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1.
Abstract Basaltic basement has been recovered by deep-sea drilling at seven sites on the linear Ninetyeast Ridge in the eastern Indian Ocean. Studies of the recovered lavas show that this ridge formed from ~ 82 to 38 Ma as a series of subaerial volcanoes that were created by the northward migration of the Indian Plate over a fixed magma source in the mantle. The Sr, Nd and Pb isotopic ratios of lavas from the Ninetyeast Ridge range widely, but they largely overlap with those of lavas from the Kerguelen Archipelago, thereby confirming previous inferences that the Kerguelen plume was an important magma source for the Ninetyeast Ridge. Particularly important are the ~ 81 Ma Ninetyeast Ridge lavas from DSDP Site 216 which has an anomalous subsidence history (Coffin 1992). These lavas are FeTi-rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic, Upper Miocene lavas in the Kerguelen Archipelago. The isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the Kerguelen plume (Weis et al. 1993a,b). Despite the overlap in isotopic ratios, there are important compositional differences between lavas erupted on the Ninetyeast Ridge and in the Kerguelen Archipelago. The Ninetyeast Ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios, such as La/Yb and Zr/Nb, which are intermediate between those of Indian Ocean MORB (mid-ocean ridge basalt) and the transitional to alkalic basalts erupted in the Kerguelen Archipelago. These compositional differences reflect a much larger extent of melting for the Ninetyeast Ridge lavas, and the proximity of the plume to a spreading ridge axis. This tectonic setting contrasts with that of the recent alkalic lavas in the Kerguelen Archipelago which formed beneath the thick lithosphere of the Kerguelen Plateau. From ~ 82 to 38 Ma there was no simple, systematic temporal variation of Sr, Nd and Pb isotopic ratios in Ninetyeast Ridge lavas. Therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume. Although Class et al. (1993) propose that some of the isotopic variations reflect such aging, we infer that most of the isotopic heterogeneity in lavas from the Ninetyeast Ridge and Kerguelen Archipelago can be explained by mixing of the Kerguelen plume with a depleted MORB-like mantle component. However, with this interpretation some of the youngest, 42–44 Ma, lavas from the southern Ninetyeast Ridge which have206pb/204Pb ratios exceeding those in Indian Ocean MORB and Kerguelen Archipelago lavas require a component with higher206Pb/204Pb, such as that expressed in lavas from St. Paul Island.  相似文献   

2.
The study of magmatism and tectonic structure of the East Indian or Ninetyeast Ridge (NER) reveals the geochemical similarity of mantle sources for the NER and Kerguelen Plateau melts. Magmas related to the Kerguelen plume were derived from an enriched mantle source, whereas the NER tholeiitic basalts originated from a source contaminated by a depleted material. While, depleted basalt varieties were not found within the NER basalts. It was shown that magmatic rocks forming the NER were generated by high degrees (30%) of partial melting within the ancient Wharton spreading ridge due to the activity of the Kerguelen plume, which was located at this time in the vicinity of the ridge. The most significant impact of the plume on the NER structures was recorded at 70–50 Ma ago.  相似文献   

3.
Using recently acquired marine magnetic data and existing magnetic and bathymetric data sets together with ODP Leg 170 age determinations we present a revised plate tectonic model for the southern Cocos and northern Nazca plate area. According to this model the formation of the southern Cocos plate was governed by spreading at different ridge axes with alternations between spreading ridges producing a complex magnetic anomaly pattern. In the Cocos and Malpelo ridge area we have identified two precursors of the recently active Cocos–Nacza spreading system which were active from 22.8 to 14.7 Ma, with a change in spreading direction from NW–SE to ENE–WSW at 19.5 Ma. The oceanic crust of these abandoned spreading systems was subsequently thickened and overprinted by hotspot volcanism that formed the Cocos and Malpelo ridges. The centre of this hotspot volcanism is about 500 km away from, but most probably related to, the Galapagos hotspot.  相似文献   

4.
Lavas from several major bathymetric highs in the eastern Indian Ocean that are likely to have formed as Early to Middle Cretaceous manifestations of the Kerguelen hotspot are predominantly tholeiitic; so too are glass shards from Eocene to Paleocene volcanic ash layers on Broken Ridge, which are believed to have come from eruptions on the Ninetyeast Ridge. The early dominance of tholeiitic compositions contrasts with the more recent intraplate, alkalic volcanism of the Kerguelen Archipelago. Isotopic and incompatible-element ratios of the plateau lavas are distinct from those of Indian mid-ocean ridge basalts; their Nd, Sr, 207Pb/204Pb and isotopic ratios overlap with but cover a much wider range than measured for more recent oceanic products of the Kerguelen hotspot (including the Ninetyeast Ridge) or, indeed, oceanic lavas from any other hotspot in the world. Samples from the Naturaliste Plateau and ODP Site 738 on the southern tip of the Kerguelen Plateau are particularly noteworthy, with εNd(T) = − 13 to −7, (87Sr/86Sr)T=0.7090 to 0.7130 and high 207Pb/204Pb relative to 206Pb/204Pb. In addition, the low-εNd(T) Naturaliste Plateau samples are elevated in SiO2 (> 54 wt%). In contrast to “DUPAL” oceanic islands such as the Kerguelen Archipelago, Pitcairn and Tristan da Cunha, the plateau lavas with extreme isotopic characteristics also have relative depletions in Nb and Ta (e.g., Th/Ta, La Nb > primitive mantle values); the lowest εNd(T) and highest Th/Ta and La Nb values occur at sites located closest to rifted continental margins. Accepting a Kerguelen plume origin for the plateau lavas, these characteristics probably reflect the shallow-level incorporation of continental lithosphere in either the head of the early Kerguelen plume or in plume-derived magmas, and suggest that the influence of such material diminished after the period of plateau construction. Contamination of asthenosphere with the type of material affecting Naturaliste Plateau and Site 738 magmatism appears unlikely to be the cause of low-206Pb/04Pb Indian mid-ocean ridge basalts. Finally, because isotopic data for the plateaus do not cluster or form converging arrays in isotope-ratio plots, they provide no evidence for either a quickly evolving, positive εNd, relatively high-206Pb/204Pb plume composition, or a plume source dominated by mantle with εNd of −3 to 0.  相似文献   

5.
跳出南海看南海——新特提斯洋闭合与南海的形成演化   总被引：6，自引：5，他引：1  
本文总结了笔者参与基金委重大研究计划"南海深海过程演变"的研究成果。我们发现南海和青藏高原都是新特提斯洋闭合的产物,而非前人所说的南海是由青藏高原碰撞导致的中南半岛逃逸所形成。与青藏高原碰撞隆升机制不同,南海是新特提斯闭合后期弧后拉张的结果。新特提斯洋位于北边的欧亚大陆与南面的非洲、印度和澳大利亚板块之间,呈东宽西窄的喇叭型。在西部,新特提斯洋向北的俯冲可能在侏罗纪就开始了,局部形成了弧后盆。约130Ma前,由于凯尔盖朗等大火成岩省的喷发,新特提斯洋脊也开始向北漂移。由于新特提斯洋东部宽度较大,弧后拉张明显,形成了古南海。新特提斯洋闭合过程中一个重大事件是洋脊俯冲:从菲律宾经福建及两广到青藏高原,均有100Ma左右的埃达克岩产出,是洋脊俯冲的产物。其中,菲律宾、福建、广东埃达克岩形成了斑岩铜金矿床;而在青藏高原,埃达克岩虽有矿化,但没有形成大规模的斑岩铜金矿床。同时期,华南出现了一次短暂的大规模挤压事件,与洋脊俯冲契合。这次挤压事件可能导致了古南海闭合的开始。与此同时,青藏高原冈底斯出现高温岩石——埃达克质紫苏花岗岩;其北面有～110Ma短时间内发生的大规模花岗岩事件。考虑到板块重建的结果,这些埃达克岩和华南短时间挤压事件的时空分布显示新特提斯洋脊在约100～110Ma,近似平行于俯冲带俯冲到了欧亚大陆之下;其前片下沉,扰动软流圈,形成大规模岩浆活动;后片则缓慢后撤,于～80Ma形成了A-型花岗岩。这些A-型花岗岩多属于A2型,受到了还原性板块俯冲的影响而普遍含锡,形成了全球60%的锡矿。俯冲板片的后撤,导致了拉张,可以合理解释南海北缘的"神狐运动"。随着俯冲板片后撤,俯冲角度加大,形成新的弧后拉张,于～33Ma出现洋壳,形成了南海。青藏高原碰撞引起的物质向东、南、北等各方向逃逸,对东亚大陆的构造格局也产生了重要的影响,但是并非南海拉张的主要控制因素。到～23Ma时,东经九十度海岭的俯冲阻挡了青藏高原下方地幔物质向东南方向逃逸,改变了东亚构造格局。同时,由于该海岭俯冲产生的向北东方向的挤压,造成印支半岛向西南挠曲,导致南海洋脊产生向南的跃迁。  相似文献   

6.
New results on the petrochemistry and geochemistry of dolerites from the Schirmacher Oasis shed light on the development of the Karoo-Maud plume in Antarctica. The basalts and dolerites are petrologically identical to the rocks of western Dronning Maud Land (DML), which were previously studied and interpreted as a manifestation of the Karoo-Maud plume in Antarctica. The spatial distribution of the dikes suggests eastward spreading of the plume material, up to the Schirmacher Oasis for at least 10 Ma. The geochemical characteristics of magmas from the Schirmacher Oasis reflect the influence of crustal contamination, which accompanied both the ascent and spreading of the plume. The magmas of the initial stage of plume activity (western DML) appeared to be the most contaminated in crustal components.It was found that the geochemical characteristics of Mesozoic magmas from the Schirmacher Oasis are identical to those of enriched tholeiites from the Afanasy Nikitin Rise and the central Kerguelen Plateau (Hole 749), which indicates that their enrichment was related to the ancient material of the Gondwana continent. This was caused by the opening of the Indian Ocean under the influence of the Karoo-Maud plume. This process was peculiar in that it occurred in the presence of nonspreading blocks of varying thickness, for instance, Elan Bank in the central Kerguelen Plateau, and was accompanied by the formation of intraplate volcanic rises, which are documented in the seafloor relief of basins around Antarctica. The geochemical characteristics of igneous rocks from the resulting rises (Afanasy Nikitin, Kerguelen, Naturaliste, and Ninetyeast Ridge) indicate the influence of processes related to crustal assimilation. The magmatism that occurred 40 Ma after the main phase of the Karoo-Maud volcanism at the margins of the adjacent continents of Australia (Bunbury basalts) and India (Rajmahal trapps) could be generated by the Karoo-Maud plume flowing along the developing spreading zone. The plume moved subsequently and was localized at the Kerguelen Plateau, where it occurs at present as an active hotspot.  相似文献   

7.
西北印度洋的洋脊系统目前以"中印度洋脊"和"卡尔斯伯格脊"分别指示南北两段,两者的分界点被认为是澳大利亚板块与印度板块的板块边界与洋脊的交点,但具体分布位置不明确.基于已有的地质、地球物理和地球化学等多方面特征,认为卡尔斯伯格脊和中印度洋脊可以统一称为"西北印度洋脊",从罗德里格斯三联点一直延伸到欧文断裂带.新的洋脊厘定将有助于更全面地了解整个西北印度洋的洋脊演化和地球动力学过程.西北印度洋脊地形上南北两端断裂较少,中间断层密集,形似吸管的弯折部位,调节洋脊的转向.重力异常显示沿脊轴方向两端高中间低的特征,表明两端岩浆供给相对充足,而中间断层密集区岩浆量少.磁异常特征显示清晰的分带性,指示多阶段的洋脊扩张历史.岩石地球化学特征显示南北两个同位素相对富集洋脊段,可能与热点作用相关,或与残留岩石圈或地壳物质对亏损软流圈地幔的富集改造有关.  相似文献   

8.
Approximately 160 Ma old basaltic lavas obtained from ODP Site 801 in the Pigafetta Basin represent the first Jurassic oceanic crust recovered in the Pacific Ocean and the oldest in situ oceanic crust discovered anywhere. The basement consists of an upper alkali olivine basalt sequence and a lower tholeiitic sequence separated by a yellow Fe-rich hydrothermal sedimentary deposit. The aphyric and sparsely plagiodase-olivine±spinel phyric tholeiites exhibit depleted, open–system fractionated characteristics with trace element abundances and Pb–Nd isotopic compositions similar to normal mid-ocean ridge basalts (N-MORB). The aphyric alkali basalts, although showing some overlap in isotopic composition with MORB, exhibit strong similarities in terms of incompatible element abundances to ocean island basalts (OIB). They could represent either OIB-type off-axis volcanism or an alkalic event possibly associated with the waning stages of spreading axis volcanism in the Pigafetta Basin. All lavas have undergone low-grade anoxic smectite–carbonate alteration, although flows underlying the Fe-rich sediments have suffered hydrothermal alteration and fracturing.  相似文献   

9.
The nature and origin of the subsurface 85°E Ridge in the Bay of Bengal has remained enigmatic till date despite several theories proposed by earlier researchers. We reinterpreted the recently acquired high quality multichannel seismic reflection data over the northern segment of the ridge that traverses through the Mahanadi offshore, Eastern Continental Margin of India and mapped the ridge boundary and its northward continuity. The ridge is characterized by complex topography, multilayer composition, intrusive bodies and discrete nature of underlying crust. The ridge is associated with large amplitude negative magnetic and gravity anomalies. The negative gravity response across the ridge is probably due to emplacement of relatively low density material as well as ∼2–3 km flexure of the Moho. The observed broad shelf margin basin gravity anomaly in the northern Mahanadi offshore is due to the amalgamation of the 85°E Ridge material with that of continental and oceanic crust. The negative magnetic anomaly signature over the ridge indicates its evolution in the southern hemisphere when the Earth’s magnetic field was normally polarized. The presence of ∼5 s TWT thick sediments over the acoustic basement west of the ridge indicates that the underlying crust is relatively old, Early Cretaceous age.The present study indicates that the probable palaeo-location of Elan Bank is not between the Krishna–Godavari and Mahanadi offshores, but north of Mahanadi. Further, the study suggests that the northern segment of the 85°E Ridge may have emplaced along a pseudo fault during the Mid Cretaceous due to Kerguelen mantle plume activity. The shallow basement east of the ridge may have formed due to the later movement of the microcontinents Elan Bank and Southern Kerguelen Plateau along with the Antarctica plate.  相似文献   

10.
The Perth Abyssal Plain (PAP), located offshore southwest Australia, formed at the centre of Mesozoic East Gondwana breakup and Kerguelen plume activity. Despite its importance as a direct and relatively undisturbed recorder of this early spreading history, sparse geophysical data sets and lack of geological sampling hamper our understanding of the evolution of the PAP. This study combines new bathymetric profiles across the PAP with petrographic and geochemical data from the first samples ever to be dredged from the flanks of the Dirck Hartog Ridge (DHR), a prominent linear bathymetric feature in the central PAP, to better constrain the formation of the early Indian Ocean floor and the influence of the Kerguelen plume. Seafloor spreading in the PAP initiated at ~ 136 Ma with spreading observed to occur at (half) rates of ~ 35 mm/yr. Changes in spreading rate are difficult to discern after the onset of the Cretaceous Quiet Zone at ~ 120 Ma, but an increase in seafloor roughness towards the centre of the PAP likely resulted from a half-spreading rate decrease from 35 mm/yr (based on magnetic reversals) to ~ 24 mm/yr at ~ 114 Ma. Exhumed gabbro dredged from the southernmost dredge site of the DHR supports a further slowdown of spreading immediately prior to full cessation at ~ 102 Ma. The DHR exhibits a high relief ridge axis and distinctive asymmetry that is unusual compared to most active or extinct spreading centres. The composition of mafic volcanic samples varies along the DHR, from sub-alkaline dolerites with incompatible element concentrations most similar to depleted-to-normal mid-ocean ridge basalts in the south, to alkali basalts similar to ocean island basalts in the north. Therefore, magma sources and degrees of partial melting varied in space and time. It is likely that the alkali basalts are a manifestation of later excess volcanism, subsequent to or during the cessation of spreading. In this case, enriched signatures may be attributed tectonic drivers and melting of a heterogeneous mantle, or to an episodic influence of the Kerguelen plume over distances greater than 1000 km. We also investigate possible scenarios to explain how lower crustal rocks were emplaced at the crest of the southern DHR. Our results demonstrate the significance of regional tectonic plate motions on the formation and deformation of young ocean crust, and provide insight into the unique DHR morphology.  相似文献   

11.
The results of analysis of the anomalous magnetic field of the Reykjanes Ridge and the adjacent basins are presented, including a new series of detailed reconstructions for magnetic anomalies 1–6 in combination with a summary of the previous geological and geophysical investigations. We furnish evidence for three stages of evolution of the Reykjanes Ridge, each characterized by a special regime of crustal accretion related to the effect of the Iceland hotspot. The time interval of each stage and the causes of the variation in the accretion regime are considered. During the first, Eocene stage (54–40 Ma) and the third, Miocene-Holocene stage (24 Ma-present time at the northern Reykjanes Ridge north of 59° N and 17–11 Ma-present time at the southern Reykjanes Ridge south of 59° N), the spreading axis of the Reykjanes Ridge resembled the present-day configuration, without segmentation, with oblique orientation relative to the direction of ocean floor opening (at the third stage), and directed toward the hotspot. These attributes are consistent with a model that assumes asthenospheric flow from the hotspot toward the ridge axis. Decompression beneath the spreading axis facilitates this flow. Thus, the crustal accretion during the first and the third stages was markedly affected by interaction of the spreading axis with the hotspot. During the second, late Eocene-Oligocene to early Miocene stage (40–24 Ma at the northern Reykjanes Ridge and 40 to 17–11 Ma at the southern Reykjanes Ridge), the ridge axis was broken by numerous transform fracture zones and nontransform offsets into segments 30–80 km long, which were oriented orthogonal to the direction of ocean floor opening, as is typical of many slow-spreading ridges. The plate-tectonic reconstructions of the oceanic floor accommodating magnetic anomalies of the second stage testify to recurrent rearrangements of the ridge axis geometry related to changing kinematics of the adjacent plates. The obvious contrast in the mode of crustal accretion during the second stage in comparison with the first and the third stages is interpreted as evidence for the decreasing effect of the Iceland hotspot on the Reykjanes Ridge, or the complete cessation of this effect. The detailed geochronology of magnetic anomalies 1–6 (from 20 Ma to present) has allowed us to depict with a high accuracy the isochrons of the oceanic bottom spaced at 1 Ma. The variable effect of the hotspot on the accretion of oceanic crust along the axes of the Reykjanes Ridge and the Kolbeinsey and Mid-Atlantic ridges adjoining the former in the north and the south was estimated from the changing obliquity of spreading. The spreading rate tends to increase with reinforcing of the effect of the Iceland hotspot on the Reykjanes Ridge.  相似文献   

12.
东经90°海岭的远洋沉积记录与晚新生代重大构造-环境事件   总被引：11，自引：0，他引：11  
东经 90°海岭的北部远洋型沉积记录是青藏高原隆升的远程监视器。文章应用有孔虫组合分析和壳体微量元素分析 ,结合相关的沉积学、地球化学、古地磁学和微体古生物学工作 ,对以ODP75 8钻孔为代表的 9Ma以来的沉积记录综合研讨 ,识别出对应于高原阶段隆升 (3 7～ 3 2Ma、0 .8～ 0 .6Ma、0 .17～ 0 .16Ma)的重大环境变化信号。文章还提出印度冬季季风强度是反映高原构造活动关键标志的观点。按照这种观点 ,海岭远洋记录提供的晚新生代最大环境转折时段位于 3.7～ 3.2Ma间。  相似文献   

13.
《International Geology Review》2012,54(14):1691-1719
This study investigates the formation of lower oceanic crust and geochemical variations of basalts along the Central Indian Ridge (CIR, lat. 7°45′–17°10′ S). Harzburgites, various gabbroic cumulates, medium- to fine-grained oxide gabbros, diabases, and pillow basalts were recovered by dredging from segment ends such as ridge-transform intersections (RTIs), non-transform discontinuities (NTDs), and transform offset areas. The occurrence of both harzburgites and gabbroic rocks with minor basalts at all segments ends, and leucogabbro intrusive into harzburgite at the 12°45′ S NTD indicates that oceanic crust at segment ends exposes mantle-derived harzburgites and gabbroic intrusions with a thin basaltic cover due to sparse magmatic activity. Basalts collected along the entire ridge show wide compositional variations between N (normal)- and E (enriched)-mid-ocean ridge basalt (MORB). T (transitional)-MORBs with enriched affinities are more prominent than N-MORBs. There is no tendency of enrichment towards specific directions. (La/Sm)N variations in MORB along the CIR (8°–21°S) fluctuates at a regional scale with local high positive anomalies reflecting compositional heterogeneity of the sub-CIR mantle domain.  相似文献   

14.
The formation and evolution of the ～600 km long arcuate Amirante Ridge and Trench Complex (ARTC) is a significant geomorphic–structural feature in the Western Indian Ocean (WIO). The WIO contains evidence of at least two major magmatic episodes followed by continental rifting within the span of a little more than 20 million years. This involved the splitting of Madagascar from India at around 85 Ma and then separation between India and the Seychelles at 64–63 Ma as a possible consequence of two powerful volcanic eruptions from the Marion and Reunion hot spots, respectively. Formation and evolution of the ARTC represents this tumultuous period in the Indian Ocean, approximately between 85 and 60 Ma (Late Cretaceous–Early Tertiary).

We integrated geophysical, palaeomagnetical, and petrological data to examine three existing models that attempt to explain the formation of ARTC. In contrast, our study hints at several stages of extension and compression responsible for its formation. Our integrated data also suggest that the Carlsberg Ridge may have played a prominent role in the evolution of the ARTC that seems to have formed through a ridge-jump process after the conjugate spreading centres – Mascarene and Palitana ridges formed earlier during the India–Madagascar separation – ceased spreading because of violent eruption of the Reunion hot spot at around 65 Ma. The eruption disturbed the plumbing system of magma ascent, resulting in cessation of spreading along the conjugate spreading centres, forcing a ridge jump.

A collage of seismic refraction and reflection, free-air gravity, magnetic anomaly data, and Ar dating of rocks indicates that as the Carlsberg Ridge swept the Seychelles towards south, the crust between Madagascar and the Seychelles was increasingly compressed, with the abandoned northern Mascarene spreading centre absorbing the maximum stress. With continued compression, the western limb of the abandoned spreading ridge was thrust below the eastern limb to a limited degree. This partial subduction agrees with the gravity and seismic results. Our new study also accounts for the anomalous presence of 14 km-thick oceanic crust beneath the ARTC and its characteristic difference in petrology with other established subduction zones in the world.  相似文献   

15.
詹美珍  孙卫东  凌明星  李贺 《岩石学报》2015,31(7):2101-2114
菲律宾吕宋岛上约5Ma以来的斑岩铜金矿床主要集中在北部的Baguio和Mankayan地区,它们在时空上与黄岩海山链密切相关。1907~2013年间的地震数据表明,在吕宋岛中部(16°N)附近存在地震稀疏带。吕宋岛上的斑岩铜金矿床分布在该地震稀疏带的两侧。收集到的相应时期埃达克岩的Sr/Y-(La/Yb)N、Sr/Y-Y和La/Yb-Yb图解表明,这些埃达克岩几乎都是洋壳部分熔融形成的。与吕宋岛北部侵入型埃达克岩相比,位于16°N附近的埃达克岩具有更高的Sr含量,这可能与南海古扩张脊俯冲撕裂形成的板片窗有关。斜长石是辉长岩的主要矿物之一,因此,撕裂的洋壳边缘的辉长岩层部分熔融,形成具有更高Sr含量的埃达克质岩浆。而位于吕宋岛南部Bataan弧中的埃达克质火山岩,可能是在南海古扩张脊俯冲之前形成的。根据已发表的斑岩铜金矿床数据,Mankayan地区的成矿年龄在约3.5~1.4Ma,Baguio地区的成矿年龄在约3.1~0.5Ma之间,有从北向南变年轻的趋势,这与黄岩海山链沿马尼拉海沟向南迁移一致。此外,吕宋岛北部Mt.Cagua到Baguio之间存在一个延伸了220km的第四纪火山活动的空隙,该区域大部分火山已经在中新世停止活动。这可能是黄岩海山链的俯冲使得俯冲倾角逐渐变缓、挤压加强而导致的。同时期的斑岩铜矿床正好分布在这一火山空隙中,是俯冲洋壳部分熔融的产物。  相似文献   

16.
Hetu C. Sheth   《Gondwana Research》2005,8(2):109-127
Deep mantle plumes supposedly incorporate deeply subducted eclogitized oceanic crust, and continental flood basalts (CFBs) are now thought by some to be derived from such eclogite-bearing peridotite plumes. Eclogite-peridotite mixtures have much lower solidi (and produce much greater melt fractions for a given temperature) than peridotite. Fe-rich (eclogite- or pyroxenite-bearing) sources have been inferred for many CFBs. However, plumes with considerable amounts of eclogite should have difficulty in upwelling owing to the high density of eclogite. Besides, CFBs are always located along pre-existing lithospheric structures (suture zones, edges of thick cratons) and commonly associated with lithospheric rifting and continental breakup. India's major late Mesozoic CFB, the Deccan Traps, erupted through rift zones and a new continental margin that had developed along ancient suture zones traversing the subcontinent. Many Deccan basalts are too Fe-rich to have been in equilibrium with a peridotite mantle source, and have commonly been considered to be significantly fractionated derivatives of picritic liquids. However, it is possible to view them as relatively less evolved liquids derived from a source with extra fertility (i.e., an Fe-rich source). A new non-plume, plate tectonic model for Icelandic hotspot volcanism involves melting of a shallowly recycled slab of eclogitized Iapetus oceanic crust formerly trapped along the Caledonian suture. The model explains the geochemical-petrological characteristics of Icelandic basalts, and is consistent with passive upper mantle upwelling under Iceland inferred from recent seismic tomography. Based on the petrological and geochemical features of the Deccan flood basalts of the type section, in the Western Ghats, I propose that old, eclogitized oceanic crust trapped in the ancient Indian suture zones could have produced voluminous basaltic melts during the Deccan event.  相似文献   

17.
方念乔  丁旋  张振国  刘豪  魏华玲 《地质科学》2009,44(4):1181-1198
东北印度洋存在两种典型的深水沉积序列,它们分别分布在孟加拉深海扇与东经90°海岭。深海扇以浊流沉积和半远洋沉积作用为主,沉积组份主要来自喜马拉雅山系直接剥蚀的物质材料; 海岭以远洋沉积作用为主,沉积组份主要来自海洋环境下生成的钙质浮游生物壳体。两类沉积序列在物质组成和形成机理上存在着根本差别,但在聚积过程中都受到以喜马拉雅隆升为代表的构造运动的显著影响。本文应用DSDP218、ODP717、ODP758及其它来自东北印度洋的资料和样品,通过沉积学、微古生物学与古海洋学的综合研究,实现了9 Ma以来扇区近源相、远源相和岭区沉积记录中的事件对比。我们的工作表明,在东北印度洋所记录的众多与喜马拉雅山系隆升有关的地质事件中, 35 Ma和08 Ma代表了最具规模和影响的两个演化阶段。  相似文献   

18.
Tectono-magmatic evolution of the west coast of india   总被引：1，自引：0，他引：1  
The west and east coasts of India (WCI & ECI) have distinct histories of their own. The WCI originated subsequent to ECI, which has the imprint of two hotspots - Marion and Reunion, evolved through several stages of rifting, magmatism and isostatic movements. Important among them are: felsic magmatism associated with doming (93 Ma); mafic magmatism related to rifting (88 Ma); origin of the Western Ghats of India and the east facing scarp of Madagascar (all the three related to separation of Madagascar from India); mafic (Deccan) volcanism in the north-western parts of India (67 Ma); rifting of Seychelles micro-continent and lava cover from the north-western parts of India along the Carlsberg ridge (62 Ma/A 27); isostatic subsidence relating to loading of Deccan basalts; subsidence of Bombay offshore region due to reactivation of SONATA rift; separation of Laccadive-Chagos ridge from the southern part of Mascarene plateau because of shifting of the Central Indian Ridge (40 Ma); buckling of South India and tilting of the Peninsula northward due to collision and subduction. These events make the WCI unique and endowed with a great deal of dynamism.  相似文献   

19.
The discovery of glaucophane relicts in the high-pressure tectonites of the Yenisei suture zone of the Yenisei Ridge suggests the manifestation of the “Chilean-type” convergent margin on the western Siberian Craton, which was controlled by subduction of oceanic crust beneath the continental margin. These rocks are restricted to the tectonic suture between the craton and the Isakovka ocean-island terrane and experienced two metamorphic stages. Petrogeochemical characteristics of the mafic tectonites indicate that their protoliths are N-MORB and E-MORB basalts. More primitive N-MORB basalts were formed at the initial spreading stages through melting of the upper depleted mantle. Higher Ti basalts were formed by melting of enriched mantle protolith at the later spreading stages. U–Pb zircon age of 701.6 ± 8.4 Ma of the metamorphosed analogues of normal basalts marks the initiation of oceanic crust in the region. Revealed sequence of spreading, subduction (640–620 Ma), and shear deformations (~600 Ma) records the early stages in the evolution of the Paleoasian ocean in its junction zone with the western margin of the Siberian craton: from formation of fragments of oceanic crust to the completion of accretionary–subduction events.  相似文献   

20.
The seafloor off the Otway/West Tasmanian Basins has an east‐west magnetic lineation attributable to seafloor spreading and notionally identified with the set of seafloor spreading anomalies A8‐A20. Anomaly A20 (45 Ma) lies immediately south of a magnetic quiet zone that extends northward past the continent‐ocean boundary (COB). The Southeast Indian Ocean has a constant angular width between the formerly conjugate margins of Australia and Antarctica, consistent with spreading that started along the entire margin about 96 Ma.The proximity of A20 to the Australian COB in some spreading ridge segments is therefore postulated as due to jumps of the spreading ridge to Australia with concomitant transfer of the older oceanic part of the Australian Plate to the Antarctic Plate. Accordingly, the age of the oldest seafloor at the COB in seven original ridge segments is estimated to step from about 96 to 82, 79, and 75 Ma. Break‐up marks a change in the subsidence of the margin from rapid, during rifting by continental extension, to slow during thermal subsidence of the seafloor. Subsequent ridge jumps to the COB are expected to cause uplift or at least still‐stand of the adjacent continental margin. The subsidence history of the Otway/West Tasmanian margin, as indicated by oil exploration wells, is sympathetic with the timing of the postulated ridge jumps in the adjacent seafloor, as is that of the Great Australian Bight Basin with adjacent seafloor to the west, and of the Bass and Gippsland Basins with the Tasman Sea adjacent to the east. The growth of structure at 80 Ma in the outer Gippsland Basin corresponds with a jump to Australia of the Tasman Sea ridge at 82 and 75 Ma, and at 65 Ma in the Great Australian Bight and Otway Basins to a ridge jump to Australia of the adjacent seafloor. The growth of structure at 60 Ma in the Bass Basin and at 55 Ma in the Gippsland Basin corresponds with the abandonment of the Tasman Sea ridge at A24 (55 Ma) during a re‐organization of spreading in the southwest Pacific.  相似文献   

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