南祁连吉尔孟岩体锆石U-Pb年代与构造环境分析

发布时间：2018-05-31 10:30

  本文选题：吉尔孟岩体 + 花岗岩　； 参考：《中国地质大学(北京)》2017年硕士论文

【摘要】：沿着柴北缘在南祁连一侧出露有一系列早古生代的侵入岩,本文研究区在南祁连青海湖北部吉尔孟地区,出露有花岗闪长岩、二长花岗岩和少量英云闪长岩等中酸性侵入岩,它们均侵位于早古生代地层,岩体年龄及成因对于南祁连地区早古生代的大地构造演化具有重要的意义。本文基于对该地区侵入岩体的野外调查、岩相学特征、锆石U-Pb年代学、全岩地球化学及Hf同位素特征研究,揭示吉尔孟岩体的结晶年龄与岩石成因类型,探讨了南祁连早古生代侵入岩的形成与大地构造演化之间的关系。利用高精度LA-ICP-MS锆石U-Pb定年测得6个样品的年龄,其中花岗闪长岩年龄为458Ma、439.6Ma、453.5Ma,二长花岗岩年龄为457.5Ma、455.8Ma,英云闪长岩年龄为448.4Ma,表明吉尔孟岩体形成于晚奥陶世-早志留世时期。地球化学特征显示吉尔孟岩体属于过铝质-强过铝质钙碱性-高钾钙碱性系列的英云闪长岩到二长花岗岩,兼具有I型和S型花岗岩的特征,为I-S过渡型花岗岩;微量元素蛛网图上富集大离子亲石元素K、Th、U等,亏损高场强元素Nb、Ta、P、Sr、Ti等,具有岛弧火山岩的特征;稀土元素球粒陨石标准化配分型式图上,呈现出轻稀土富集、重稀土相对平坦的右倾“V”字型模式。通过对吉尔孟岩体的锆石Hf同位素研究,获得两件花岗闪长岩样品的εHf(t)值分别分布于-1.4~-12.0及-7.8~-11.8之间,两阶段模式年龄TDM C(Ma)变化范围分别为1.50~2.17Ga(集中于1.70~2.17Ga)及1.92~2.17Ga。前者的Hf同位素组成变化范围较大,暗示其有幔源岩浆的混入,而后者具有较稳定的Hf同位素组成,暗示其来源于古老地壳基底的深熔。通过以上对南祁连吉尔孟岩体的研究,结合构造背景认为该地区花岗岩与南祁连洋向北俯冲消减具有密切的关系,岩浆来自于南祁连洋的俯冲到碰撞的构造体制转换背景下古元古代地壳基底的部分熔融与地幔底侵物质的不均匀混合作用。因此,吉尔孟地区的花岗岩是处于同一俯冲带内的不同时期形成于不同背景下的两类花岗岩,揭示了450~460Ma之间南祁连洋仍然处于向北俯冲的构造背景之下,而439Ma时南祁连洋已经处于闭合阶段,本文所研究的花岗岩形成于洋陆转换阶段。
[Abstract]:Along the northern margin of Qaidam, a series of early Paleozoic intrusive rocks were exposed on the side of South Qilian. In this study area, there are granodiorite, monzogranite and a small amount of dolomite and other intermediate-acid intrusive rocks in the Germeng area north of Qinghai Lake in South Qilian. Both of them are located in the early Paleozoic strata and the age and genesis of the rock mass are of great significance for the tectonic evolution of the early Paleozoic in the South Qilian area. Based on the field investigation, petrographic characteristics, zircon U-Pb chronology, whole rock geochemistry and HF isotopic characteristics of intrusive rock bodies in this area, this paper reveals the crystallization age and petrogenetic types of the Kilmeng rock mass. The relationship between the formation of early Paleozoic intrusive rocks and tectonic evolution in South Qilian is discussed. The age of 6 samples was determined by LA-ICP-MS zircon U-Pb dating with high accuracy. The age of granodiorite is 458 Ma-439.6Ma-453.5Ma., the age of monzogranite is 457.5 Ma-455.8Ma. the age of Yingyun diorite is 448.4 Ma. which indicates that the Gillmonite was formed in the late Ordovic-early Silurian period. The geochemical characteristics show that the Jilmeng rock body is a type of dolomite diorite to monzogranite of peralumina-strong peraluminous calc-alkaline high-potassium calc-alkaline series, which has the characteristics of both I-type and S-type granites, and is I-S transitional granite. The trace element cobwebs are enriched in large ion lithophile elements K _ (+) Thu and so on, and the depletion of high field strength elements, such as Nbtadao _ Ta-Ph _ (Sr) Ti and so on, are characterized by island arc volcanic rocks, and the normalized distribution pattern of chondrites of rare earth elements is characterized by light rare earth enrichment. Heavy rare earth relatively flat right-leaning "V" pattern. Based on the study of zircon HF isotopes of the Gillmund massif, the 蔚 HF ~ (t) values of two granodiorite samples are found to be between -1.4 ~ (-12.0) and -7.8 ~ 11.8, respectively, and the variation range of the two-stage model age TDM _ (C) Mais is 1.50 ~ 2.17Ga (concentrated at 1.702.17GaA) and 1.92kW _ (2.17Gaa), respectively. The HF isotopic composition of the former has a large variation range, which implies that it is mixed with mantle magma, while the latter has a more stable HF isotope composition, implying that it originated from the deep penetration of the basement of the ancient crust. Based on the above study of the South Qilian Gil Meng pluton and combined with the tectonic background, it is concluded that the granite in this area is closely related to the subduction of the South Qilian Ocean toward the north. The magma originated from the subduction to collision tectonic regime transformation of the South Qilian Ocean. The partial melting of the Gu Yuan ancient crustal basement and the inhomogeneous mixing of the mantle submersion material. Therefore, the granites in the Kilmeng area are two types of granites formed in different settings at different times in the same subduction zone, which reveals that the South Qilian Ocean between 450~460Ma is still in the tectonic setting of northward subduction. The South Qilian Ocean was already closed by 439Ma, and the granite studied in this paper was formed in the stage of oceanic transition.
【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】：P597.3;P583
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本文编号：1959304