB-type geochemical characteristics. The Dupal isotope anomaly and presence of high-magnesian olivine phenocrysts suggests their possible derivation from the Hainan mantle plume. The temporal and spatial distribution of Cenozoic volcanism in the SCS and its surroundings may be accounted for either by plate stress re-organization before and after SCS spreading, or by ridge suction of plume flow during opening of the SCS. If the latter is the case, the volcanic rocks within the SCS basin may not be typical mid-ocean ridge basalts (MORB). It remains puz-zling, however, that the transition between the South China continental margin and the SCS basin does not have features typical of a volcanic rifted margin. Clearly, the relationship between mantle plume and SCS opening needs further evaluation. A better un-derstanding of the link between deep processes and opening of the SCS not only requires enhanced studies on igneous petrogene-sis, but also is heavily dependent on systematic sampling of seafloor rocks.     

Mantle wedge control on back-arc crustal accretion

At mid-ocean ridges, plate separation leads to upward advection and pressure-release partial melting of fertile mantle material; the melt is then extracted to the spreading centre and the residual depleted mantle flows horizontally away. In back-arc basins, the subducting slab is an important control on the pattern of mantle advection and melt extraction, as well as on compositional and fluid gradients. Modelling studies predict significant mantle wedge effects on back-arc spreading processes. Here we show that various spreading centres in the Lau back-arc basin exhibit enhanced, diminished or normal magma supply, which correlates with distance from the arc volcanic front but not with spreading rate. To explain this correlation we propose that depleted upper-mantle material, generated by melt extraction in the mantle wedge, is overturned and re-introduced beneath the back-arc basin by subduction-induced corner flow. The spreading centres experience enhanced melt delivery near the volcanic front, diminished melting within the overturned depleted mantle farther from the corner and normal melting conditions in undepleted mantle farther away. Our model explains fundamental differences in crustal accretion variables between back-arc and mid-ocean settings.     

Ancient, highly heterogeneous mantle beneath Gakkel ridge, Arctic Ocean

The Earth's mantle beneath ocean ridges is widely thought to be depleted by previous melt extraction, but well homogenized by convective stirring. This inference of homogeneity has been complicated by the occurrence of portions enriched in incompatible elements. Here we show that some refractory abyssal peridotites from the ultraslow-spreading Gakkel ridge (Arctic Ocean) have very depleted 187Os/188Os ratios with model ages up to 2 billion years, implying the long-term preservation of refractory domains in the asthenospheric mantle rather than their erasure by mantle convection. The refractory domains would not be sampled by mid-ocean-ridge basalts because they contribute little to the genesis of magmas. We thus suggest that the upwelling mantle beneath mid-ocean ridges is highly heterogeneous, which makes it difficult to constrain its composition by mid-ocean-ridge basalts alone. Furthermore, the existence of ancient domains in oceanic mantle suggests that using osmium model ages to constrain the evolution of continental lithosphere should be approached with caution.     

Mineralogy of the mid-ocean-ridge basalt source from neodymium isotopic composition of abyssal peridotites

Inferring the melting process at mid-ocean ridges, and the physical conditions under which melting takes place, usually relies on the assumption of compositional similarity between all mid-ocean-ridge basalt sources. Models of mantle melting therefore tend to be restricted to those that consider the presence of only one lithology in the mantle, peridotite. Evidence from xenoliths and peridotite massifs show that after peridotite, pyroxenite and eclogite are the most abundant rock types in the mantle. But at mid-ocean ridges, where most of the melting takes place, and in ophiolites, pyroxenite is rarely found. Here we present neodymium isotopic compositions of abyssal peridotites to investigate whether peridotite can indeed be the sole source for mid-ocean-ridge basalts. By comparing the isotopic compositions of basalts and peridotites at two segments of the southwest Indian ridge, we show that a component other than peridotite is required to explain the low end of the (143)Nd/(144)Nd variations of the basalts. This component is likely to have a lower melting temperature than peridotite, such as pyroxenite or eclogite, which could explain why it is not observed at mid-ocean ridges.     

Water-rich basalts at mid-ocean-ridge cold spots

Although water is only present in trace amounts in the suboceanic upper mantle, it is thought to play a significant role in affecting mantle viscosity, melting and the generation of crust at mid-ocean ridges. The concentration of water in oceanic basalts has been observed to stay below 0.2 wt%, except for water-rich basalts sampled near hotspots and generated by 'wet' mantle plumes. Here, however, we report unusually high water content in basaltic glasses from a cold region of the mid-ocean-ridge system in the equatorial Atlantic Ocean. These basalts are sodium-rich, having been generated by low degrees of melting of the mantle, and contain unusually high ratios of light versus heavy rare-earth elements, implying the presence of garnet in the melting region. We infer that water-rich basalts from such regions of thermal minima derive from low degrees of 'wet' melting greater than 60 km deep in the mantle, with minor dilution by melts produced by shallower 'dry' melting--a view supported by numerical modelling. We therefore conclude that oceanic basalts are water-rich not only near hotspots, but also at 'cold spots'.     

The propagation of seafloor spreading in the southwestern subbasin, South China Sea

On the basis of the summary of basic characteristics of propagation, the dynamic model of the tectonic evolution in the South-western Subbasin (SWSB), South China Sea (SCS), has been established through high resolution multi-beam swatch bathymetry and multi-channel seismic profiles, combined with magnetic anomaly analysis. Spreading propagates from NE to SW and shows a transition from steady seafloor spreading, to initial seafloor spreading, and to continental rifting in the southwest end. The spreading in SWSB (SCS) is tectonic dominated, with a series of phenomena of inhomogeneous tectonics and sedimentation.     

Evolution of the South China Sea and monsoon history revealed in deep-sea records

As the third summary report of ODP Leg 184 to the South China Sea (SCS), this paper discusses the evolution of the East Asian monsoon and the SCS basin. A multi-proxy approach, involving geochemistry, micropale-ontology, pollen and other analyses, was adopted for reconstructing the evolutionary history of the East Asian monsoon, which was characterized by a series of paleo-climate events especially at 8, 3.2, 2.2 and 0.4 Ma. The new record indicates similar stages in the development of the East and South Asian monsoons, with an enhanced winter monsoon over East Asia being the major difference. The rich spectrums of monsoon variability from the southern SCS also reveal other characteristic features of the low latitude ocean. Evidence for the evolution of the SCS includes the hemipelagic Oligocene sediments, implying the existence of deep water environments during the early seafloor spreading stage of the SCS basin. The four major unconformities and some remarkabl ediagenetic features in upper Oligocene deposits indicate the strongest tectonic events in the region. From a careful comparison of lithologies and sedimentation rates, we conclude that the prominent differences in sedimentary environments between the southern and northern SCS were established only by ～3 Ma.     

Rifting process and formation mechanisms of syn-rift stage prolongation in the deepwater basin,northern South China Sea

Based on the latest seismic and geological data, tectonic subsidence of three seismic lines in the deepwater area of Pearl River Mouth Basin （PRMB）, the northern South China Sea （SCS）, is calculated. The result shows that the rifting process of study area is different from the typical passive continental margin basin. Although the seafloor spreading of SCS initiated at 32 Ma, the tectonic subsidence rate does not decrease but increases instead, and then decreases at about 23 Ma, which indicates that the rifting continued after the onset of seafloor spreading until about 23 Ma. The formation thickness ex- hibits the same phenomenon, that is the syn-rift stage prolonged and the post-rift thermal subsidence delayed. The formation mechanisms are supposed to be three： （1） the lithospheric rigidity of the northern SCS is weak and its ductility is relatively strong, which delayed the strain relaxation resulting from the seafloor spreading; （2） the differential layered independent extension of the lithosphere may be one reason for the delay of post-rift stage; and （3） the southward transition of SCS spreading ridge during 24 to 21 Ma and the corresponding acceleration of seafloor spreading rate then triggered the initiation of large-scale thermal subsidence in the study area at about 23 Ma.     

The influence of ridge migration on the magmatic segmentation of mid-ocean ridges

The Earth's mid-ocean ridges display systematic changes in depth and shape, which subdivide the ridges into discrete spreading segments bounded by transform faults and smaller non-transform offsets of the axis. These morphological changes have been attributed to spatial variations in the supply of magma from the mantle, although the origin of the variations is poorly understood. Here we show that magmatic segmentation of ridges with fast and intermediate spreading rates is directly related to the migration velocity of the spreading axis over the mantle. For over 9,500 km of mid-ocean ridge examined, leading ridge segments in the 'hotspot' reference frame coincide with the shallow magmatically robust segments across 86 per cent of all transform faults and 73 per cent of all second-order discontinuities. We attribute this relationship to asymmetric mantle upwelling and melt production due to ridge migration, with focusing of melt towards ridge segments across discontinuities. The model is consistent with variations in crustal structure across discontinuities of the East Pacific Rise, and may explain variations in depth of melting and the distribution of enriched lavas.     

Origin of a 'Southern Hemisphere' geochemical signature in the Arctic upper mantle

The Gakkel ridge, which extends under the Arctic ice cap for approximately 1,800 km, is the slowest spreading ocean ridge on Earth. Its spreading created the Eurasian basin, which is isolated from the rest of the oceanic mantle by North America, Eurasia and the Lomonosov ridge. The Gakkel ridge thus provides unique opportunities to investigate the composition of the sub-Arctic mantle and mantle heterogeneity and melting at the lower limits of seafloor spreading. The first results of the 2001 Arctic Mid-Ocean Ridge Expedition (ref. 1) divided the Gakkel ridge into three tectonic segments, composed of robust western and eastern volcanic zones separated by a 'sparsely magmatic zone'. On the basis of Sr-Nd-Pb isotope ratios and trace elements in basalts from the spreading axis, we show that the sparsely magmatic zone contains an abrupt mantle compositional boundary. Basalts to the west of the boundary display affinities to the Southern Hemisphere 'Dupal' isotopic province, whereas those to the east-closest to the Eurasian continent and where the spreading rate is slowest-display affinities to 'Northern Hemisphere' ridges. The western zone is the only known spreading ridge outside the Southern Hemisphere that samples a significant upper-mantle region with Dupal-like characteristics. Although the cause of Dupal mantle has been long debated, we show that the source of this signature beneath the western Gakkel ridge was subcontinental lithospheric mantle that delaminated and became integrated into the convecting Arctic asthenosphere. This occurred as North Atlantic mantle propagated north into the Arctic during the separation of Svalbard and Greenland.