【作者】 闫晶晶；

【导师】 赵志丹；

【作者基本信息】 中国地质大学(北京) ， 矿物学、岩石学、矿床学， 2016， 硕士

【摘要】 位于青藏高原西南部的拉萨地块,大面积地出露中生代岩浆岩,它们较好地记录了与高原构造演化与深部过程有关的的一系列重要信息,一直以来作为研究陆陆碰撞和造山运动的最佳的天然实验室而被广泛关注。近年来,对拉萨地块白垩纪岩浆作用已经有较好的研究,而关于侏罗纪岩浆活动的研究还相对较少,缺乏系统的岩石学、年代学和地球化学研究,对于侏罗纪岩浆活动的性质与地球动力学背景还存在一定的争议。本文对采自中部拉萨地块许如错岩体的花岗质岩及其闪长质包体样品进行了锆石LA-ICP-MS U-Pb定年、全岩元素地球化学以及锆石原位Hf同位素研究,通过分析岩浆形成的源区性质、岩石的成因过程等,试图揭示该区岩浆作用的深部动力学机制,对拉萨地块中生代与班公湖-怒江洋有关的构造岩浆演化提供更多证据。本文锆石U-Pb定年结果表明,寄主花岗质岩年龄为155.1±0.7Ma,闪长质包体为155.7±0.7Ma,指示二者同期侵位。在原始地幔标准化微量元素蜘蛛图上,寄主花岗质岩富集Rb、K等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,并且具负δEu异常;闪长质包体的原始地幔标准化微量元素蜘蛛图型式与寄主花岗质岩相似。寄主花岗质岩属未分异的I型偏铝质—弱过铝质高钾钙碱性系列,锆石εHf(t)值为-16.6~-6.6,地壳模式年龄(TDM C)为1.6Ga~2.3Ga,表明其可能来源于古老下地壳物质的重熔。闪长质包体为偏铝质钙碱性系列,锆石εHf(t)值-8.9~-3.8,地壳模式年龄(TDM C)为1.5Ga~1.8Ga,可能起源于富集岩石圈地幔的部分熔融作用。对已报道的中部及北部拉萨地块其他地区产出的中-晚侏罗世岩浆岩也作了系统的对比分析,结果表明:(1)岩石类型从偏铝质到过铝质均有出现,大部分A/CNK<1.1,为I型花岗岩;小部分A/CNK>1.1,为S型花岗岩;(2)北部拉萨地块产出的岩性较多样,有安山岩、英安岩、流纹岩以及花岗岩,部分εHf(t)>0,而中部拉萨地块主要为酸性岩(Si O2含量62%~78%),εHf(t)<0,中北部的εHf(t)变化范围大,表明可能有幔源岩浆的加入;(3)中部拉萨地块εHf(t)值表现出随年龄由老到新(从180Ma到130Ma)逐渐增长的趋势,而北部拉萨地块岩石的年龄记录出现在中部拉萨地块年龄记录范围的两端(135Ma~125Ma和180Ma~160Ma),在140Ma~160Ma范围内缺失,即在本文所研究的时间段内(从180Ma到130Ma),早于中部拉萨地块岩浆活动以及在中部拉萨地块岩浆活动的晚期出现。在135Ma~125Ma和180Ma~160Ma区间内,εHf(t)值大致表现出随年龄由老到新而逐渐减少的趋势;(4)中部拉萨地块样品的地壳模式年龄为古元古代—中元古代,北部拉萨地块样品的地壳模式年龄除了有中元古代外,还显示出较年轻的地壳模式年龄(<1000Ma),表明北部拉萨地块既发生了新生地壳的重熔,也包含有古老基底或者是卷入的中部拉萨地块古老基底的重熔或者深熔。应用本文获得的年代学及地球化学数据,结合区域上已发表的相关成果,本论文认为中部以及北部拉萨地块中侏罗世-早白垩早期岩浆活动更有可能是班公怒江洋南向俯冲至拉萨地块之下的产物,而非新特提斯洋壳的向北俯冲导致的。更多还原

【Abstract】 There are many Mesozoic magmatic rocks in the Lhasa Terrane, Tibet that has recorded many important informations of plateau tectonic evolution and deep process. The Lhasa Terrane has received considerable attentions by scholars as a natural laboratory of the continental-continental collision research. This years, many study of magma source regions and petrogenesis of the Cretaceous magmatites has been well conducted, while the study of the Late Jurassic granitoids in the central Lhasa Terrane, Tibet is still limited. In this paper, we focus on the Xuru Tso area, central Lhasa Terrane, select the host granites and dioritic enclaves and carry out a systematic petrography, zircon U-Pb gepchronology and _Hf isotopes,whole rocks elements study, attempting to provide more evidence for petrogenesis, tectonic setting and deep process in the area.According to the result of zircon U-Pb LA-ICP-MS dating of the Xuru Tso batholith, the age of the host granite（155.1±0.7Ma） is identical to that of the dioritic enclave（155.7±0.7Ma）, implying that they were the products of coeval magmatic events. The host granitic rocks are metaluminous to slightly peraluminous and high K calc-alkaline I-type granites, enrichment in LILE and LREE, lack of HFSE, and negative zircon ε_Hf（t）（-16.6^-6.6） values, indicating that they were derived from antaxis of ancient lower crust materials. The dioritic enclaves are metaluminous calc-alkaline and are characterized by negative zircon ε_Hf（t）（-8.9^-3.8） values, indicating that these enclaves were likely derived from partial melting of the ancient enrichment lithospheric mantle materials.Together with the published datas of the magmatic rocks of other areas of the central and northern Lhasa Terrane, this paper attempts to draw the conclusion that the granitoids from Xuru Tso area were formed by the varing extents mixing of mantle materials-dirved melts and crust-derived silicic melts from ancient basement of the central Lhasa Terrane in response to southward subduction beneath Lhasa Terrane of Bangong-Nujiang Ocean seafloor in Middle-Late Jurassic. And also, we believe that the coeval magmatic rocks of other areas of the central and northern Lhasa Terrane are the products of Bangong-Nujiang Ocean seafloor’s southward subduction.更多还原