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11.
翁通爪哇高原、凯尔盖朗高原与沙茨基海隆是全球三大洋底高原, 是大量岩浆喷发到地表的结果, 火山面积分别达1.90×106、1.25×106、0.53×106km2。本文详细分析了该三大洋底高原的地形、剩余地幔布格重力异常(residual mantle Bouguer anomaly, RMBA)与重力反演的相对地壳厚度, 并结合地质与地球化学特征约束进行对比研究。结果显示, 翁通爪哇高原、凯尔盖朗高原与沙茨基海隆分别高出周围海底约4.3、5、4km, 相应的地幔布格重力异常最大变化值分别为250、330、200mGal, 以及相应的相对地壳厚度变化分别为11、13、9km, 表明形成三大洋底高原的岩浆量远远大于正常洋中脊的岩浆量。此外, 三大洋底高原皆形成于洋中脊附近。Nd、Pb、Hf同位素比值分析表明, 翁通爪哇高原的玄武岩组分为洋岛玄武岩; 凯尔盖朗高原大部分类似于洋岛玄武岩, 并含有洋中脊玄武岩组分; 沙茨基海隆的玄武岩组分主要为东太平洋海隆正常洋中脊玄武岩, 却又存在少量位于全球洋岛玄武岩范围内。这些特征揭示了三大洋底高原可能形成于“地幔柱-洋中脊相互作用”。对此本文提出了两种模式: 一为洋中脊被地幔柱拖拽至其上方; 二为洋中脊之下的软流圈受到地幔柱影响, 从而产生超常熔融与超厚地壳。 相似文献
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Intertidal Invertebrate and Algal Communities on the Rocky Shores of the Bay of Morbihan, Kerguelen (South Indian Ocean) 总被引：3，自引：0，他引：3
Abstract. Both the emergent surfaces and tide pools of the rocky intertidal shores of the Bay of Morbihan, Kerguelen are depauperate in number of macroinvertebrate and macroalgal species. Diversity (H') was high and similar for macroinvertebrates in tide pools from the high to low intertidal and on the emergent surface exposed at extreme low-tide when calculated in terms of numbers of individuals. H' was lower when calculated in terms of energy and twice as high for the tide pool in the high intertidal than the lower tide-pools and the emergent surface. H' for macroalgae was 0 in the highest tide-pool and highest in the lower tide-pools. The density/species-rank curves for the macroinvertebrates and the macroalgae are logarithmic but vary in linearity. Algal biomass was 37 % of the macroinvertebrate biomass on the emergent surface at the extreme low-tide, but 106% in the highest tide-pool. Twenty-three months after denudation of the tide pools, the ratios of total densities of the macroinvertebrates (ind m-2 ) to the original densities were 48 % for the pool in the high intertidal, 25 % for the middle tide-pool, and 4 % for the pool in the low intertidal. The diversity indexes for the macroinvertebrates were similar to the original ones. Recolonization at Kerguelen is slow and shows an intertidal gradient. The composition of the rocky intertidal community at Kerguelen results from the rigorous physical environment and the small number and functional types of species. In the absence of major predators on macroinvertebrates and of herbivores on macroalgae, interaction among the species present seems to be competitive in nature. 相似文献
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The early Cretaceous (Albian–Aptian) Sung Valley ultramafic–alkaline–carbonatite complex is one of several alkaline intrusions that occur in the Shillong Plateau, India. This complex comprises calcite carbonatite and closely associated ultramafic (serpentinized peridotite, pyroxenite and melilitolite) and alkaline rocks (ijolite and nepheline syenite). Field relationship and geochemical characteristics of these rocks do not support a genetic link between carbonatite and associated silicate rocks. There is geochemical evidence that pyroxenite, melilitolite and ijolite of the complex are genetically related. Stable (C and O) and radiogenic (Nd and Sr) isotope data clearly indicate a mantle origin for the carbonatite samples. The carbonatite Nd (+0.7 to +1.8) and Sr (+4.7 to +7.0) compositions overlap the field for Kerguelen ocean island basalts. One sample of ijolite has Nd and Sr isotopic compositions that also plot within the field for Kerguelen ocean island basalts, whereas the other silicate–carbonatite samples indicate involvement with an enriched component. These geochemical and isotopic data indicate that the rocks of the Sung Valley complex were derived from and interacted with an isotopically heterogeneous subcontinental mantle and is consistent with interaction of a mantle plume (e.g. Kerguelen plume) with lithosphere. A U–Pb perovskite age of 115.1±5.1 Ma obtained for a sample of Sung Valley ijolite also supports a temporal link to the Kerguelen plume. The observed geochemical characteristics of the carbonatite rocks indicate derivation by low-degree partial melting (0.1%) of carbonated mantle peridotite. This melt, containing a substantial amount of alkali elements, interacted with peridotite to form metasomatic clinopyroxene and olivine. This process could progressively metasomatize lherzolite to form alkaline wehrlite. 相似文献
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白垩纪中期地球磁场与全球地质现象(英文) 总被引：5，自引：0，他引：5
白垩纪超静磁带(CNS)是指白垩纪中期约40 Ma内(Aptian Santonian, 121~83 Ma)地球磁场没有出现倒转的异常现象。这一现象引起地球学界的极大关注,原因在于它与白垩纪中期(124~90 Ma)其他一系列事件在时间上非常吻合,包括洋底扩张速率的快速增加和洋底高原、海山链、大型火成岩省等生产速率的快速增加。过去20多年许多研究强调所有这些现象与下地幔对流的联系。近期研究的一个重要的进展在于把地球磁场的变化(反转频率和古强度)与许多重要的地质和地球物理过程结合起来,如周期性地幔对流、地幔柱活动、全球热流、真极移、大型火成岩省的产生和生物群集绝灭。文中回顾了白垩纪地球磁场研究以及与白垩纪中期所发生的全球性的地质事件之间的可能存在的关联,并讨论在这一重要研究领域内未来研究的发展方向。 相似文献
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Numerous early Cretaceous mafic and alkaline dykes, mostly trending in N-S direction, are emplaced in the Archaean gneissic
complex of the Shillong plateau, northeastern India. These dykes are spatially associated with the N-S trending deep-seated
Nongchram fault and well exposed around the Swangkre-Rongmil region. The petrological and geochemical characteristics of mafic
dykes from this area are presented. These mafic dykes show very sharp contact with the host rocks and do not show any signature
of assimilation with them. Petrographically these mafic dykes vary from fine-grained basalt (samples from the dyke margin)
to medium-grained dolerite (samples from the middle of the dyke) having very similar chemical compositions, which may be classified
as basaltic-andesite/andesite. The geochemical characteristics of these mafic dykes suggest that these are genetically related
to each other and probably derived from the same parental magma. Although, the high-field strength element (+rare-earth elements)
compositions disallow the possibility of any crustal involvement in the genesis of these rocks, but Nb/La, La/Ta, and Ba/Ta
ratios, and similarities of geochemical characteristics of present samples with the Elan Bank basalts and Rajmahal (Group
II) mafic dyke samples, suggest minor contamination by assimilation with a small amount of upper crustal material. Chemistry,
particularly REE, hints at an alkaline basaltic nature of melt. Trace element modelling suggests that the melt responsible
for these mafic dykes had undergone extreme differentiation (∼ 50%) before its emplacement. The basaltic-andesite nature of
these rocks may be attributed to this differentiation. Chemistry of these rocks also indicates ∼ 10–15% melting of the mantle
source. The mafic dyke samples of the present investigation show very close geochemical similarities with the mafic rocks
derived from the Kerguelen mantle plume. Perhaps the Swangkre-Rongmil mafic dykes are also derived from the Kerguelen mantle
plume. 相似文献
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N. G. Direen B. E. Cohen R. Maas F. A. Frey J. M. Whittaker M. F. Coffin 《Australian Journal of Earth Sciences》2017,64(7):851-869
Volcanism associated with the Kerguelen Large Igneous Province is found scattered in southwestern Australia (the ca 136 to ca 130 Ma Bunbury Basalts, and ca 124 Ma Wallaby Plateau), India (ca 118 Ma Rajmahal Traps and Cona Basalts), and Tibet (the ca 132 Ma Comei Basalts), but apart from the ～70 000 km2 Wallaby Plateau, these examples are spatially and volumetrically minor. Here, we report dredge, geochronological and geochemical results from the ～90 000 km2 Naturaliste Plateau, located ～170 to ～500 km southwest of Australia. Dredged lavas and intrusive rocks range from mafic to felsic compositions, and prior geophysical analyses indicate these units comprise much of the plateau substrate. 40Ar/39Ar plagioclase ages from mafic units and U–Pb zircon ages from silicic rocks indicate magmatic emplacement from 130.6 ± 1.2 to 129.4 ± 1.3 Ma for mafic rocks and 131.8 ± 3.9 to 128.2 ± 2.3 Ma for silicic rocks (2σ). These Cretaceous Naturaliste magmas incorporated a significant component of continental crust, with relatively high 87Sr/86Sr (up to 0.78), high 207Pb/204 Pb ratios (15.5–15.6), low 143Nd/144Nd (0.511–0.512) and primitive-mantle normalised Th/Nb of 11.3 and La/Nb of 3.97. These geochemical results are consistent with the plateau being underlain by continental basement, as indicated by prior interpretations of seismic and gravity data, corroborated by dredging of Mesoproterozoic granites and gneisses on the southern plateau flank. The Cretaceous Naturaliste Plateau igneous rocks have signatures indicative of extraction from a depleted mantle, with trace-element and isotopic values that overlap with Kerguelen Plateau lavas reflect crustal contamination. Our chemical and geochronological results therefore show the Naturaliste Plateau contains evidence of an extensive igneous event representing some of the earliest voluminous Kerguelen hotspot magmas. Prior work reports that contemporaneous correlative volcanic sequences underlie the nearby Mentelle Basin, and the Enderby Basin and Princess Elizabeth Trough in the Antarctic. When combined, the igneous rocks in the Naturaliste, Mentelle, Wallaby, Enderby, Princess Elizabeth, Bunbury and Comei-Cona areas form a 136–124 Ma Large Igneous Province covering >244 000 km2. 相似文献
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西藏夏瓦地区位于特提斯喜马拉雅构造带中东部,广泛发育大量近东西向的基性脉岩,这些基性脉岩对于研究该区域地质构造演化具有重要的意义。本文对夏瓦地区基性脉岩开展了系统的岩石学、年代学、地球化学等研究。夏瓦基性脉岩的岩石类型以辉绿岩和辉长玢岩为主。锆石U-Pb年代学揭示基性脉岩结晶年龄为146～145 Ma。夏瓦基性脉岩具有低SiO2(47.20%～50.54%)和高Mg~#值(39.78～53.79)特征,富集Ti、Fe、P元素,属碱性系列,富集轻稀土元素(LREE),高场强元素(Nb、Ta、Zr、Hf、Th)相对富集,显示出似OIB的地球化学特征,指示夏瓦基性脉岩来源于软流圈地幔的部分熔融,形成于大陆边缘裂谷背景下。结合区域大火成岩省基性岩的发育,认为夏瓦地区基性脉岩是大陆边缘裂谷背景下Kerguelen地幔柱作用的产物。夏瓦基性脉岩的结晶年龄虽然早于Kerguelen地幔柱活动的峰期(132 Ma),但可能属于地幔柱峰期之前的小规模岩浆活动。 相似文献