西藏泽当桑格达角闪辉长岩体的发现：雅鲁藏布江闭合时间的证据
发布时间：2019-01-06 14:51
【摘要】：印度板块与欧亚大陆碰撞导致了新特提斯洋的闭合,但是对于其闭合的时间前人从不同的角度提供了多种解释。泽当蛇绿岩套是雅鲁藏布江缝合带东段最大的蛇绿岩块体,本文研究的泽当桑格达角闪辉长岩体侵位于蛇绿岩中,主要由细粒角闪辉长岩、斑状角闪辉长岩、巨晶角闪辉长岩、堆晶角闪辉长岩和角闪岩这5个单元组成。泽当地区产出的角闪辉长岩体前人有多种命名,例如玄武岩、玄武岩、埃达克岩等,我们发现这些岩体时间是同一个岩体的不同部分。本论文通过详细的研究表明其属于侵入岩,并且在TAS图解中,角闪辉长岩投影点几乎全部落入辉长岩区内,属于基性岩类。在泽当角闪辉长岩的哈克图解中可知,角闪石是由其它岩浆带入岩体,岩体中存在的大量角闪石与岩浆不平衡。岩体各岩石样品微量元素总体分布趋势相近,Cs、Rb、Th、K、Ba等大离子亲石元素和La元素相对富集,Ta、Nb、Ti、Th等高场强元素亏损,Sr正异常,具有板块俯冲环境中的岛弧岩浆岩特征。球粒陨石标准化稀土配分型式较为一致,表明这些角闪辉长岩具有同源岩浆系列的特征。对于新特提斯洋的闭合时间,一直没有取得一直的认识,而本文所发现的桑格达角闪辉长岩可以用来判断印度板块与亚洲板块碰撞启动和完成碰撞的时间。角闪辉长岩体侵位于蛇纹石化橄榄岩(被动陆缘)和安山岩(主动陆缘火山-沉积建造)之间,说明它形成于碰撞以后;角闪辉长岩富含大量挥发分,是弧岩浆的特点,据此可以证明其形成在碰撞之前的俯冲环境中。利用这二者的矛盾可以判断,其形成于板块碰撞早期,也就是新特提斯洋刚刚闭合的时间。锆石U-Pb测年得到桑格达岩体的年龄,也就是新特提斯洋闭合的时间为47.6±2.2Ma。根据前人研究,西藏泽当蛇绿岩套形成于板块俯冲环境,且锆石的U-Pb年龄是154.9Ma±2.0Ma,说明板块俯冲一直持续了至少110Ma左右,也就是新特提斯洋在这110Ma时间范围内一直在闭合。以北大西洋的平均扩张速率近34 mm/年作为新特提斯洋闭合的速率下限,计算得新特提斯洋规模下限为3470公里。
[Abstract]:The collision between the Indian plate and Eurasia resulted in the closure of the New Tethys Ocean, but many explanations were provided for the time of its closure from different angles. The Zedang ophiolite suite is the largest ophiolite block in the eastern segment of the Yalu Zangbo River suture zone. The Zedang Sangoda amphiolite intrusion in this paper is located in the ophiolite, mainly composed of fine-grained hornblende gabbro, porphyry hornblende gabbro, megacrystalline hornblende gabbro. The five units are composed of amphibolite and amphibolite. There are many names of the hornblende gabbro body in Zedang area, such as basalt, adakite and so on. We find that these rocks are different parts of the same rock mass in time. In this paper, it is shown by detailed research that it belongs to intrusive rocks, and in the TAS diagram, almost all the projection points of hornblende gabbro fall into the gabbro area and belong to the basic rock category. In the Huck diagram of Zedang hornblende gabbro it can be seen that hornblende is brought into the rock body by other magma and a large number of hornblende and magma are not in equilibrium in the rock mass. The general distribution trend of trace elements in rock samples is similar, the large ion lithophile elements and La elements such as Cs,Rb,Th,K,Ba are relatively enriched, the high field strength elements such as Ta,Nb,Ti,Th are depleted, and the Sr anomaly is positive. It has the characteristics of island arc magma in plate subduction environment. The standardized REE distribution patterns of chondrites are consistent, indicating that these hornblende gabbro have the characteristics of homologous magma series. The closing time of the New Tethys Ocean has not been always known, and the Sangda diorite found in this paper can be used to judge the time of initiation and completion of the collision between the Indian plate and the Asian plate. The amphiolite intrusion is between serpentine peridotite (passive continental margin) and andesite (active continental margin volcanic-sedimentary formation), indicating that it was formed after collision. The hornblende gabbro is rich in volatile matter and is characterized by arc magma, which can be proved to be formed in the subduction environment before collision. This contradiction can be used to judge that it was formed in the early stage of plate collision, when the New Tethys Ocean was just closed. Zircon U-Pb dating shows that the age of the Sangeda rock body, i.e., the closing time of the NeoTethys Ocean is 47.6 卤2.2 Ma. According to previous studies, the Zedang ophiolite suite in Tibet was formed in the subduction environment of the plate, and the U-Pb age of zircon is 154.9Ma 卤2.0 Ma, indicating that the subduction of the plate has lasted at least about 110Ma. That is, the New Tethys Ocean has been closed in this 110Ma time range. Taking the average expansion rate of the North Atlantic for nearly 34 mm/ years as the lower limit of the closure rate of the New Tethys Ocean, it is calculated that the lower limit of the New Tethys Ocean scale is 3470 km.
【学位授予单位】：中国地质大学(北京)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P588.124;P542
，
本文编号：2402939
[Abstract]:The collision between the Indian plate and Eurasia resulted in the closure of the New Tethys Ocean, but many explanations were provided for the time of its closure from different angles. The Zedang ophiolite suite is the largest ophiolite block in the eastern segment of the Yalu Zangbo River suture zone. The Zedang Sangoda amphiolite intrusion in this paper is located in the ophiolite, mainly composed of fine-grained hornblende gabbro, porphyry hornblende gabbro, megacrystalline hornblende gabbro. The five units are composed of amphibolite and amphibolite. There are many names of the hornblende gabbro body in Zedang area, such as basalt, adakite and so on. We find that these rocks are different parts of the same rock mass in time. In this paper, it is shown by detailed research that it belongs to intrusive rocks, and in the TAS diagram, almost all the projection points of hornblende gabbro fall into the gabbro area and belong to the basic rock category. In the Huck diagram of Zedang hornblende gabbro it can be seen that hornblende is brought into the rock body by other magma and a large number of hornblende and magma are not in equilibrium in the rock mass. The general distribution trend of trace elements in rock samples is similar, the large ion lithophile elements and La elements such as Cs,Rb,Th,K,Ba are relatively enriched, the high field strength elements such as Ta,Nb,Ti,Th are depleted, and the Sr anomaly is positive. It has the characteristics of island arc magma in plate subduction environment. The standardized REE distribution patterns of chondrites are consistent, indicating that these hornblende gabbro have the characteristics of homologous magma series. The closing time of the New Tethys Ocean has not been always known, and the Sangda diorite found in this paper can be used to judge the time of initiation and completion of the collision between the Indian plate and the Asian plate. The amphiolite intrusion is between serpentine peridotite (passive continental margin) and andesite (active continental margin volcanic-sedimentary formation), indicating that it was formed after collision. The hornblende gabbro is rich in volatile matter and is characterized by arc magma, which can be proved to be formed in the subduction environment before collision. This contradiction can be used to judge that it was formed in the early stage of plate collision, when the New Tethys Ocean was just closed. Zircon U-Pb dating shows that the age of the Sangeda rock body, i.e., the closing time of the NeoTethys Ocean is 47.6 卤2.2 Ma. According to previous studies, the Zedang ophiolite suite in Tibet was formed in the subduction environment of the plate, and the U-Pb age of zircon is 154.9Ma 卤2.0 Ma, indicating that the subduction of the plate has lasted at least about 110Ma. That is, the New Tethys Ocean has been closed in this 110Ma time range. Taking the average expansion rate of the North Atlantic for nearly 34 mm/ years as the lower limit of the closure rate of the New Tethys Ocean, it is calculated that the lower limit of the New Tethys Ocean scale is 3470 km.
【学位授予单位】：中国地质大学(北京)
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P588.124;P542
，
本文编号：2402939
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