东南沿海晚中生代镁铁质岩的Re-Os同位素组成     

周金城蒋少涌  王孝磊杨竞红 张孟群 《岩石学报》2006,22(2):407-413

从浙东、闽东南的早白垩世“双峰式火山岩”和“复合岩体”中选择了有代表性的玄武岩和角闪辉长岩，分离出钛-磁铁矿，进行Re—Os同位素分析。角闪辉长岩的γos（130Ma）为-83．7～ -47．1，其（^187 Os／^188 Os）。过低，为0．0667和0．0205。这低得不合理的（^187 Os／^188 Os）i表明成岩之后Re-Os体系可能受到扰动，某些地质过程导致外加Re的进入，使岩石的^187 Re／^188 Os比值偏高、（^187 Os／^188 Os）;及γOs（t）值偏低。玄武岩的γOs（130Ma）=21．4—267．8，其（^187 Os／^188 Os）。为0．1531～0．4639。Re—Os、Sm—Nd同位素及微量元素地球化学研究表明，早白垩世玄武岩浆不是来自富集地幔，而是来自亏损的地幔源区，在其演化过程中经受了地壳岩石和熔体的混染。晚中生代时期，东南沿海“大陆弧”下的地幔可能不是富集的岩石圈地幔，具有亏损特性，因而是软流圈地幔。  相似文献   

铙钹寨岩体是蛇绿岩吗？——评“也谈铙钹寨超镁铁岩体的成因和构造类型的归属问题”   总被引：1，自引：2，他引：1  

张旗 《岩石学报》2006,22(12):3079-3084

王希斌等根据铙钹寨岩体恢复的原岩由亏损强烈的方辉橄榄岩、纯橄岩和弱亏损的二辉橄榄岩组成以及有豆荚状铬铁矿存在,认为铙钹寨岩体属于大洋岩石圈地幔,是蛇绿岩的成员;并且还根据岩体存在两种地幔橄榄岩组合进一步推断铙钹寨岩体"可能经历了洋内扩张(形成 MOR 型的地幔残余)和洋内俯冲两个阶段的演化过程"。我们认为,铙钹寨岩体是交代的地幔橄榄岩,它不大可能是蛇绿岩;铙钹寨岩体的特征比较接近大陆岩石圈地慢而非大洋岩石圈地幔;铬铁矿不是判别蛇绿岩的标志;不能根据岩体存在强烈亏损和弱亏损的两种橄榄岩而推断其形成于两种环境。  相似文献   

鄂尔多斯块体西缘及西南缘深部电性结构与该区地质构造的关系   总被引：6，自引：0，他引：6  

屈健鹏 《内陆地震》1998,(4)

对在地理位置上具有一定代表性的鄂尔多斯块体西缘及西南缘的３ 条大地电磁剖面进行了分析。盐池—阿拉善左旗剖面：整条剖面上均有壳内低阻层和上地幔低阻层分布,低阻层在银川断陷盆地上隆。定边—景泰剖面：壳内低阻层仅出现在弧形断裂带区,但上地幔低阻层在整个剖面上都有分布。在弧形断裂带区上地幔低阻层埋藏深度加大,但并不上隆,这与北面银川断陷盆地的上地幔上隆形成反照。分析认为,银川断陷盆地属于拉张性质,而弧形断裂带区属于挤压性质,由于均衡调整作用,造成了两者上地幔结构的反差。成县—西吉剖面：以天水太京测点为界,其南、北两段的电性结构差异较大,这为划分南、北两个地质单元提供了深部结构上的依据  相似文献   

Fractionation of platinum group elements in upper mantle: Evidence from peridotite xenoliths from Wangqing     

Yigang Xu  B. Orberger  S. J. Reeves 《中国科学D辑(英文版)》1998,41(4):354-361

The peridotite xenoliths from Wangqing display a downward-concave PGE pattern with Pt at maximum due to Pt-Pd fractionation, which is different from the flat or negatively sloped patterns commonly observed for worldwide peridotite xenoliths and massifs. Taking into account the difference in partition coefficients of Pt and Pd between alloy and sulfide melt, it is suggested that some of the Wangqing peridotites must have been equilibrated with a melt in which alloy and sulfide coexisted. Segregation of Pt-rich alloy from Pd-sulfide melts due to density contrast and extraction of residual melts without sulfide saturation resulted in the fractionation of Pt from Pd. Project supported by the Ministry of Human Affairs and a part of an international collaboration program between the Chinese Academy of Sciences and Centre National de Recherche Scientifique (CNRS, France), and Australian Institute of Nuclear Sciences and Engineering (AINSE) grant to SJR.  相似文献   

Is there a first-order discontinuity in the lowermost mantle?     

Xian-Feng Liu  Jeroen Tromp  Adam M Dziewonski 《Earth and Planetary Science Letters》1998,160(3-4):343-351

In 1983, Lay and Helmberger [Geophys. J. R. Astron. Soc. 75 (1983) 799–837] reported the detection of a precursor to the seismic phase ScS. They attributed this precursor to a sharp seismic discontinuity located several hundred kilometers above the core–mantle boundary. Such a lowermost mantle discontinuity implies the existence of a sharp phase change or a chemical boundary. Precursors to ScS and, less frequently, PcP have since been observed in numerous locations, but are not a global phenomenon. Frequently, PcP precursors are weak or absent when ScS precursors are observed in the same location, and vice versa. There can be significant variations in the amplitude and arrival time of the precursor relative to the main phase. The presence or absence of these precursors has led to speculations about the nature of the lowermost mantle. Here we demonstrate that ScS or PcP precursors may be produced by gradients in seismic wave speed associated with large-scale lowermost mantle heterogeneity. Rather than a phase or chemical boundary with substantial topography, such gradients require lateral variations in temperature and, close to the core–mantle boundary, composition.  相似文献   

Helium isotope composition of the early Iceland mantle plume inferred from the Tertiary picrites of West Greenland     

D.W. Graham  L.M. Larsen  B.B. Hanan  M. Storey  A.K. Pedersen  J.E. Lupton 《Earth and Planetary Science Letters》1998,160(3-4):241-255

Picrites from the 61 million year old Vaigat Formation of the Nuussuaq Peninsula in West Greenland have ³He/⁴He ratios trapped in olivine phenocrysts which range up to 30 times the atmospheric ratio. These high values, measured during gas extraction by crushing in vacuum, are similar to the highest magmatic ³He/⁴He ratios found in young terrestrial volcanic rocks. By analogy with young basalts, in which crushing selectively extracts magmatic helium, any significant cosmogenic ³He appears to be absent in these picrites. Additional evidence for the absence of cosmogenic helium is provided by fusion results on the crushed olivine powders and by a single stepwise crushing experiment, in which only magmatic and radiogenic helium components are resolvable. The West Greenland picrites have Pb, Nd and Sr isotope compositions which overlap those found in picrites from Iceland and in basalts from Loihi Seamount, localities which today also have high ³He/⁴He ratios. Isotopic variations in He, Pb, Nd and Sr for the West Greenland picrites are interpreted to largely result from interaction of the early Iceland mantle plume with the upper mantle during plume ascent and dispersion beneath the continental lithosphere. The presence of high ³He/⁴He ratios in West Greenland, and the onset of magmatism across the North Atlantic Volcanic Province near 62 Ma, supports the hypothesis for very rapid dispersion (>1 m/year) of mantle plume head material during the earliest stages of plume impact, as predicted in recent numerical simulations of plume behavior during thermal mantle convection with non-Newtonian rheology.  相似文献   

Edge-driven convection   总被引：23，自引：0，他引：23  

Scott D King  Don L Anderson 《Earth and Planetary Science Letters》1998,160(3-4):289-296

We consider a series of simple calculations with a step-function change in thickness of the lithosphere and imposed, far-field boundary conditions to illustrate the influence of the lithosphere on mantle flow. We consider the effect of aspect ratio and far-field boundary conditions on the small-scale flow driven by a discontinuity in the thickness of the lithosphere. In an isothermal mantle, with no other outside influences, the basic small-scale flow aligns with the lithosphere such that there is a downwelling at the lithospheric discontinuity (edge-driven flow); however, the pattern of the small-scale flow is strongly dependent on the large-scale thermal structure of a much broader area of the upper mantle. Long-wavelength temperature anomalies in the upper mantle can overwhelm edge-driven flow on a short timescale; however, convective motions work to homogenize these anomalies on the order of 100 million years while cratonic roots can remain stable for longer time periods. A systematic study of the effect of the boundary conditions and aspect ratio of the domain shows that small-scale, and large-scale flows are driven by the lithosphere. Edge-driven flow produces velocities on the order of 20 mm/yr. This is comparable to calculations by others and we can expect an increase in this rate as the mantle viscosity is decreased.  相似文献   

Neotectonic Crustal Uplift on the Continents and its Possible Mechanisms. The Case of Southern Africa     

Eugene V. Artyushkov  Albrecht W. Hofmann 《Surveys in Geophysics》1998,19(5):369-415

According to a large volume of data an intensive crustal uplift began in the Oligocene over most of the continental areas after a long period of relative tectonic stability. This Neotectonic uplift formed most of the present positive topographic features on the continents, and its strong acceleration took place during the last several million years. In many regions the uplift was associated with magmatism. The main methods of studying the Neotectonic uplift are considered together with the data on the uplift of Southern Africa. In this area the uplift took place in the Early Miocene (up to 300 m) and in the Late Pliocene and Pleistocene (up to 900 m). It occurred without stretching or shortening of the crust. Rapid erosion of the lower part of mantle lithosphere by a plume material is proposed as a mechanism of the uplift. This material ascended from below and rapidly spread along the base of the lithosphere. Its spreading for 1000 km during a few million years is possible only under a low viscosity of normal asthenosphere (1019 Pa s) and a much lower viscosity of a plume material (2 × 1016 Pa s). As in Southern Africa, in most of the regions the Neotectonic uplift was associated with insignificant shortening or stretching of the crust. This indicates that in some regions a plume material ascended from below and rapidly spread along the base of the lithosphere and eroded the mantle lithosphere in vast areas beneath the continents. In regions with a hot asthenosphere a strong weakening of the mantle lithosphere which allows its erosion can be associated with a high temperature of the plume material. In regions where the asthenosphere is at moderate temperature weakening of the mantle lithosphere can result from infiltration of volatiles from the plume material.  相似文献   

Petrology and geochemistry of potassic rocks in the Gölcük area (Isparta, SW Turkey): genesis of enriched alkaline magmas     

P Al&#x;c&#x;  A Temel  A Gourgaud  G Kieffer  M.N Gündodu 《Journal of Volcanology and Geothermal Research》1998,85(1-4)

The Lower Pliocene volcanic rocks occurring in the Gölcük area of SW Turkey exhibit alkaline major element trends with a general potassic character. The development of volcanism can be divided into 2 major stages such as trachytic ancient lavas/domes and tephriphonolitic, trachyandesitic to trachytic Gölcük eruptions (ignimbrites, lava/dome extrusions, phreatomagmatic deposits, and finally, young domes). Volcanic rocks consist primarily of plagioclase, clinopyroxene (which ranges in composition from diopside to augite and are commonly zoned), biotite, and phlogopite. Amphibole phenocrysts are restricted to the pyroclastic deposits. Pseudoleucites are also seen only in the lava/dome extrusions. Oxides and apatites are common accessory phenocryst phases. As would be expected from their potassic–alkaline nature, the volcanic rocks of the Gölcük area contain high amounts of LILE (Ba, Sr, Rb and K), LREE, and Zr. Concentrations of compatible elements such as Cr, Ni and V are very low, possibly indicating fractionation of olivine and clinopyroxene. Correlation of SiO₂, Rb/Sr and MgO with ⁸⁷Sr/⁸⁶Sr (0.703506–0.704142) exhibit an increasing trend in the direction of crustal contamination. However, the isotopic compositions of Sr are not as high to indicate a high level of crustal contamination. Geochemical data are consistent with the derivation of Gölcük volcanic rocks from a metasomatized and/or enriched lithospheric mantle source during crustal extension in the area. This metasomatism was probably occurred by fluids released from the northward subduction between African and Eurasian plates during Tertiary, as the Gölcük volcanic rocks display features of island-arc magmas with having high Ba/Nb (>28) ratios, and Nb and Ti depletions. Lower Pliocene volcanism in the Gölcük was response to extensional tectonics.  相似文献   

Upper mantle P wave velocity structure of the northern part of China and Mongolia     

JIAN PING WU RONG SHENG ZENG YUE HONG MING Institute of Geophysics  China Seismological Bureau  Beijing  China 《地震学报(英文版)》1998,11(6):677-686

ｎｔｒｏｄｕｃｔｉｏｎＴｈｅｄｅｔｅｒｍｉｎａｔｉｏｎｏｆｆｉｎｅｒａｄｉａｌｖｅｌｏｃｉｔｙｓｔｒｕｃｔｕｒｅｏｆｕｐｐｅｒｍａｎｔｌｅｐｌａｙｓａｎｉｍｐｏｒｔａｎｔｒｏｌｅｉｎｉｎｖｅｓｔｉｇａｔｉｏｎｏｆｍａｎｔｌｅｃｏｍｐｏｓｉｔｉｏｎａ...  相似文献