南海新生代碱性玄武岩的特征及其地球动力学意义

IOCAS-IR > 海洋环流与波动重点实验室

	南海新生代碱性玄武岩的特征及其地球动力学意义
其他题名	Geochemistry of Cenozoic Alkali Basalts from the South China Sea and its Geodynamical significance
	鄢全树
学位类型	博士
	2008-06-12
学位授予单位	中国科学院海洋研究所
学位授予地点	海洋研究所
关键词	地质年代学矿物化学橄榄石地球化学 Sr-nd-pb同位素 Dupal异常海南地幔柱南海形成演化南海
摘要	南海是西太平洋最大的边缘海之一，位于欧亚板块、印-澳板块以及太平洋板块之间。南海海底扩张停止(15.5Ma, Briais et al., 1993) 后的板内火山作用，影响着中国南部、中南半岛、大洋基底和分裂的微大陆片段的广大地区。对南海新生代玄武岩进行地球化学研究，不仅对于理解南海板内火山作用的深部地幔过程有着重要意义，而且对南海形成演化及含油气盆地形成的深部动力学机制有着深远意义。南海新生代玄武岩K-Ar/Ar-Ar年龄为3.8-7.9Ma, 表明为晚中新世以来的岩浆活动产物，与周边地区的碱性火山岩在年龄上的一致性。岩石学特征表明，南海新生代玄武岩的矿物组合为橄榄石、单斜辉石、斜长石，与特征的碱性玄武岩的矿物组合一致。由橄榄石所计算的南海底潜在地幔温度(Tp)平均值为1661℃, 暗示南海地区下的地幔可能存在热量异常，为海南地区存在地幔柱的观点提供了证据。单斜辉石富钙、钛，由单斜辉石-熔体平衡温压计计算的岩浆房深度分别为：碱玄岩岩浆房深度约49km（对应压力为1.46～1.48 GPa）；粗面玄武岩岩浆房约25km（对应压力为0.76 GPa）；玄武岩岩浆房约15km（对应压力为0.44GPa）。由碱玄岩→粗面玄武岩→玄武岩，平衡温度(K)依次降低:从1535～1498→1429→1369。由斜长石微晶所计算的岩浆喷出地表的温度为989℃。主量元素特征表明，岩石类型主要为碱玄岩，有少量的粗面玄武岩和玄武岩，属于碱性系列。微量元素方面，大离子亲石元素(LILE)以及高场强元素（HFSE）特别是Nb、Ta、Ti、Y等元素均呈现富集现象，Yb、Sc、Sr以及K、U、Th等生热元素相对亏损，微量元素及稀土元素分布岩石类似板内OIB微量元素的全球平均值。同位素地球化学研究表明，源区存在两个混合端员并具Dupal Pb异常：一个为DMM，位于软流圈或岩石圈地幔中；另一个为EM2源区，可能来自位于核-幔边界处的海南地幔柱而非大陆底岩石圈地幔。研究表明，南半球Dupal异常不存在全球范围内的地区专属性，本区存在的Dupal异常与南半球Dupal异常可能不存在联系。在南海新生代玄武岩的成因过程中，海南地幔柱在为岩石圈地幔的部分熔融作用提供所需的热量同时，也在物质上作出了贡献。南海盆新生代碱性玄武岩由不同程度的部分熔融作用，以及岩浆在上升期间或者在高位岩浆房中的橄榄石等矿物分离结晶作用所形成，同时还可能发生了堆晶作用。构造环境判别表明，玄武岩浆在上升到地表过程中几乎未受到地壳混染。南海新生代玄武岩的地球化学研究表明，在玄武质岩浆的深部地幔演化过程中，海南地幔柱可能起着重要的作用。通过引入海南岛地幔柱这个概念，本文建立了一个新的有关南海形成演化的初步的概念性模型：（1）50-32Ma，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，此伸展环境为地幔柱物质的上升提供了通道；（2）32-21Ma，当地幔柱柱头到达软流圈时, 通过侧向物质流开始同扩张中心发生相互作用，促进了南海的扩张，并在26-24Ma期间发生了洋脊重新就位事件，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）21-15.5Ma, 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21Ma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-现在，由于印澳板块前缘与巽他大陆碰撞，使得南海大约在15.5Ma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，在地表处直到第四纪还有碱性玄武岩喷出。
其他摘要	The South China Sea is one of the largest marginal basins in the west Pacific, which lies at triple junction among three large plates, i.e., Eurasian plate, Indo- Australia plate and Pacific plate (or Philippine plate ). A large scale intraplate volcanism after the cessation of seafloor spreading, affects a broad areas including the South China Sea itself, southeast China, Indochina peninsula and several rifted micro-continental segments in the South China Sea. A comprehensive study on Cenozoic alkali basalts is not only helpful to understand the deep mantle process of the intraplate volcanism occurred in the South China Sea, but also significant for understanding of the deep geodynamical regime for the formation and evolution of the South China Sea and a series of oil-bearing basins. The whole rock K-Ar/Ar-Ar ages for basaltic rock from the South China Sea range from 3.8 Ma to 7.9 Ma, which suggestS that they are products of magmatism since late Miocene, as is consistent with those of alkali basalt from around the South China Sea. Petrographic studies suggest that the mineral assemblage is composed of olivine, clinopyroxene and plagioclase, which is consistent with common mineral assemblage of a characteristic alkali basalt. The average value for mantle potential temperatures (Tp) beneath the South China Sea, calculated from olivine-melt equilibrium, is 1661℃, which imply that there may exist thermally anomaly in the mantle beneath the South China Sea, and provide a important evidence for the Hainan Plume. During the ascent form the magma origin to the surface, the temperatures and pressures of evolved magmas in different high level magma chambers calculating from clinopyroxene - melt equilibrium are as follows, (1) equilibrium temperatures (°K) for tephrite, trachybasalt and basalt are 1535～1498, 1429 and 1369, respectively; (2) equilibrium pressure for tephrite is 1.46～1.48 GPa , which corresponds to magma chamber at depth of ~ 49km (the middle-lower part of oceanic lithosphere); (3) equilibrium pressure for trachybasalt is 0.76 GPa , which corresponds to magma chamber at depth of ~ 25 km (the middle-upper part of oceanic lithosphere); (4) equilibrium pressure for basalt is 0.44 GPa , which corresponds to magma chamber at depth of ~ 15 km (the upper part of oceanic lithosphere). The quenching temperature of magma erupted on the surface, calculated from plagioclase – melt equilibrium, is 989℃. Major element compositions suggest that rock types are mainly tephrite, trachybasalt and basalt in subordination, and all rocks belong to alkali series. Large ion lithophile elements (LILE) and high field strength elements (HFSE), e.g., Nb, Ta, Ti and Y, are enriched, and Yb, Sc, Sr and heat-producing elements, e.g. K, Th, U, are relatively depleted. In a whole, the distributional patterns for trace elements and rare earth elements are similar to those of global intraplate OIB average value. Sr-Nd-Pb isotopic data of these basaltic rocks strongly suggest there is a binary mixing model in origin with Dupal Pb anomaly, one is a depleted mantle end-member (DMM), and the other is EM2 component which may be not result from subcontinental lithospheric mantle, but the Hainan plume originated from core-mantle boundary. Dupal anomaly is not only limited to South Hemisphere, and there have no relationship in Dupal anomaly between the South China Sea and South Hemisphere. The petrogenesis for basaltic rock described below, the Hainan plume contributes to partial melting of source rock thermally and materially. During the ascent of magma from the origin to the surface, they experienced fractional crystallization and/or cumulation to different extent in different level magma chamber, and there have no obvious mixing inprint with continental lithospheric mantle and crust. Hainan plume are introduced into a preliminary model about the formation and evolution of the South China Sea which this study newly builds. The model is as follows, (1), 50-32Ma. Integrated effects of the collision between Indian ocean plate and Euro-Asian plate and its resulting in the retrogression of Pacific plate, created a extensional tectonic setting, which provided a channel for the ascent of mantle plume. (2), 32-21Ma. 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 26-24Ma, there took place 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), 21-15.5Ma. With the mantle plume effect gradually enhancing, the hotspot-spreading center interaction is more and more intensive, and about 21Ma there took place ridge jump again, and induced the open of SW sub-basin. (4), 15.5-0 Ma. Due to the collision between the predestined affinity of Indo-Australian plate and Sunda continent, the spreading stopped. And subsequently earlier formed oceanic crust subducted along Nansha trench and Manila trench. However, the mantle plume still existed up to now. Since Pliocene A large amount of alkali basalt erupted in the South China Sea and its adjacent areas are a actual evidence.
页数	148
语种	中文
文献类型	学位论文
条目标识符	http://ir.qdio.ac.cn/handle/337002/611
专题	海洋环流与波动重点实验室
推荐引用方式 GB/T 7714	鄢全树. 南海新生代碱性玄武岩的特征及其地球动力学意义[D]. 海洋研究所. 中国科学院海洋研究所,2008.

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