[an error occurred while processing this directive] 高校地质学报 2007, 13(2) 311-322 DOI:     ISSN: 1006-7493 CN: 32-1440/P

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海南地幔柱与南海形成演化
鄢全树1, 2,石学法1
1.海洋沉积与环境地质国家海洋局重点实验室 国家海洋局,第一海洋研究所, 青岛 266061;2.中国科学院 海洋研究所,青岛 266071
摘要: 东南亚上地幔地震层析成像表明,海南岛周围之下存在地幔柱,近垂直的低波速柱体位于海南岛及南海之下,从浅部向下穿越660 km的不连续面处(上下地幔的分界面)并一直延伸到1 900 km。南海及周边地区包括雷琼半岛、海南岛、北部湾盆地、广西北海涠洲岛、以及中南半岛等地,分布有一定量的新生代碱性玄武岩,它们的地球化学数据显示出OIB的特点并具有DUPAL异常,表明其源区较深。此外,由南海新生代碱性玄武岩中的橄榄石-流体平衡所推导的南海底地幔潜在温度( 1 661℃)位于夏威夷(1 688℃)与冰岛热点(1 637℃)相应值之间,为海南岛地幔柱的存在提供了岩石学及矿物化学方面的约束。基于以上地球物理学、地球化学及矿物化学方面的证据,结合数字模拟实验资料,表明在海南岛及邻近区域之下存在地幔柱。建立了一个南海形成演化的初步模型:(1)50~32 Ma,印度洋板块-欧亚板块碰撞及其所导致的太平洋板块后退的综合效应为南海地区提供了一个伸展环境,进而为地幔柱物质的上升提供了通道;(2)32~21 Ma,当地幔柱柱头到达软流圈时, 由于侧向物质流与扩张中心发生相互作用,促进了南海的扩张,并在26~24 Ma期间发生了洋脊重新就位,使扩张中心从原来的18°N附近(即现今西北海盆的中心)调整到15.5°N附近(即现今的东部亚盆);(3)21~15.5 Ma, 随着地幔柱效应的逐渐增强,热点-洋脊相互作用越来越强烈,在大约21 Ma发生了洋脊的再次重新就位事件,诱发了西南海盆的扩张;(4)15.5 Ma~现在,由于印澳板块前缘与巽他大陆碰撞,使得南海大约在15.5 Ma停止扩张,并沿着南沙海槽及吕宋海沟向菲律宾岛弧及巴拉望地块之下俯冲,而南海热点继续活动,直到第四纪还有碱性玄武岩喷出 地表。
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Hainan Mantle Plume and the Formation and Evolution of the South China Sea
YAN Quan-shu1,2 and SHI Xue-fa1
1. Key Lab of Marine Sedimentary and Environment Geology, The First Institute of Oceanography, State Oceanic Administration,Qingdao 266061,China;2. Institute of Oceanology, Chinese Academy of Sciences,Qingdao 266071, China
Abstract: Seismic tomographic images obtained from the mantle under the southeast Asia region indicate there may exist a mantle plume beneath and around the Hainan island. A sub-vertical low-velocity column is imaged beneath the Hainan and the South China Sea, and extends from shallow depths to 660-km seismic discontinuity (i.e., the interface between upper mantle and lower mantle), and continuously to a depht of 1900 km. There is a large quantity of Cenozoic alkali basalts distributed in the South China Sea and its adjacent areas which include Leiqiong Peninsula, Hainan Island, Beibuwan Basin, Weizhou Island in Guangxi province and Indochina block. The geochemical data for these basalts show the characteristics of OIB-type basalt and DUPAL-like isotopic anomaly, and imply its deepseated origin. In addition, the average value of Tp (mantle potential temperature) for the South China Sea inferred from olivine-fluid equilibrium, is 1661℃, which is higher than that of MORB and lies between the corresponding values of Hawaii hotspot and Iceland hotspot. Based on evidences mentioned above, combined with numerical model experimental data, it shows that there does exist a mantle plume beneath the Hainan Island and adjacent areas. Until recently, scholars have developed many models about the formation and evolution of the South China Sea, and the debating issue is the geodynamic source. We suggest that the Hainan plume may be a significant geodynamical source for the formation and evolution of the South China Sea. Here the Hainan plume is introduced into our preliminary model about the formation and evolution of the South China Sea. The model is as follows: (1) 50-32 Ma, Integrated effects of collision between thd Indian Ocean plate and Euro-Asian plate resulted in retrogression of Pacific plate, created a extensional tectonic setting, and provided a channel for ascent of the mantle plume; (2) 32-21 Ma. When the head of mantle plume arrived at asthenosphere, it immediately interacted with the spreading center of the South China Sea by lateral material flow, which enhanced spreading spead. During 26-24 Ma, there took place a ridge jump, which adjusted the spreading center from nearby 18。N (i.e., present-day center of NW sub-basin) to nearby 15.5。N (i.e., present-day center of East sub-basin); (3) 21-15.5 Ma. With the mantle plume effect gradually enhancing, the hotspot-spreading center interaction became more and more intensive, and at about 21 Ma, there took place a ridge jump again, and induced the opening of SW sub-basin; (4) 15.5-0 Ma. Due to collision between the Indo-Australian plate and the Sunda continent, the spreading stopped. Subsequently, the earlier formed oceanic crust subducted along Nansha trench and Manila trench. However, the mantle plume still existed up to now. An actual evidence is: since the Pliocene a large amount of alkali basalt erupted in the South China Sea and its adjacent areas.
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收稿日期 2007-06-20 修回日期 2007-06-20 网络版发布日期  
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