全球洋中脊系统的构造特征及其运动学意义

本文将全球洋中脊系统作为研究整体,根据洋中脊的全球分布、运动学特征及其初始形成时与泛大陆的构造几何关系,将全球现今的洋中脊系统划分为内、外支洋中脊。外支洋中脊为探索者洋中脊-太平洋洋隆-东南印度洋中脊-西北印度洋中脊,起源于泛大洋及冈瓦纳大陆内部;内支洋中脊为西南印度洋中脊-大西洋中脊-北冰洋加科尔洋中脊,起源于泛大陆内部。两者之间通过俯冲带、转换断层以及弥散性板块边界实现全球板块构造在运动上的平衡,并保持地球的球形几何形态恒定。外支洋中脊在全球板块构造上造成泛大洋缩减,并持续被太平洋取代,直接推动了环太平洋俯冲带的形成;内支洋中脊造成大西洋盆、印度洋盆中生代以来持续扩张。中生代以来,外支洋中脊和内支洋中脊共同作用引起非洲板块、印度澳大利亚板块向北运动,新特提斯洋盆关闭,形成特提斯(阿尔卑斯山-喀尔巴阡山-扎格罗斯山-喜马拉雅山)碰撞造山带,并通过洋中脊扩张平衡了相关的岩石圈缩短。     

The geologic history of the Nicoya Ophiolite Complex, Costa Rica, and its geotectonic significance

Costa Rica forms part of an intra-oceanic arc between the Pacific and Caribbean oceans; the Nicoya Ophiolite Complex is located along its Pacific border. In this study, evidence is given that the Nicoya Complex is composed of ridge-formed oceanic crust that suffered a strong compressional stress during Late Santonian times. As a result of this, isoclinal folding and large-scale nappe emplacement occurred at a shallow crustal depth. The principal component of this compressional stress was E-W-directed. It is also demonstrated that, from this time, the complex was situated between a subducting plate and a volcanic arc. From that Campanian until the Middle Eocene the zone was undulated, and generally at a great depth below sea level. During the Eocene—Oligocene epoch a new tectonic stress affected the area. It produced open folding with upthrusting in the ophiolite complex and overthrust folding of the overlying rock series. As a result of crustal thickening during this tectonic phase, the area was uplifted. From Miocene times, the zone was shaped into a dome and a synform. These undulations are attributed to compression of the subducting Coco Plate, west of the area.The Upper Santonian tectonic phase demonstrates how compressional stress produced the break-up of the Caribo-Pacific plate west of the study area, as a result of which, a Caribbean plate without an associated oceanic ridge and a Pacific plate originated. The compressional stress in question was presumably generated by the opposed spreading directions of the new Mid-Atlantic Ridge and an older ridge to the west of the study area.Furthermore, it is argued that the Cretaceous obduction of the ophiolite belt along the Pacific coast of the American continents, was produced by the directional change of these continents during the birth of the Mid-Atlantic Ridge. This created intra-plate compressional stress and converted originally passive continental margins into active zones, where thrusting of oceanic crust on to a continental margin (obduction) could occur. When the Mid-Atlantic Ridge started spreading, the obduction phase ended due to subduction of the oceanic plate below the leading edge of the continent.     

Tectonic evolution of the Pacific Ocean since 74 ma

A recent re-evaluation of the Late Mesozoic and Cenozoic sea-floor spreading data in the eastern Pacific has allowed us to make a new interpretation of the timing and sequence of the tectonic events which produced the present configuration of the plates (Whitman and Harrison, 1981; Whitman, 1981). Rotation parameters specifying the relative motion between all pairs of plates in the ocean basin have been calculated from the best fit of oceanic magnetic anomalies, with additional input from bathymetry and crustal ages of the Deep Sea Drilling Project sites. The rotation parameters for the relative motion between the Pacific and Antarctic plates are taken from Weissel et al. (1977) and the continental rotation parameters are from Barron et al. (1981).Plate motions have been determined back to 74 Ma. This time marks the initiation of spreading at the Pacific-Antarctic Ridge which caused the separation of the Campbell Plateau from Antarctica (Barron et al., 1981). Thus, this time is the earliest fix on the position of the Pacific plate relative to the continents surrounding the Pacific Ocean basin using sea-floor spreading. Since it is not possible to derive quantitative information about the relative motion between two plates separated by a trench, all rotations for the oceanic plates of the Pacific basin have been calculated relative to the Pacific plate and then relative to North America through the plate circuit: Pacific-Antarctica-Africa-North AmericaSince we also know the relative position of North America with respect to the other continents, we can show the relative position of the Pacific plate and the other oceanic plates with respect to all of the continental plates surrounding the Pacific Ocean basin.     

热点作用背景下的洋中脊跃迁和扩展作用:印度洋盆地张开过程探讨

在《印度洋底大地构造图》的基础上,分析了印度洋盆构造格局和洋盆演化重大事件序列,并从印度洋盆初始裂解机制、扩张中心跃迁与热点作用、洋中脊扩展作用等方面讨论了印度洋盆的张开过程,提出以下几点认识:(1)现今印度洋洋中脊可分为两个系统:东南印度洋中脊-中印度洋中脊-卡斯伯格洋脊系统(东支)和西南印度洋中脊系统(西支),前者是太平洋洋中脊扩展作用的产物,后者是太平洋-东南印度洋中脊与大西洋中脊之间构造调节的产物;(2)印度洋盆最初裂解受地幔柱垂向挤压-水平伸展作用控制,沿前寒武造山带等地壳薄弱带发育;(3)印度洋盆经历两次扩张中心的跃迁,其趋向性跃迁方向与热点相对板块的运动方向具有一致性,显示两者存在内在联系。(4)大西洋和太平洋洋中脊在印度洋交汇,于古近纪连通,末端伴随陆块持续发生碎裂化、裂解化,可称为鱼尾构造模式,表明印度洋盆衔接和调节了三大洋盆的发育和演化过程,具有全球洋盆枢纽的关键意义。     

Paleocene on-spreading-axis hotspot volcanism along the Ninetyeast Ridge: An interaction between the Kerguelen hotspot and the Wharton spreading center

Investigations of three plausible tectonic settings of the Kerguelen hotspot relative to the Wharton spreading center evoke the on-spreading-axis hotspot volcanism of Paleocene (60-54 Ma) age along the Ninetyeast Ridge. The hypothesis is consistent with magnetic lineations and abandoned spreading centers of the eastern Indian Ocean and seismic structure and radiometric dates of the Ninetyeast Ridge. Furthermore, it is supported by the occurrence of oceanic andesites at Deep Sea Drilling Project (DSDP) Site 214, isotopically heterogeneous basalts at Ocean Drilling Program (ODP) Site 757 of approximately the same age (59-58 Ma) at both sites. Intermix basalts generated by plume-mid-ocean ridge (MOR) interaction, exist between 11° and 17°S along the Ninetyeast Ridge. A comparison of age profile along the Ninetyeast Ridge between ODP Sites 758 (82 Ma) and 756 (43 Ma) with similarly aged oceanic crust in the Central Indian Basin and Wharton Basin reveals the existence of extra oceanic crust spanning 11° latitude beneath the Ninetyeast Ridge. The extra crust is attributed to the transfer of lithospheric blocks from the Antarctic plate to the Indian plate through a series of southward ridge jumps at about 65, 54 and 42 Ma. Emplacement of volcanic rocks on the extra crust resulted from rapid northward motion (absolute) of the Indian plate. The Ninetyeast Ridge was originated when the spreading centers of the Wharton Ridge were absolutely moving northward with respect to a relatively stationary Kerguelen hotspot with multiple southward ridge jumps. In the process, the spreading center coincided with the Kerguelen hotspot and took place on-spreading-axis volcanism along the Ninetyeast Ridge.     

Petrological and geochemical variations along the Mid-Atlantic Ridge between 46°S and 32°S: Influence of the Tristan da Cunha mantle plume

Basalts from a section of the Mid-Atlantic Ridge close to the active volcanic island of Tristan da Cunha in the South Atlantic have been analysed to investigate the influence of the mantle plume on the geochemistry of basalts being erupted at the spreading center. Although petrographically the rocks show only limited variation, two basaltic types were determined to be erupting in this region based on their major, trace and REE compositions. One group shows depletion in the incompatible and LRE elements, and can be characterised as N-type mid-ocean ridge basalts. The second group shows “enriched” geochemical characteristics and is similar to T-type MORBs.Mixing hyperbolae for the incompatible element and REE ratios suggest that extensive mixing of an end-member, characteristic of a plume region with an end-member of normal depleted MORB, canaccount for the occurrence of the T-type MORBs in this region.Based on the nature and development of the Tristan da Cunha mantle plume over the past 100 Ma, a composite model of evolution is suggested,in which a ridge-centered hotspot progressed to a near ridge hotspot, and finally to a totally intraplate situation. The fact that Tristan da Cunha is highly alkalic now, but that an irregular geochemical anomalyis also present on the Mid-Atlantic Ridge at this latitude would suggest an intermediate stage between the near-ridge and totally intraplate situation. This model leads to the conclusion that, as the Mid-Atlantic Ridge migrated away from the Tristan hotspot, a preferential sublithospheric flow towards the Ridge was established. This discontinuous feature can explain the geochemical variations seen along the Mid-Atlantic Ridge by providing a mechanism for mixing of a depleted N-type MORB component with an enriched component originating through processes active at the Tristan da Cunha mantle plume.     

The Lead, Neodymium and Strontium Isotopic Structure of Ocean Ridge Basalts

Pb-, Nd- and Sr-isotope compositions and U, Pb, Sm, Nd, Rb andSr concentrations are reported for samples of basaltic glassand altered substrates from spreading centres in the Atlantic,Indian and Pacific Oceans. Correlations are shown to exist between^{208, 207, 206}Pb/²⁰⁴Pb ratios, and ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Ndratios in basaltic glasses, but they are dominated by samplesfrom the Mid-Atlantic Ridge. Whereas basaltic glasses from EastPacific spreading centres exhibit smaller variability in Pb,Sr and Nd isotope compositions than Atlantic samples, seamountsamples from the E. Pacific have a similar range of Pb-isotopecompositions as Mid-Atlantic Ridge glasses. Contamination ofbasaltic magmas by altered oceanic crust or sediments is notconsidered to be of prime importance in determining the isotopicstructures of MORB glasses. It is proposed that the isotopicheterogeneity in the mantle beneath the Pacific and Atlanticis similar, but magma generation processes associated with fastspreading ridges of the East Pacific more effectively eradicateheterogeneities in the erupted basalts. Alteration of oceanic crust is further investigated with respectto the relative response of the U–Pb, Sm–Nd andRb–Sr systems, and the role of recycled oceanic crustin producing the mantle heterogeneities is evaluated.     

Recent tectonics in the northern part of the Knipovich Ridge,Atlantic ocean

The walls of the Knipovich Ridge are complicated by normal and reverse faults revealed by a high-frequency profilograph. The map of their spatial distribution shows that the faults are grouped into domains a few tens of kilometers in size and are a result of superposition of several inequivalent geodynamic factors: the shear zone oriented parallel to the Hornsunn Fault and superposed on the typical dynamics of the midocean ridge with offsets along transform fracture zones and rifting along short segments of the Mid-Atlantic Ridge (MAR). According to the anomalous magnetic field, the Knipovich Ridge as a segment of the MAR has formed since the Oligocene including several segments with normal direction of spreading separated by a multitransform system of fracture zones. In the Quaternary, the boundary of plate interaction along the tension crack has been straightened to form the contemporary Knipovich Ridge, which crosses the previously existing magmatic spreading substrate and sedimentary cover at an angle of about 45° relative to the direction of accretion. The sedimentary cover along the walls of the Knipovich is Paleogene in age and has subsided into the rift valley to a depth of 500–1000 m along the normal faults.     

Mesozoic and Cenozoic reconstructions of the South Atlantic

The movement of Antarctica with respect to South America has a number of implications for paleocirculation as well as for the reconstructions of Gondwanaland. Recent papers on the Southwest Indian Ridge have published new or revised poles of opening for Africa and Antarctica which can be combined with the poles of opening between South America and Africa to give resultant motions between South America and Antarctica.The first indication of a complete closure between South America and the Antarctic Peninsula is at anomaly 28 time (64 Ma) as the two continents are now configured. Between anomaly 28 time (64 Ma) and anomaly M0 time (119 Ma) the amount of closure does not change greatly, and the small computed overlap can be explained by minor uncertainties in the rotation poles used for the reconstructions or some slight extension between East and West Antarctica. By 135 Ma some rotation or translation of the Antarctic Peninsula with respect to East Antarctica must be postulated in addition to any presumed extension between East and West Antarctica in order to avoid an overlap of South America with the Antarctic Peninsula.Having determined what we feel to be a viable reconstruction of Western Gondwanaland and holding South America fixed, we rotated Africa and Antarctica, with respect to South America, for eight different times during the past. Africa moved away from South America in a more or less consistent manner throughout the time period, closure to present, while Antarctica moved away from Africa in a consistent manner only between 160 Ma and 64 Ma. At 64 Ma its motion changed abruptly: it slowed its north-south motion with respect to Africa and began slow east-west extension with respect to South America. This change supports the hypothesis that a major reorganization of the triple junction between Africa, Antarctica and South America occurred between 60 and 65 Ma. The triple junction changed from ridge-ridge-ridge to ridge-fault-fault at the time of the major westward jump of the Mid-Atlantic Ridge just south of the Falkland-Agulhas Fracture Zone.The Mesozoic opening of the Somali Basin moved Madagascar from its presumed original position with Africa in Gondwanaland. The closure of Sri Lanka with India produces a unique fit for India and Sri Lanka with respect to Africa, Madagascar and Antarctica. This fit juxtaposes geological localities in Southeast India against similar localities in Enderhy Land. East Antarctica. The late Jurassic opening in the Somali Basin is tied to opening of the same age in the Mozambique Basin. Since this late Jurassic movement represents the initial break-up of Gondwanaland, it is assumed that similar movement must have occurred in what is now the western Weddell Sea and may also explain the opening evidenced by the Rocas Verdes region of southern South America.     

The evolution of marginal basins and adjacent shelves in east and southeast Asia

The structural elements on the shallow (Sunda Shelf) and deep seas of east and south—east Asia are interpreted as the result of past interaction between lithospheric plates. During the Mesozoic the western Pacific Ocean and the eastern Indian Ocean were parts of the Tethys Sea and were moving to the north relative to Antarctica. A Mesozoic ridge system trending east—west produced east—west trending magnetic anomalies throughout the entire area. The ridge system was bisected by large north—south transform faults which divided the eastern Indian Ocean—western Pacific Ocean into sub-plates traveling at different speeds. The Mesozoic evolution of the Sunda Shelf and the deep seas resulted from such horizontal differential movement in a north—south direction. During Late Cretaceous—Eocene the various segments of the spreading ridge gradually submerged beneath the deep sea trenches to the north, causing a gradual change in the direction of motion of the Pacific plate. The change in motion of the Pacific plate resulted in the separation between the Pacific and the eastern Indian Ocean plates, the formation of large northeast—southwest tectonic elements on the Sunda Shelf and elsewhere in south—east Asia, the formation of the western Philippine Basin and the rapid northward motion of Australia. The only remnant of the Mesozoic ridge system exists today at the western Philippine Basin.     

全球空间测地站矢量场对板块运动的描述及地幔的经、纬向流

从 2 0 0 1年IERS公布的ITRF参考框架系下的 4 32个空间测地站中以面积平均随机选用了111个站 ;这些点的站速度矢量图描述了北半球欧亚 (EA)、北美 (NA)和北太平洋 (NP)三大板块的运动态势 :以北大西洋洋脊为轴 ,欧亚板块向NE—E—SE运动 ;北美板块向NW—W—SW运动 ,呈绵羊角式张裂运动 ,两者在大西洋两侧的N向运动越过极区后 ,汇集指向阿留申 ;北太平洋板块以超过北美板块和欧亚板块 6倍左右的速度向NWW运动 ,与欧亚板块呈正面碰撞型关系 ,表现为大洋壳的深俯冲与大陆壳仰冲 ;北太平洋板块与北美板块以圣安德烈斯断层为界呈剪切拉张型关系。文中描述了南半球南美 (SA)、非洲 (AF)、阿拉伯 (AR)、印—澳 (IN—AU)和东南太平洋 (SP)的运动态势 :以东南太平洋洋脊为轴 ,其东侧的纳兹卡 (NZ)板块高速向东运动 ,而其前方的南美、非洲、阿拉伯和印—澳等板块则向NE—NNE运动 ,速度依次增高 ,可能与其间的大西洋洋脊、印度洋洋脊和红海裂谷张裂的正、反向叠加作用有关。西南太平洋与澳大利亚板块的东北缘和东缘均呈强碰撞强剪切型运动关系 ,而纳兹卡板块与北美板块呈追尾型运动关系。南极板块 (Ant)从环南极洋脊的东南太平洋段为起点向南运动 ,过南极后继续向北、向非洲、印度洋方向运动 ,并略显右旋。上述板?     

Multibeam bathymetry and sidescan imaging of the Rivera Transform–Moctezuma Spreading Segment junction, northern East Pacific Rise: New constraints on Rivera–Pacific relative plate motion

To better understand the recent motion of the Pacific plate relative to the Rivera plate and to better define the limitations of the existing Rivera–Pacific plate motion models for accurately predicting this motion, total-field magnetic data, multibeam bathymetric data and sidescan sonar images were collected during the BART and FAMEX campaigns of the N/O L'Atalante conducted in April and May 2002 in the area surrounding the Moctezuma Spreading Segment of the East Pacific Rise, located offshore of Manzanillo, Mexico, at 106°16′W, between 17.8°N and 18.5°N. Among the main results are: (1) the principle transform displacement zone of the Rivera Transform is narrow and well defined east of 107o15′W and these azimuths should be used preferentially when deriving new plate motion models, and (2) spreading rates along the Moctezuma Spreading Segment should not be used in plate motion studies as either seafloor spreading has been accommodated at more than one location since the initiation of seafloor spreading in the area of the Moctezuma Spreading Segment, or this spreading center is not a Rivera–Pacific plate boundary as has been previously assumed. Comparison of observed transform azimuths with those predicted by the best-fit poles of six previous models of Rivera–Pacific relative motion indicate that, in the study area, a significant systematic bias is present in the predictions of Rivera–Pacific motion. Although the exact source of this bias remains unclear, this bias indicates the need to derive a new Rivera–Pacific relative plate motion model.     

Mesozoic interaction of the Kula plate and the western margin of north america

Oceanic crust west of North America at the beginning of the Jurassic belonged to the Kula plate. The development of the western margin of North America since the Jurassic reflects interaction with the Kula plate, the Kula-Farallon spreading center and the Farallon plate. The Kula plate ceased to exist in the Paleocene and later developments were caused by interaction of the Farallon plate and, subsequently, collision with the East Pacific Rise.At the beginning of the Jurassic, when spreading between North and South America began, the Kula-Farallon-Pacific triple junction moved to the north relative to North America, and the eastern end of the Kula-Farallon spreading center swept northwards along the continental margin.During the Paleocene, Kula-Pacific spreading ceased and the Kula plate fused to the Pacific plate. Throughout the Mesozoic, subduction of the Kula plate took place along the Alaskan continental margin. When the Kula plate joined the Pacific plate a new subduction zone formed along the line of the present Aleutian chain.Wrangellia and Stikinia, anomalous terrains in Alaska and northwestern Canada respectively, were emplaced by transport on the Kula plate from lower latitudes. Hypotheses which require transport of these plates in the Mesozoic from the “far reaches of the Pacific” ignore the problem of transport across either the Kula-Pacific or Kula-Farallon spreading centers. The interaction of the Kula plate and western North America throughout the Jurassic and the Cretaceous should result in emplacement of these terrains by motion oblique to the continental margin. Tethyan faunas in Stikinia must come from the western end of Tethys between North and South America, not the Indonesian region at the eastern end of Tethys.As the northeastern end of the Kula-Farallon ridge moved northward, the sense of motion changed from right lateral shear between the Kula and North American plates to collision or left lateral shear between the Farallon and North American plates. Left lateral shear along zones analogous to the Mojave-Sonora megashear may have been the means by which anomalous terrains were transported to the southeast into the gap between North and South America forming present day Central America. Such a model overcomes the overlap difficulties suffered in previous attempts to reconstruct the Mesozoic paleogeography of Central America.     

Active Motion of Tectonic Blocks in East Asia: Evidence from GPS Measurement

The relative Euler vectors of the Pacific, Philippine, Amurian, Okhotsk, N. Honshu and South China plates or blocks are deduced from earthquake slip vectors, transform fault azimuths and spreading rates, which are consistent with new results derived from the International Terrestrial Reference Frame ITRF2000 velocity field, the velocity field of GPS stations in China and the GPS measurement data of the GEONET network in Japan. Based on the two groups of Euler vectors, analysis and comparative study of the relative motions and deformations of the tectonic blocks in East Asia reveal the present-day motion characteristics of the blocks.     

Seismotectonic of the Azores-Alboran region

New seismicity and focal-mechanism data from the area of the Azores Islands, in the Mid-Atlantic Ridge, to the Alboran Sea and the southern part of Spain are presented.As a consequence of the different characters in the focal-mechanism solutions and b-values associated, the area has been divided in four different parts, namely, Mid-Atlantic and Terceira Ridge, Azores—Gibraltar fault, Gulf of Cadiz, Alboran Sea and Betica. The last two form the interaction between the Eurasian and African continental plates.The fracture zone is the locus of very large earthquakes with mechanisms showing a predominant right-lateral horizontal motion. Seismic foci in the continental interaction zone are spread over the whole region with mechanisms changing in character from west to east. It is suggested that this may be consequence of the behaviour of the Spanish Peninsula as a partly independent subplate. In the eastern part of the studied zone, the so-called Alboran plate may be considered as a buffer plate.     

Plate boundaries in the Far East region of Russia (from GPS measurement,seismic-prospecting,and seismological data)

The recent geodynamics of the Far East region of Russia is considered, where three large tectonic plates converge—Eurasian, North American, and Pacific, as well as several microplates—Okhotsk, Bering, and Amurian—have been hypothesized to exist. The available data on the position of the plate boundaries, the relative plate rotation poles, and the regional seismicity were analyzed, and parameters of plate motion models for northeastern Russia were determined in this study. The regional deep structure was investigated, using data obtained by different geophysical methods. The results of observations of the Magadan–Vrangel Island profile by deep seismic sounding (DSS), common-depth point (CDP) method, and correlation refraction method (CRM) are presented.     

The oblique intersection of the Mid-Atlantic Ridge with Charlie-Gibbs transform fault

The junction angle between the western Charlie-Gibbs transform fault and the spreading axis of the Mid-Atlantic Ridge diverges by 40° from the orthogonal intersection assumed in many studies of plate boundaries. This has been established by a surface-ship reconnaissance and by mapping fault trends in a transponder-navigated deep-tow survey of the fracture valley 25 km from the intersection. One set of normal faults trends 325–330°, parallel to the obliquely spreading ridge axis, and another set trends 275°, parellel to the direction of relative plate motion. Although the near-bottom survey was in the theoretically inactive part of the fracture zone, beyond the transform fault section, there is evidence for recent motion on faults that cut the thick sediment fill of the fracture valley.Oblique spreading of a ridge axis near a transform fault may result from distortion of the regional stress field by a strike-slip couple. Tension parallel to the long axis of the strike-slip strain ellipse, which is responsible for oblique normal faulting in transform valleys, causes oblique dike injection and oblique faulting in the axial rift valley. These effects extend further from transfrom fault intersections on slow-spreading ridges than on fast-spreading rises.     

New visualizations of global tectonic plate motions and plate boundary interactions

Clear understanding of detailed lithospheric plate motions has been impeded by lack of a suitable means of graphical representation. A series of coloured global maps are presented that reveal more detail in the patterns of both absolute and relative global plate motions. The use of continuous colour to represent velocities overcomes the limitations of earlier maps that used isolated vectors at selected points to indicate plate velocities. Velocity magnitudes and directions for entire surfaces of plates were computed at a resolution of 0.5°, and are shown on two separate maps. Relative motions between plates were decomposed into their shear and normal components, and are plotted on separate maps. Continuous colour is again used to indicate both the directions and magnitudes of sinistral/dextral and convergent/divergent motions for all plate boundaries. A final map of normalized velocity magnitudes for all plates reveals a global, fast 'belt' of plate motion that parallels a great circle aligned with the fastest portion of the Pacific Plate and orthogonal to the East Pacific Rise.     

大西洋中脊26°S玄武岩岩石成因及构造意义: 来自Hf同位素的约束

大西洋中脊是慢速扩张洋脊的典型代表。本文以大西洋中脊26°S地区脊轴及海山玄武岩代表性样品为研究对象, 开展系统的Sr-Nd-Pb-Hf同位素研究, 并结合已发表的数据, 探讨研究区玄武岩成因及地幔源区性质和演化, 旨在为认识地幔不均一性和地幔柱-洋脊相互作用方式提供关键证据。样品主-微量元素与Sr-Nd-Pb-Hf同位素分析结果表明, 所有样品均显示同位素富集的N-MORB特征。此外, 大西洋26°S玄武岩主微量元素和同位素具有较大的变化范围, 且同位素之间呈现出良好的相关关系, 表明其是亏损软流圈地幔熔融的结果, 但有富集组分参与。结合元素和同位素特征以及Sr-Nd-Hf同位素定量模拟结果, 富集组分可能为Tristan da Cunha地幔柱残余组分, 显示EMⅠ型富集地幔特征。同位素定量模拟结果表明: 海山玄武岩地幔源区组成为约90%~95%的亏损组分和10%~5%的富集组分; 而脊轴玄武岩地幔源区富集组分较少(< 5%)。点位6、7海山玄武岩样品显示高放射成因Pb同位素组成, 符合Dupal异常边界条件。定量计算表明, 造成其异常的原因可能与EMⅠ型组分参与有关, 这与同位素定量模拟结果相吻合。本文研究的同位素不同程度富集N-MORB可能的成因机制为: 远端地幔柱-洋脊相互作用, 即Tristan da Cunha地幔柱距离洋脊>1000km, 地幔柱在运移至大西洋中脊的过程中, 岩石圈厚度明显变薄, 为减压熔融的发生提供了良好条件, 使残余地幔柱物质不相容元素亏损, 但同位素组成保留源区富集的特征。地幔柱残余物质到达大西洋中脊下方后, 参与洋脊地区减压熔融, 最终形成研究区不相容元素亏损且同位素富集的N-MORB。因此, 本文研究的同位素富集的N-MORB可能记录了远端柱-脊相互作用和洋脊之下富集地幔柱物质再熔融的过程, 为认识地幔不均一性提供了新的岩石学和地球化学证据。因此, 地幔柱-洋脊相互作用不仅是E-MORB的可能成因, 对理解N-MORB形成也有十分重要的意义。

     

Hydrous partial melting within the lower oceanic crust

We studied more than 60 oceanic gabbros from the recent oceanic crust and from ophiolites (East Pacific Rise, Mid-Atlantic Ridge, Southwest Indian Ridge, Oman ophiolite) by scanning electron microscopy and found in nearly all samples microstructures suggesting that hydrous partial melting reactions proceeded. The characteristic paragenesis consists of orthopyroxene and pargasite rimming olivine and clinopyroxene primocrysts in intimate contact with neoblastic plagioclase strongly enriched in anorthite. This is in agreement with recent water-saturated melting experiments on a variety of natural gabbros between 900 and 1000 °C. The observed microtextures in the natural gabbros imply the propagation of water-rich fluids on grain boundaries in a ductile regime causing hydrous partial melting. Thus, this type of hydrothermal activity proceeds within the deep oceanic crust at very high temperatures (900–1000 °C) without a crack system, a prerequisite in current models for enabling hydrothermal circulation.