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藏北羌塘中西部红脊山蛇绿岩的岩石学、地球化学、年代学特征及构造意义

发布时间:2018-10-18 18:35
【摘要】:红脊山蛇绿岩是笔者等厘定出的各单元出露齐全的岩石组合,前人对这套蛇绿岩的研究几乎是一片空白,尤其缺少蛇绿岩年代学和地球化学的资料,这严重制约了龙木错-双湖-澜沧江板块缝合带在羌塘中西部的深入研究。红脊山蛇绿岩的岩石学、年代学、地球化学特征及构造意义等一系列问题均有待解决,针对这些问题,本文从蛇绿岩的岩石学、年代学和地球化学特征等方面进行了详细的研究,以期解决这些问题。 岩石学研究结果显示,该套蛇绿岩总体呈NW-SE向带状断续展布,长约30km,宽约5km,由变质橄榄岩,变质堆晶辉长岩,变质基性岩墙,变质玄武岩,放射虫硅质岩和斜长花岗岩组成。 蛇绿岩锆石U-Pb年代学研究结果显示,变质堆晶辉长岩21个测点得出的加权平均年龄为274.8±2.6Ma(MSWD=0.21);斜长花岗岩12个测点得出的加权平均年龄值为271.7±2.3Ma(MSWD=0.38);红脊山变质堆晶辉长岩和斜长花岗岩两个年龄值非常接近,代表了红脊山蛇绿岩的形成时代,即红脊山蛇绿岩形成于早二叠世。 本文以变质橄榄岩、变质堆晶辉长岩、变质基性岩墙和变质玄武岩为研究对象,对红脊山蛇绿岩进行了详细的岩石学和地球化学研究,其地化特征分析结果表明,各蛇绿岩单元主量元素含量与典型的N-MORB非常相似,稀土元素总量较低,具有轻稀土略亏损的近平坦左倾型配分型式,铕异常不显著,各蛇绿岩单元微量元素蛛网图总体也与N-MORB相似,表明变质基性岩源于N-MORB型亏损地幔源区。但是,与典型的大洋中脊玄武岩相比,各蛇绿岩单元微量元素蛛网图上都显示了部分大离子亲石元素(Ra、Ba、U)的明显富集和部分高场强元素(Nb、Ta)的明显亏损,尤其是Nb的亏损较为明显,这又与典型的大洋中脊MORB型玄武岩有明显区别,,显示红脊山蛇绿岩兼具N-MORB和IAB特征,表明本区蛇绿岩受到了俯冲带物质的影响,很可能形成于俯冲带之上的弧后盆地次级扩张的大地构造背景,是特提斯大洋岩石圈在俯冲过程中引发弧后盆地次级扩张的产物,属于SSZ型蛇绿岩范畴。 综合以上研究结果,笔者认为二叠纪古特提斯大洋演化已经进入了弧后拉张阶段,由于弧后拉张,二叠纪红脊山地区出现了一个小洋盆,而红脊山蛇绿岩就是这一期构造运动的记录。本文对红脊山蛇绿岩的研究为龙木错-双湖-澜沧江板块缝合带中西段的构造演化研究提供了重要依据和基础资料,进而为探讨青藏高原特提斯演化以及冈瓦纳和欧亚大陆的界线提供了新的资料和认识。
[Abstract]:The Hongjishan ophiolite is a fully exposed rock assemblage of various units determined by the author and the previous study of the ophiolite is almost blank, especially the lack of information on the geochronology and geochemistry of the ophiolite. This seriously restricted the deep study of Longmu-Shuanghu-Lancang River plate suture zone in the central and western Qiangtang. A series of problems such as petrology, geochronology, geochemical characteristics and tectonic significance of ophiolite in Hongji Mountain need to be solved. The geochronology and geochemical characteristics are studied in detail in order to solve these problems. The petrological results show that the ophiolite is distributed in a NW-SE zonal pattern, about 30km long and 5km wide, consisting of metamorphic peridotite, metamorphic heap pyroxenite, metamorphic basalt dike, metamorphic basalt, radiolarian siliceous rock and plagioclase granite. The results of U-Pb chronological study on zircon from ophiolite show that the weighted average age of metamorphic pyroxene is 274.8 卤2.6Ma (MSWD=0.21), that of clinopyroclase is 271.7 卤2.3Ma (MSWD=0.38), that of clinopyroxenite is 271.7 卤2.3Ma (MSWD=0.38), that of metamorphic pyroxene is 274.8 卤2.6Ma (MSWD=0.21), that of amphiolite is 271.7 卤2.3Ma. The two ages of Hongjishan metamorphic heap pyroxenite and clinopyranite are very close, which represents the formation age of Hongji mountain ophiolite, that is, Hongjishan ophiolite was formed in early Permian. In this paper, metamorphic peridotite, metamorphic heap pyroxene, metamorphic basic rock dike and metamorphic basalt are taken as the objects of study. The detailed petrology and geochemistry of Hongjishan ophiolite are studied in detail. The main element contents of each ophiolite unit are very similar to those of typical N-MORB, the total amount of rare earth elements is relatively low, and the ophiolite unit has a near-flat left-dip distribution pattern with slight depletion of light rare earth elements, and europium anomaly is not significant. The trace element cobweb map of each ophiolite unit is similar to that of N-MORB, indicating that the metamorphic basic rocks originated from the N-MORB type depleted mantle source region. However, compared with the typical basalt in the mid-oceanic ridge, the trace element cobwebs of each ophiolite unit show the obvious enrichment of some large ion lithophile elements (Ra,Ba,U) and the obvious depletion of some high field strength elements (Nb,Ta), especially the depletion of Nb. This is obviously different from the typical MORB basalt in the mid-oceanic ridge, which indicates that the ophiolite in Hongji Mountain possesses both N-MORB and IAB characteristics, indicating that the ophiolite in this area is affected by subduction zone material. It is very likely that the tectonic setting of the secondary extension of the back-arc basin over the subduction zone is the product of the secondary expansion of the back-arc basin caused by the Tethys oceanic lithosphere during the subduction and belongs to the category of SSZ ophiolite. Based on the above results, the authors believe that the evolution of the Permian PaleoTethys ocean has entered the post-arc extensional stage. As a result of the back-arc extension, a small oceanic basin appeared in the Red Ridge Mountains of the Permian period. The Hongji Mountain ophiolite is the record of this period of tectonic movement. In this paper, the study of ophiolite in Hongji Mountain provides important basis and basic data for the study of tectonic evolution in the western segment of the Longmao-Shuanghu-Lancangjiang plate suture zone. It provides new data and understanding for the Tethys evolution of the Qinghai-Tibet Plateau and the boundary between Gondwana and Eurasia.
【学位授予单位】:吉林大学
【学位级别】:硕士
【学位授予年份】:2015
【分类号】:P597.3;P588

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