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1.
Highly vesicular, microporphyritic basaltic rocks have been dredged from the slow-spreading Spiess Ridge segment of the Southwest Indian Ridge. All the samples recovered are hyalocrystalline with plagioclase, clinopyroxene and olivine as phenocryst and microphenocryst phases. Titanomagnetite occurs as euhedral microphenocrysts in some of the more evolved samples. In terms of bulk rock and quench glass chemistry the lavas are characterised by highly evolved compositions(e.g. FeO*=10.3−14.2%;TiO 2=2.0−3.4%;K 2O=0.50−1.1%;MgO=6.0−3.5%;Zr=160−274ppm;Nb=14−32ppm) and can be classified as ferrobasalts. Isotopic and incompatible element ratios of the lavas(e.g. 87Sr/ 86Sr=0.70325−0.70333;Zr/Nb=8.4−11.3;Y/Nb=2.3−1.4) indicate their strongly “enriched” nature (see also Dickey et al. [6]). Quantitative major and trace element modelling indicates that most of the compositional variations observed can be attributed to low-pressure fractional crystallisation of plagioclase, clinopyroxene and minor olivine and titanomagnetite. The range in composition can be accounted for by up to 65% fractional crystallisation. We suggest that the extreme differentiation of the Spiess Ridge lavas is related not to spreading rate, but to rate of magma supply. The basaltic melts appear to have evolved in a newly established zone of magmatic activity, associated with the most recent northward jump of the Bouvet triple junction, where they were effectively isolated from significant admixture of primitive magmas. 相似文献
2.
西南印度洋中脊(SWIR)增生的洋壳面积仅占印度洋的15%左右,但其具有比东南印度洋中脊和西北印度洋中脊更悠久而复杂的演化历史.基于已有的地质、地球物理和地球化学等资料,系统总结了SWIR的地质构造特征,并讨论了SWIR的演化过程、洋脊地幔的不均一性、洋脊周边海底高原成因等核心问题.SWIR地形中段高、东西两段低,空间重力异常基本与地形变化一致.按转换断层一级边界可将SWIR划分为20个一级段.SWIR的磁异常条带呈现两端渐进式分布和中段带状分布特征,对应洋脊的三期演化历史.SWIR的地幔源区极不均一,尤其是中新元古代造山带根部集中拆离的中段.源区地幔的不均一性与大陆裂解和洋脊演化过程密切相关.SWIR的东端与西北印度洋中脊和东南印度洋中脊的邻近洋脊段具有地球化学亲缘性,西端与大西洋中脊和南美洲—南极洲洋中脊的邻近洋脊段具有地球化学亲缘性,这与SWIR的渐近式扩张有关.SWIR周边海底高原普遍具有较大的地壳厚度,其成因除了陆壳基底之外,可能与热点火山作用、热点-洋脊相互作用或热点-三联点相互作用有关,目前尚未形成统一的认识.SWIR的形成演化及其作用域内的熔融异常(如海底高原)是冈瓦纳大陆裂解、残留岩石圈地幔、软流圈地幔和深部地幔热柱物质共同作用的结果.了解SWIR的演化过程对揭示冈瓦纳大陆的裂解过程和印度洋的演化具有重要意义. 相似文献
3.
西南印度洋中脊(SWIR)增生的洋壳面积仅占印度洋的15%左右,但其具有比东南印度洋中脊和西北印度洋中脊更悠久而复杂的演化历史.基于已有的地质、地球物理和地球化学等资料,系统总结了SWIR的地质构造特征,并讨论了SWIR的演化过程、洋脊地幔的不均一性、洋脊周边海底高原成因等核心问题.SWIR地形中段高、东西两段低,空间重力异常基本与地形变化一致.按转换断层一级边界可将SWIR划分为20个一级段.SWIR的磁异常条带呈现两端渐进式分布和中段带状分布特征,对应洋脊的三期演化历史.SWIR的地幔源区极不均一,尤其是中新元古代造山带根部集中拆离的中段.源区地幔的不均一性与大陆裂解和洋脊演化过程密切相关.SWIR的东端与西北印度洋中脊和东南印度洋中脊的邻近洋脊段具有地球化学亲缘性,西端与大西洋中脊和南美洲—南极洲洋中脊的邻近洋脊段具有地球化学亲缘性,这与SWIR的渐近式扩张有关.SWIR周边海底高原普遍具有较大的地壳厚度,其成因除了陆壳基底之外,可能与热点火山作用、热点-洋脊相互作用或热点-三联点相互作用有关,目前尚未形成统一的认识.SWIR的形成演化及其作用域内的熔融异常(如海底高原)是冈瓦纳大陆裂解、残留岩石圈地幔、软流圈地幔和深部地幔热柱物质共同作用的结果.了解SWIR的演化过程对揭示冈瓦纳大陆的裂解过程和印度洋的演化具有重要意义. 相似文献
4.
Li isotopic compositions of magmatic rocks have gained considerable attention recently as probes of mantle-scale processes. However, the concentrations and isotopic composition of Li in mantle minerals from mid-ocean ridges remain relatively unconstrained. This is largely because of the general presence of seawater alteration in abyssal peridotites. Lithium elemental and isotopic compositions for mineral separates of coexisting olivine, clinopyroxene, orthopyroxene and bulk rocks of serpentine-free Gakkel Ridge peridotites were investigated. Bulk rocks have Li contents of 1.6 to 2.7 ppm and δ 7Li values of 3 to 5‰, which fall within the range of reported normal pristine “MORB mantle” values. Lithium concentrations vary in the order cpx (2.1–4.7 ppm) > opx (0.9–1.7 ppm) ≥ olivine (0.4–0.9 ppm), the opposite found in “equilibrated” mantle peridotite xenoliths (Seitz and Woodland, 2000). The Li isotopic compositions indicate a systematic mineral variation with δ 7Li olivine (7.14‰–15.09‰) > δ 7Li opx (1.81‰–3.66‰) > δ 7Li cpx (?2.43‰ ? ?0.39‰). The δ 7Li values of cpx are negatively correlated with their Li concentrations with the lightest value for the most enriched cpx grains. There is a first order negative linear correlation between Δ olivine–cpx (δ 7Li olivine ? δ 7Li cpx) and ol/cpxD (Li olivine/Li cpx).Numerical simulations indicate that the observed systematic inter-mineral variations of Li concentrations and isotopic compositions could be explained by a cooling driven diffusive redistribution between minerals in a closed system if there is a temperature dependent partitioning of Li between olivine and clinopyroxene. The studied Gakkel Ridge abyssal peridotites may alternatively have cooled under a variable cooling rate with a rapid cooling before the Li system was closed, which is less likely given the tectonic setting. Our calculations confirm that Li systematics in minerals, especially in coexisting mineral phases could potentially be used as a mantle geospeedometer, even for slowly cooled mantle rocks. 相似文献
5.
西南印度洋洋中脊(SWIR)是超慢速扩张洋脊的代表,是海洋地学研究热点.本文从SWIR多波束水深数据、重、磁数据和地震结构等几方面,阐述了SWIR热液活动区(49°39′E)的综合地质地球物理特征.SWIR热液活动不仅与扩张速率有关,构造作用更是一个重要控制因素;热液活动区位于Indomed和Gallieni转换断层之间,从水深地形上看,该区段洋脊是SWIR上水深最浅的区域之一,水深与MBA存在良好的镜像关系,MBA和RMBA低值意味着较厚的地壳厚度与较高的地幔温度,洋脊段27地壳厚度大于9km,可能是受到Crozet热点的影响;磁条带数据表明,此区段洋脊南北两翼呈不对称扩张,形成南翼的浅离轴域比北翼宽;在洋脊段28发现的活动热液喷口刚好位于热液蚀变形成的低磁强区内,具有良好的硫化物资源.这些认识必将为在该区首次实施的三维地震探测研究的地质地球物理解释及活动热液喷口的动力学机制研究打下坚实基础. 相似文献
6.
The hydrothermal vent in Area A(37.78°S,49.65°E)is the first active hydrothermal vent discovered on the Southwest Indian Ridge(SWIR).Heat source and adequate bulk permeability are two necessary factors for the formation of a hydrothermal vent.Along the SWIR 49.3°E to 51.2°E,the gravity-derived crustal thickness is up to 9.0 km,much thicker than the average thickness of the global oceanic crust.This characteristic indicates that the magma supply in this area is robust,which is possibly affected by a hotspot.The large-scale residual mantle Bouguer anomalies(RMBA)reveal prominent negative-gravity anomalies between the first-order ridge segment(from Indomed to Gallieni,46.0°E to 52.0°E)and the Marion-Del Cano-Crozet region.These anomalies indicate the channel of the hotspot-ridge interaction.The tomography data corrected with theoretical thermal model indicate that the low-velocity anomalies corresponding to this channel can reach the base of the lithosphere.Near the hydrothermal vent area,the topography and crustal thickness at the off-axis area are extremely asymmetrical.South of the SWIR,the high topography corresponds to the thinning crustal thickness.The residual isostatic topography anomalies indicate that Area A is a deviation from the local isostatic equilibrium,similar to the characteristics of the transform fault inside corner.The forward profiles of the magnetic data indicate that the thinning magnetic layer at the south side of Area A corresponds to the shallow,high-velocity area revealed by the OBS,which is the result of tectonic extension of a detachment fault.The active tectonic processes in Area A can provide sufficient crustal permeability to the hydrothermal circulation and may form massive sulfide deposits. 相似文献
7.
超慢速扩张西南印度洋中脊岩浆的集中供给在空间维度上表现为岩浆扩张段(NVR)与相邻的非转换断层不连续带(NTD)地壳结构的差异,而在时间维度上表现为离轴与沿轴地壳结构的差异.为了进一步揭示岩浆集中供给的时空分布特征,本文选取西南印度洋中脊热液区2010年海底地震仪深部探测中平行于洋中脊距轴部偏北约10 km的离轴测线d0d10,使用射线追踪正演和反演的方法,得到了NVR和NTD北侧离轴区域的地壳及上地幔P波速度结构,并与轴部速度结构进行了对比分析.研究结果表明:(1)NTD北侧离轴区域的地壳厚度约5.2 km,其厚度明显大于轴部NTD下方地壳厚度(~3.2 km),由此推测洋脊轴部NTD区域形成的地壳在不断减薄;(2)NVR北侧离轴区域的地壳厚度约7.0 km,其厚度亦大于轴部NVR地壳厚度(~5.8 km),表明在洋中脊演化过程中洋脊轴区域的岩浆供给在不断减少,其活动性在不断减弱. 相似文献
9.
A statistical downscaling model is built for the late-winter rainfall over Southwest China(SWC).A partial-correlation method is used for selecting factors.The results show that the selected factors for late-winter rainfall in SWC are sea level pressure in Western Europe(SNAO)and sea surface temperature in Western Pacific(WPT).SNAO is related to the southern pole of North Atlantic Oscillation(NAO)and excites Southern Eurasian teleconnection,which influences the development of the southern branch trough and the water vapor transport to SWC.WPT indicates the variability of ENSO in the tropical Western Pacific.WPT excites Pacific-East Asia teleconnection and an anticyclone(cyclone)is formed in the southern part of China and suppresses(enhances)rainfall over SWC.A regression statistical downscaling model using SNAO and WPT shows good performance in fitting the variability of late-winter rainfall in the whole SWC region and every observation station,and the model also shows strong robustness in the independent validation.The statistical model can be used for downscaling output from seasonal forecast numerical models and improve the SWC late winter rainfall prediction in the future. 相似文献
10.
Floating pumice masses have been observed several times near the central part of Ninety-East Ridge, Indian Ocean, in 1879 and in 1883. The author is of the opinion that the source of these pumice masses cannot have been the volcano Krakatau in the Sunda Strait - as has been suggested earlier by other authors -, but that they originated in situ. Examples of floating pumice observed over submarine volcanoes lying at depths of 1500 m or even more, are mentioned briefly by other authors, suggesting that eruptions in deep water may indeed lead to its formation.The suspected submarine volcano may coincide with an elongated but rather flat elevation a little north of the central part of Ninety-East Ridge. The center of this elevation is located approximately at 6° 05′ S, 89° 10 E. A roughly NW-SE profile of this area shows the presence of this shield-like feature very near the site where floating pumice was seen on three occasions. The inferred submarine volcanic center is lying within the belt of heat flow anomaly (called belt I). A particular “hot spot” with an unusually high heat flow value (7.61 HFU) is located southeast of the feature in question. The heat flow values measured in belt I are well above the world-average. This belt may be the result of the presence of linearly arranged magma chambers at a relatively shallow depth, probably within the oceanic crust. This belt has a length of 1100 km.The seismic activity of the area investigated is similarly remarkable. Numerous powerful shocks have taken place there. Belt I represents the axis of the seismic zone, extending from points b to c (Fig. 5). The seismic belt is regarded as a zone of weakness within which fractures can originate, permitting rock-melts to emerge from depth towards the surface. 相似文献
11.
The three-dimensional (3D) geoelectric model of the Kuznetsk-Alatau folded area is reconstructed by magnetotelluric inversion using 3D fitting. It is established that the zones of ore mineralization within the Batenevsky massif are confined to the subvertical faults characterized by the electric resistivity of 100–300 Ω m. Blocks with ρ ≈ 10−100 Ω m are identified at a depth below 10 km in the western part of the model. The blocks are located close to the areas marked by the increased thermal flux, reduced seismic velocities, and elevated Moho boundary. This is probably associated with the presence of the rift zone in this area. 相似文献
12.
The three-dimensional (3D) geoelectric model of the Kuznetsk-Alatau folded area is reconstructed by magnetotelluric inversion using 3D fitting. It is established that the zones of ore mineralization within the Batenevsky massif are confined to the subvertical faults characterized by the electric resistivity of 100?C300 ?? m. Blocks with ?? ?? 10?100 ?? m are identified at a depth below 10 km in the western part of the model. The blocks are located close to the areas marked by the increased thermal flux, reduced seismic velocities, and elevated Moho boundary. This is probably associated with the presence of the rift zone in this area. 相似文献
13.
New analyses of He, Ne, Ar and CO 2 trapped in basaltic glasses from the Southeast Indian Ridge (Amsterdam-St. Paul (ASP) region) show that ridge magmas degas by a Rayleigh distillation process. As a result, the absolute and relative noble gas abundances are highly fractionated with 4He/ 40Ar* ratios as high as 620 compared to a production ratio of ∼3 (where 40Ar* is 40Ar corrected for atmospheric contamination). There is a good correlation between 4He/ 40Ar* and the MgO content of the basalt, suggesting that the amount of gas lost from a particular magma is related to the degree of crystallization. Fractional crystallization forces oversaturation of CO 2 because CO 2 is an incompatible element. Therefore, crystallization will increase the fraction of gas lost from the magma. The He-Ar-CO 2-MgO-TiO 2 compositions of the ASP basalts are modeled as a combined fractional crystallization-fractional degassing process using experimentally determined noble gas and CO 2 solubilities and partition coefficients at reasonable magmatic pressures (2-4 kbar). The combined fractional crystallization-degassing model reproduces the basalt compositions well, although it is not possible to rule out depth of eruption as a potential additional control on the extent of degassing. The extent of degassing determines the relative noble gas abundances ( 4He/ 40Ar*) and the 40Ar*/CO 2 ratio but it cannot account for large (>factor 50) variations in He/CO 2, due to the similar solubilities of He and CO 2 in basaltic magmas. Instead, variations in CO 2/ 3He (≡C/ 3He) trapped in the vesicles must reflect similar variations in the primary magma. The controls on C/ 3He in mid-ocean ridge basalts (MORBs) are not known. There are no obvious correlated variations between C/ 3He and tracers of mantle heterogeneity ( 3He/ 4He, K/Ti etc.), implying that the variations in C/ 3He are not likely to be a feature of the mantle source to these basalts. Mixing between MORB-like sources and more enriched, high 3He/ 4He sources occurs on and near the ASP plateau, resulting in variable 3He/ 4He and K/Ti compositions (and many other tracers). Using 4He/ 40Ar* to track degassing, we demonstrate that mixing systematics involving He isotopes are determined in large part by the extent of degassing. Relatively undegassed lavas (with low 4He/ 40Ar*) are characterized by steep 3He/ 4He-K/Ti mixing curves, with high He/Ti ratios in the enriched magma (relative to He/Ti in the MORB magma). Degassed samples (high 4He/ 40Ar*) on the other hand have roughly equal He/Ti ratios in both end-members, resulting in linear mixing trajectories involving He isotopes. Some degassing of ASP magmas must occur at depth, prior to magma mixing. As a result of degassing prior to mixing, mixing systematics of oceanic basalts that involve noble gas-lithophile pairs (e.g. 3He/ 4He vs. 87Sr/ 86Sr or 40Ar/ 36Ar vs. 206Pb/ 204Pb) are unlikely to reflect the noble gas composition of the mantle source to the basalts. Instead, the mixing curve will reflect the extent of gas loss from the magmas, which is in turn buffered by the pressure of combined crystallization-degassing and the initial CO 2 content. 相似文献
14.
The cooling history and therefore thermal structure of oceanic lithosphere in slow-spreading environments is, to date, poorly constrained. Application of thermochronometric techniques to rocks from the very slow spreading SW Indian Ridge provide for the first time a direct measure of the age and thermal history of in situ lower oceanic crust. Crystallization of felsic veins (∼850°C) drilled in Hole 735B is estimated at 11.93±0.14 Ma, based on U-Pb analyses of zircon by ion probe. This crystallization age is older than the ‘crustal age’ from remanence inferred from both sea surface and near-bottom magnetic anomaly data gathered over Hole 735B which indicate magnetization between major normal polarity chrons C5n.2n and C5An.1n (10.949-11.935 Ma). 40Ar/ 39Ar analyses of biotite give plateau ages between 11 and 12 Ma (mean 11.42±0.21 Ma), implying cooling rates of >800°C/m.y. over the first 500,00 years to temperatures below ∼330-400°C. Fission-track ages on zircon (mean 9.35±1.2 Ma) and apatite reveal less rapid cooling to <110°C by ∼7 Ma, some 4-5 m.y. off axis.Comprehensive thermochronometric data from the structurally intact block of gabbro between ∼700 and 1100 m below sea floor suggest that crust traversed by ODP Hole 735B mimics conductive cooling over the temperature range ∼900-330°C, characteristic of a 2-D plate-cooling model for oceanic lithosphere. In contrast, lower temperature chronometers (fission track on zircon, titanite, and apatite; T≤280°C) are not consistent with these predictions and record anomalously high temperatures for crust >700 m below sea floor at 8-10 Ma (i.e. 2-4 m.y. off axis). We offer two hypotheses for this thermal anomaly: - (i)
- Off-axis (or asymmetric) magmatism that caused anomalous reheating of the crust preserved in Hole 735B. This postulated magmatic event might be a consequence of the transtension, which affected the Atlantis II transform from ∼19.5 to 7.5 Ma.
- (ii)
- Late detachment faulting, which led to significant crustal denudation (2.5-3 km removed), further from the ridge axis than conventionally thought.
相似文献
15.
We investigate the role of crustal architectures playing in controlling the genesis and nature of supergiant Jiaodong orogenic gold system via seismic imaging. We deployed an NWW-SEE-trending broadband seismic linear array traversing the province to construct a shear wave velocity(V S) transect based on ambient noise tomography. Our crustal V S transect reveals a prominent low velocity zone(LVZ) at 12–20 km depths across the whole province and high V S anomalies at ... 相似文献
16.
在海上实施三维地震探测过程中,人工震源枪阵中心与船上GPS的距离及地震探测作业中的船行方向造成炮点实际位置与预设位置有一定偏差;自由落体投放的OBS由于海流的影响会偏离原定设计位置(投放点),因此,炮点与海底地震仪(OBS)的位置校正是三维地震结构研究中的基本环节.本文利用艏向信息校正了炮点位置;采用蒙特卡洛和最小二乘法方法对海底地震仪的位置进行了校正,并探讨了直达水波曲线特征.结果表明 OBS位置一般偏离设计点1 km左右,其误差范围在20 m以内,校正后的OBS记录剖面展示了真实的记录情况.该研究结果为下一步西南印度洋的三维层析成像研究提供了坚实数据基础,同时为今后南海的三维深部地壳结构探测提供经验与借鉴. 相似文献
17.
利用西南印度洋脊中段Indomed-Gallieni洋段49—51°E区段全覆盖高分辨率多波束水深地形资料,应用构造地貌学分析方法,结合区域地形及其他地球物理等资料,在分段分析49—51°E区段岩浆-构造动力学模式的基础上,进一步探讨了约10 Ma以来Indomed-Gallieni洋段的演化史.28、29洋段目前岩浆供应不足,在轴部不对称深断层的控制之下不对称扩张,属于超慢速扩张洋脊较常见的演化方式.轴部火山建造主要向北翼增生,发育与火山脊相关的火山地貌;南翼构造拉张作用强烈,地貌上可观察到大量断块,拆离断层可能大量存在.而27洋段水深浅、火山密集、轴部缺失裂谷,超慢速扩张下却具有较高的岩浆通量.Indomed-Gallieni洋段地形高地建造于一次岩浆增强事件,但应该不是因为Crozet热点的影响.27洋段为目前仍受该岩浆增强事件影响的唯一区段,但其强度和规模也在逐渐减小;包括28、29洋段在内的Indomed-Gallieni段其他部分,已重新恢复到岩浆供应不足的正常超慢速扩张洋脊演化模式.28、29洋段和27洋段岩浆供应均存在岩浆通量由多至少的周期,周期内岩浆供应较多时期轴部建脊,减少时期轴部火山建造裂离.但27洋段由于仍受岩浆增强事件的影响,与28、29洋段表现形式不同,主要表现为火山建造裂离方式、岩浆供应周期长短以及构造活动强烈程度的不同. 相似文献
18.
The orbital and interior climatic cycles can be found both in the Bengal Deep Sea Fan and Ninetyeast Ridge, North Indian Ocean. The periodicity of the Quaternary glacio-eustacy by 100 ka gave a strong impact on the sedimentation in the fan area and the monsoon signals controlled by the obliquity and precession were easily picked up. This paper discusses the possible correlation between the environmental elements on the basis of the ETP phase wheels. A rapid change with short-periods develops during the past 60 ka in the region under study as well. The variability of paleoproductivity has a nonlinear response to the Indian summer monsoon. As contrasted to the Northwest Indian Ocean, here an abundance of Globigerina bulloides, a proxy to indicate upwelling current, does not imply so much a promotion of the summer monsoon as its decrease. The record from the ridge area shows in a longer-scale a climatic evolutionary feature corresponding to that of the fan area. A special and great event arising at around 165 kaBP and meaning a catastrophe for ecological environment is reported in this paper. It is also regarded as a result induced by the monsoon. 相似文献
19.
通过对大量地质及物化探资料分析,运用地质学、矿床学以及地球物理勘探学的基本理论和方法,分析了金平白马寨铜镍矿床的地质与地球物理特征,从而提出了本区铜镍矿床为岩浆熔离脉冲贯入型矿床和“三高一低”,即重力高、磁力高、极化率高、电阻率低的地球物理成矿预测模式. 相似文献
20.
南岭东段地区是世界著名的钨多金属矿集区。近年来中国地质调查局在该区实施钨多金属矿调查评价,找到了一批具有中大型远景的与花岗岩有关的新类型钨锡多金属矿床,特别是在复式岩体中的花岗岩型新类型钨钼多金属矿床(简称"体中体"式钨钼多金属矿床)和受中、上泥盆统地层控制的层控矽卡岩型钨锡多金属矿床的发现,不仅实现了南岭东段地区找矿新突破,而且对整个南岭乃至我国华南地区钨锡多金属矿找矿具有重要指导作用。本文根据近年来南岭东段地区钨锡多金属矿找矿新成果,通过该区钨锡多金属矿床地质特征的总结,建立了南岭东段地区钨锡多金属矿成矿模式,提出了南岭东段地区今后钨锡多金属矿找矿方向应重点注意与燕山期复式花岗岩有关的"体中体"式钨钼多金属矿床和中、上泥盆统控制的层控矽卡岩型钨锡多金属矿床的寻找。 相似文献
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