南海玳瑁海山与相邻陆域玄武岩的地球化学特征及其构造意义

发布时间：2018-01-23 13:02

本文关键词： 玳瑁海山及邻近陆域玄武岩地化分析构造演化　出处：《中国地质大学(北京)》2015年博士论文　论文类型：学位论文

【摘要】：玳瑁海山位于南海海盆北部,其16.7-14.4Ma的喷发年龄与南海海盆扩张的结束时段(～15.5Ma)大体相当,而且提供了现存南海海山唯一的MORB型玄武岩样品,从年代学和岩石地球化学角度衡量都具有难以替代的分析价值。南海及其邻近陆域在新生代存在两次重大构造-岩浆事件：发生于古新世末至晚始新世(60-～40Ma)的三水盆地和中中新世以后(15Ma)的我国雷琼地区与越南中部的嵬嵩-呵叻地区。本文为考察南海演化各阶段的基本特点,利用当前能够获得的南海扩张前(三水盆地)、扩张晚期(玳瑁海山)和扩张停止之后(雷琼、]嵩地区)的玄武岩样品,在建立基于LA-ICP-MS技术的高精度分析方法,对样品中70个元素进行分析,其中特别包括含量为ppb级别的亲铁元素和亲硫元素。力图将不同时段和不同地域的分析结果纳入同一过程体系,通过对比研究建立它们在深部和构造上的关联。玳瑁海山的MOBR型样品来自DM亏损端元,而所有OIB型玄武岩样品均源自一个DM和EM2的双端元混合源区。其中,DM源区类似MORB亏损地幔源区而EM2源区则来自俯冲的洋壳物质再循环。研究区火山岩源区的部分熔融深度和潜在温度均低于深部地幔柱成因的夏威夷或冰岛玄武岩；而相较于南海北、西部陆缘玄武岩源区,玳瑁海山则更低。此外,样品中亲铁及亲硫元素,特别是W的含量也明显低于深源热点型OIB,进一步说明研究区幔源EM2组分并非来自下地幔深部或核幔边界。对应南海及其相邻陆域火山岩岩石地球化学特征所约束的源区深部过程,本研究认为：在中新生代之交,由于华南大陆岩石圈的大规模减薄和水平位移,诱使研究区的某些地点软流圈显著抬升,加大上地幔熔融量并形成大量岩浆在三水盆地广泛喷发,构成浅源地幔柱和主动型大陆裂谷表象,与其后的南海张裂本来存在机制关联。但是南海海盆的扩张最终还是由印度板块挤入欧亚大陆的“挤出效应”所控制,强烈的大陆块体的迁移运动“压制”并取代浅源地幔柱的“热点效应”成为南海开裂的主控因素；强势的构造拉张运动使得岩石圈发生分熔,并对洋壳和海山的形成提供物质来源。南海扩张停止后,区域边界条件发生改变,导致类似三水与软流圈关系密切的“浅源热点”现象在雷琼、昆嵩、呵叻等地“复苏”。
[Abstract]:The tortoiseshell seamounts are located in the northern part of the South China Sea basin, and the eruption age of 16.7-14.4 Ma is roughly similar to that of 15.5Ma at the end of the South China Sea basin extension. The only MORB basalt samples are provided in the seamounts of the South China Sea. There are two major tectonic-magmatic events in the South China Sea and its adjacent continental areas during the Cenozoic: from the end of Paleocene to the late Eocene (from the end of Paleocene to the late Eocene) (from the late Paleocene to the late Eocene). The Sanshui basin of 60-40 Ma and the Raiqiong area of China and the Wei Song-Kalat area of central Vietnam. The basic characteristics of the evolution stages in the South China Sea are discussed in this paper. Basalt samples from the South China Sea before (Sanshui basin), late expansion (hawksbill seamounts) and after the extension (Leiqiong,] Yunsong area) are used. A high precision analysis method based on LA-ICP-MS was established to analyze 70 elements in the sample. In particular, it includes the content of ppb level of iron and sulfur elements, trying to bring the results of different time and different regions into the same process system. The MOBR type samples of the tortoiseshell seamounts are derived from DM depleted end elements. All OIB basalt samples are derived from a mixed source of DM and EM2. The DM source region is similar to the MORB depleted mantle source region, while the EM2 source region is derived from the subducted oceanic crust material recycling. The partial melting depth and potential temperature of the volcanic source region in the study area are lower than those in Hawaii, where the deep mantle plume was formed. Or Icelandic basalt; Compared with the northern part of the South China Sea, the tortoiseshell seamounts are lower than those in the northern part of the South China Sea. In addition, the contents of Fe and S, especially W, in the samples are obviously lower than those in the deep-source hot spot type OIB. It is further explained that the EM2 component of mantle source in the study area is not derived from the lower mantle or the core mantle boundary, and corresponds to the deep source process restricted by the geochemical characteristics of volcanic rocks in the South China Sea and its adjacent continental regions. This study suggests that at the turn of Mesozoic and Cenozoic, the large scale thinning and horizontal displacement of the continental lithosphere in South China induced a significant uplift of the asthenosphere in some locations in the study area. Increasing the amount of melting of the upper mantle and forming a large amount of magma erupting widely in the Sanshui basin, forming the shallow mantle plume and active continental rift surface. However, the expansion of the South China Sea basin is ultimately controlled by the "crowding effect" of the Indian plate into Eurasia. The strong migration movement of continental block "suppresses" and replaces the "hot spot effect" of shallow mantle plume as the main controlling factor of South China Sea cracking. The strong tectonic extensional movement caused the lithosphere to melt and provided material source for the formation of oceanic crust and seamounts. After the South China Sea extension stopped the regional boundary conditions changed. Leading to similar Sanshui and soft circle close relationship between the "shallow hot spot" phenomenon in Lei Qiong, Kunsong, Kulat, and other places, "recovery."
【学位授予单位】：中国地质大学(北京)
【学位级别】：博士
【学位授予年份】：2015
【分类号】：P588.145;P736.1

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