青藏高原后碰撞钾质—超钾质岩石的地球化学特征与岩石成因

发布时间：2018-05-27 21:23

  本文选题：Sr-Nd-Pb-Os-Mg-O同位素 + 锆石U-Pb-Hf同位素　； 参考：《中国地质大学(北京)》2017年博士论文

【摘要】：青藏高原南部的拉萨地块在后碰撞构造-岩浆演化阶段发育了一套与东西向伸展构造伴生的渐新世-中新世钾质-超钾质火山岩和埃达克质含矿斑岩。作为青藏高原演化过程中深部动力学过程的岩浆活动响应,这套后碰撞岩浆活动记录为探讨高原隆升机制、揭示岩石圈深部物质组成、以及示踪特提斯洋壳俯冲消减和印度-欧亚大陆汇聚过程中的沉积物俯冲交代作用提供了重要视角,具有很高的科研价值。然而,对于藏南幔源超钾质岩石的成因、其地幔源区交代富集作用和中新世藏南深部动力学过程的理解却明显不足。与此同时,钾质中酸性火山岩被认为具有和后碰撞埃达克质侵入岩相似的地球化学特征,因此被视为一种特殊的埃达克质岩石,但是这种观点显然忽略了两者之间可能存在的成因差异。本文以藏南幔源超钾质火山岩(包括捕获的地幔橄榄岩包体和壳源包体)和同时代的钾质中酸性岩石为主要研究对象,进行了系统的锆石U-Pb年代学和原位Lu-Hf同位素研究,并对全岩Sr-Nd-Pb-Os和Mg-O同位素组成进行了准确测定。借助锆石U-Pb定年、微量元素和Lu-Hf同位素分析,本文从藏南超钾质火山岩中发现了大量大陆地壳起源的元古代-新生代锆石捕掳晶。通过对比拉萨地块的碎屑锆石和岩浆活动记录,本文发现超钾质岩浆在演化过程中受到了拉萨地块地壳物质的混染。超钾质岩石锆石捕掳晶的DyN/YbN和U/Yb从~55 Ma开始发生系统性的升高,这种现象被解读为记录了印度-欧亚大陆汇聚导致拉萨地块地壳自早古近纪以来的持续加厚过程,表明幔源岩浆捕获的锆石捕掳晶可以用来反演上覆地壳的演化。全岩Os-Sr-Mg同位素分析进一步证实了地壳混染作用对后碰撞幔源超钾质岩浆活动的同位素组成具有深远影响,同时还发现新特提斯洋壳富含碳酸盐的沉积物在北向俯冲过程中对藏南岩石圈地幔的交代与改造作用。受到地壳混染影响较小的超钾质岩石样品具有低的187Os/188Os,这些低187Os/188Os样品的δ~(26)Mg与~(87)Sr/~(86)Sr呈负相关关系,与Hf/Sm则为正相关关系,表明高镁碳酸盐(例如白云石)的交代作用能够有效改变地幔源区的Mg同位素组成。拉萨地块超钾质岩石的δ~(26)Mg和Hf/Sm在空间分布上向北逐渐降低的趋势表明地幔碳酸盐交代作用会随着新特提斯洋板片俯冲深度的增加而逐渐增强。尽管钾质火山岩和埃达克质侵入岩均展现出高Sr/Y和La/Yb、低Y和Yb的地球化学特征,但是这两种壳源后碰撞岩浆岩具有不同的时空分布和同位素变化特征。后碰撞埃达克质侵入岩主要出露在南部拉萨地块,起源于加厚下地壳的部分熔融,其非放射性成因的~(143)Nd/~(144)Nd以及其同岩浆锆石εHf(t)从35Ma开始迅速减低的趋势早于幔源超钾质岩浆活动的开始,表明俯冲印度大陆地壳物质对埃达克质岩浆活动有重要贡献。另一方面,同时代的钾质中酸性火山岩与幔源超钾质岩石拥有相同的时空分布特征,这两种岩石的全岩Pb同位素和锆石Hf同位素组成不仅证实了两者与拉萨地块的亲缘性,还表明深俯冲的印度大陆地壳物质在岩石形成过程中并未扮演主要角色。通过对比拉萨地块不同地区的后碰撞岩浆活动记录,本文发现早中新世钾质-超钾质岩石在野外呈现类似双峰式火山岩的分布形式,两者在同位素组成上的差异表明幔源超钾质岩浆对同时代钾质火山岩的直接贡献很少;随着拉萨地块后碰撞岩浆活动向东部迁移,晚中新世的钾质火山岩拥有与超钾质岩石相近的同位素组成,暗示两者具有更加密切的成因联系。考虑到与青藏高原加速隆升相关的一系列地质事件在中新世集中爆发,包括剥蚀-沉积速率加剧、海水~(87)Sr/~(86)Sr比值剧烈变化、以及构造应力格架转变等,拉萨地块渐新世-中新世的钾质-超钾质岩浆活动可能与该时期藏南加厚岩石圈地幔底部对流减薄或拆沉作用有关。
[Abstract]:The Lhasa massif in the southern part of the Qinghai Tibet Plateau developed a set of Oligocene Miocene potash and hyper kaliplic volcanic rocks associated with east-west extensional tectonics in the post collisional tectonic magmatic evolution stage. As a response to the magmatic activity of the deep dynamic process during the evolution of the Qinghai Tibet Plateau, the post collisional magmatism record was recorded. In order to explore the mechanism of uplift of the plateau, to reveal the composition of the deep lithosphere, and to trace the subduction and subduction of the subduction of the Tethys oceanic crust and the subduction and metasomatism of the sediment in the India Eurasia continent, it is of great value for scientific research. However, the genesis of the mantle source rocks in the mantle source in Zangnan, and the metasomatism and enrichment of the mantle source region. The deep understanding of the deep dynamic process in the middle Miocene of Zangnan is obviously inadequate. At the same time, the acid volcanic rocks in the potash are considered to have similar geochemical characteristics with the post collision of the edacitic intrusive rocks, so they are considered a special kind of edacic rocks, but this view obviously ignores the possible cause of the difference between the two. In this paper, a systematic study of zircon U-Pb chronology and in situ Lu-Hf isotopes was carried out on the mantle derived ultra potassic volcanic rocks (including the captured mantle peridotite xenoliths and crust source enclaves) and the intermediate acid rocks of the same epoch, and the composition of Sr-Nd-Pb-Os and Mg-O isotopes of the whole rock was accurately measured with the aid of zirconium. The dating of stone U-Pb, trace elements and Lu-Hf isotopes found a large number of Proterozoic Cenozoic zircon captive crystals from the Zangnan super potassic volcanic rocks. By comparing the detrital zircon and magmatic records of the Lhasa massif, this paper found that the super potassic magma was subjected to the crustal substance of the Lhasa massif during the evolution process. The DyN/YbN and U/Yb of the zircon captive crystals of the super potassic rocks began to rise systematically from ~55 Ma. This phenomenon was interpreted as a record of the continuous thickening of the crust of the Lhasa massif since the early Palaeogene of India and Eurasia, indicating that the captive zirconium captive crystals captured by the mantle derived magma could be used to invert the overlying crust. The Os-Sr-Mg isotopic analysis of the whole rock further confirms that the crustal contamination has a profound influence on the isotopic composition of the ultra potassic magmatism of the post collision mantle source. Meanwhile, it is also found that the carbonate deposits of the new Tethys oceanic crust have been metasomatic and reformed in the Zangnan lithosphere mantle during the North subduction. The smaller super potash rock samples have low 187Os/188Os, and the delta ~ (26) Mg of these low 187Os/188Os samples is negatively correlated with ~ (87) Sr/~ (86) Sr, and is positively correlated with Hf/Sm, indicating that the metasomatism of high magnesium carbonate (such as dolomite) can effectively change the Mg isotopic composition of the mantle source region. (26) the trend of decreasing the spatial distribution of Mg and Hf/Sm indicates that the mantle carbonate metasomatism will gradually increase with the increase of the subduction depth of the new Tethys plate. Although the potassium volcanic rocks and the intrusive rocks all show high Sr/Y and La/Yb, low Y and Yb geochemistry characteristics, but these two shells collide with magma. The post collisional EDK intrusive rocks are mainly exposed in the southern Lhasa block, which originated from partial melting of the thickened lower crust. The non radioactive ~ (143) Nd/~ (144) Nd and its magmatic zircon epsilon Hf (T) rapidly decreased from 35Ma to the mantle derived ultra potassic magmatism. At the beginning, it shows that the subduction of the crust in the continental crust of India has an important contribution to the arkic magmatic activity. On the other hand, the same epoch potassium medium acid volcanic rocks have the same temporal and spatial distribution characteristics with the mantle derived super potassic rocks. The total rock Pb isotopes of the two rocks and the zircon Hf isopsin not only confirmed the relationship between the two types of rocks and the Lhasa massif. It is also shown that the deep subduction of the crust of the India continental crust did not play a major role in the process of rock formation. By comparing the records of post collision magma activities in different regions of the Lhasa massif, it was found that the early Miocene potassic super potassic rocks were distributed in the field similar to the Shuangfeng type volcanic rocks in the field, both of which were on the isotopic composition. The difference shows that the direct contribution of the mantle derived super potassic magma to the same epoch potassium volcanic rocks is very little. With the migration of the post collisional magmatic activity to the East, the late Miocene potassic volcanic rocks have a similar isotopic composition with the super potassic rocks, suggesting that the two have a more close genetic connection. The acceleration uplift of the Qinghai Tibet Plateau is considered. A series of related geological events erupted in the Miocene, including the intensification of erosion and deposition, the violent change of the ~ (87) Sr/~ (86) Sr ratio and the tectonic stress transition, and the potassium and super potassium magmatic activity of the Oligocene Miocene in the Lhasa massif may be thinned or removed from the bottom of the thick lithosphere mantle at the time of Zangnan. It's relevant.
【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】：2017
【分类号】：P588.1
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本文编号：1943792