西藏罗布莎砾岩火山岩夹层年代学、地球化学及其地质意义

发布时间：2018-04-01 05:06

本文选题：埃达克质岩　切入点：地球化学　出处：《东华理工大学》2015年硕士论文

【摘要】：雅鲁藏布江缝合带是青藏高原一条重要的构造界线,带内地质现象复杂对于反演青藏高原演化具有重要意义。本文报道了缝合带东段泽当地区罗布莎砾岩中的火山岩及相关岩石的年代学、岩石地球化学和同位素地球化学特征,试图为反演青藏高原演化及印亚碰撞提供更多岩石学及地球化学证据。泽当地区罗布莎砾岩为一套磨拉石沉积建造,不整合沉积于冈底斯花岗岩之上。野外地质调查证实其沉积物源确实存在明显变化,反映了缝合带两侧不同的构造运动。在砾岩近底部笔者发现以英安质晶屑玻屑凝灰岩为主的火山岩夹层有局部向熔岩端元过渡的现象,含有含砾英安岩,呈层状产出。LA-ICP-MS锆石U-Pb同位素结果显示罗布莎砾岩中该火山岩喷发时代为18.7±0.2Ma,属于中新世前中前期。罗布莎砾岩下伏的冈底斯花岗岩锆石U-Pb定年显示其侵位时代为29.9±0.4Ma。综合野外地质关系及磨拉石快速沉积的特点,大约在18.7Ma~29.9Ma之间罗布莎砾岩开始沉积。罗布莎砾岩中火山岩全岩地球化学显示其为钾玄岩系列,样品富集轻稀土元素、亏损重稀土元素,具有弱的铕负异常,大离子亲石元素富集,高场强元素亏损;Sr含量很高(平均405×10-6),Y含量很低(平均10.05×10-6),Sr/Y=36.40~43.21,属于埃达克质岩,来源于加厚下地壳部分熔融。全岩Sr-Nd-Pb同位素显示样品具有亲特提斯洋壳属性,εNd(t)介于-2.58~-3.36之间,年轻亏损地幔模式年龄TDM(Ga)=0.84Ga~0.91Ga,具有初生地壳特征,锆石Lu-Hf同位素也显示出相似特征,说明亏损地幔对源区贡献较大,全岩Sr-Nd-Pb同位素及锆石Lu-Hf同位素特征都表明该火山岩具有亲特提斯洋属性。泽当地区罗布莎砾岩中火山岩亲新特提斯洋洋壳属性说明俯冲的印度板块在18.7±0.2Ma还没有影响到该地区。通过与拉萨地块西部超钾质岩的对比,可以进一步说明拉萨地块东西差异性,暗示印度板块在该时期内可能经历了一次快速的俯冲过程。
[Abstract]:The Yalu Zangbo River suture zone is an important tectonic boundary of the Qinghai-Tibet Plateau, and the complexity of geological phenomena in the belt is of great significance for inversion of the evolution of the Qinghai-Tibet Plateau.This paper reports the geochronological, petrogeochemical and isotopic geochemical characteristics of volcanic rocks and related rocks in the Luobusa conglomerate in the Zedang area of the eastern segment of the suture zone.This paper attempts to provide more petrological and geochemical evidence for inversion of the evolution of the Qinghai-Tibet Plateau and the India-Subcollision.The Luobusa conglomerate in Zedang area is a set of morlite sedimentary formation, which is unconformity deposited on Gangdis granite.The field geological investigation confirmed that the sediment source changed obviously, which reflected the different tectonic movements on the two sides of the suture zone.Near the bottom of the conglomerate, it is found that the intercalation of volcanic rock, which is dominated by vitreous tuff, has the phenomenon of partial transition to the end of molten rock, and contains conglomerate dolomite.The U-Pb isotopic results of LA-ICP-MS zircon show that the eruption age of the volcanic rocks in the Luobusa conglomerate is 18.7 卤0.2 Ma.It belongs to the pre-Miocene period.The U-Pb dating of zircon from the Gangdese granite underlying the Luobusa conglomerate shows that its emplacement age is 29.9 卤0.4 Ma.Combined with the field geological relationship and the characteristics of rapid deposition of molars, the Luobusa conglomerate began to deposit between 18.7Ma~29.9Ma.The whole rock geochemistry of the volcanic rocks in the Luobusa conglomerate shows that it is a series of potassium metamorphic rocks. The samples are rich in light rare earth elements, depleted in heavy rare earth elements, with weak europium negative anomalies and large ion lithophile elements.The high field strength element depleted Sr content is very high (average 405 脳 10-6) and the content of Y is very low (average 10.05 脳 10-6) Sr-P / Y ~ (36.40) ~ (43.21), which belongs to adakitic rock and originated from partial melting of thickened lower crust.The Sr-Nd-Pb isotopes of the whole rock show that the samples have the characteristics of pro-Tethys oceanic crust, 蔚 Ndndnt) between -2.58 and 3.36, and the young depleted mantle model age is 0.84Ga0.91 Ga, which shows the characteristics of the primary crust. The zircon Lu-Hf isotopes also show similar characteristics, indicating that the depleted mantle contributes a great deal to the source region.The Sr-Nd-Pb isotopic and zircon Lu-Hf isotopic characteristics of the whole rock indicate that the volcanic rock is characterized by the Tethys oceanic property.The neo-Tethys oceanic crust properties of volcanic rocks in the Robesa conglomerate in the Zedang area indicate that the subducted Indian plate has not affected the area at 18.7 卤0.2Ma.The comparison with the ultrakalitic rocks in the western Lhasa block can further explain the east-west difference of the Lhasa block suggesting that the Indian plate may have experienced a rapid subduction process in this period.
【学位授予单位】：东华理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：P588.14;P597.3

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