藏南冈底斯中段南缘构造演化

发布时间：2018-01-22 07:21

本文关键词： 冈底斯花岗岩类韧性剪切带裂变径迹构造演化　出处：《中国地质科学院》2016年博士论文　论文类型：学位论文

【摘要】：冈底斯岩浆带位于拉萨地体南缘,是新特提斯洋盆演化和印度-亚洲陆陆碰撞的岩浆产物,对冈底斯岩浆带的研究方兴未艾,但一些基本问题仍然存在着激烈的争论(比如晚三叠到早-中侏罗世新特提斯洋演化的极性,冈底斯岩浆带中大型韧性剪切带形成的时代、动力学机制以及冈底斯中段新生代的隆升-剥蚀模式等等),这都直接影响了对新特提斯域构造和青藏高原形成演化的理解,以及对冈底斯构造成矿带的资源评价。本文选择冈底斯岩浆带中段南缘地区作为研究对象,开展了详细的野外地质调查,首次在日喀则南木林和拉萨曲水地区发现和厘定了晚三叠纪的侵入岩体,并开展了详细的岩石学、矿物学、地球化学、锆石Lu-Hf同位素分析和年代学研究；对早-中侏罗世的花岗岩类及次火山岩开展了岩石学、年代学和锆石Lu-Hf同位素研究；对冈底斯中段高海拔地区的始新世花岗岩类进行了地球化学和年代学研究；对冈底斯中段谢通门-曲水韧性剪切带和曲水地区色甫-鸡公韧性剪切带进行了详细的野外地质调查、显微薄片鉴定、EBSD研究、云母40Ar-39Ar和锆石U-Pb定年等；在前人研究的基础上,对冈底斯中段进行选择性采样,补充了新的磷灰石裂变径迹数据；综合分析上述研究资料,并结合前人研究成果以及区域地质特征,试图建立和探讨冈底斯中段南缘的构造演化模式。1.南木林卡孜乡糜棱质花岗岩形成于206-212Ma之间,形成环境为活动大陆边缘,锆石的εHf(t)值从8.95到12.91,一阶段模式年龄326-486Ma;曲水其奴角闪辉长岩侵位结晶年龄为-210Ma,锆石的εHf(1)值从9.56到14.75,一阶段模式年龄为256～459Ma;曲水达嘎花岗岩体的侵位结晶年龄为225-230Ma,具有正的εHf(t)值(13.91到15.54);地质年代学、岩石学、矿物学、地球化学以及锆石Lu-Hf同位素等显示,晚三叠纪的花岗岩、角闪辉长岩和新特提斯洋的向北俯冲有关,其中角闪辉长岩和俯冲板片流体交代地幔楔的部分熔融有关,花岗质岩石和初生地壳的部分熔融有关,并且地幔物质在花岗质岩石的形成过程中扮演有重要的角色。晚三叠纪的岩浆作用暗示了新特提斯洋的俯冲应该不晚于230Ma。2.通过对冈底斯中段南缘日喀则奴玛地区和尼木地区的闪长岩、花岗闪长岩、花岗岩以及次火山岩的地质年代学、地球化学、矿物岩石学以及锆石Lu-Hf同位素等综合研究,获得了如下认识：花岗岩类及次火山岩主要形成于169～191Ma,为早-中侏罗世晚期岩浆作用的产物;在化学组成上,花岗岩类及次火山岩富集轻稀±(LREEs)和大离子亲石元素(LILEs),强烈亏损Nb、Ta等高场强元素,具有岛弧型花岗岩的地球化学特征；花岗岩类为亚碱性,属中钾钙碱性I型花岗岩类;样品的εHf(t)值为10.10-15.44,具有明显亏损锆石的Hf同位素组成；综合研究表明,早中侏罗世应该和晚三叠纪具有一样的大地构造环境,即藏南冈底斯地区(活动大陆边缘)应属于新特提斯洋板片俯冲的构造背景,花岗质岩石的形成主要是来自中下地壳物质的部分熔融。3.对冈底斯中段始新世的花岗岩类(闪长岩到花岗岩斑岩)进行了锆石U-Pb年代学、Lu-Hf同位素和地球化学分析。分析结果显示,花岗岩类锆石的LA-ICP-MS U-Pb定年结果为40-55Ma之间,该年龄代表了花岗岩类的侵位结晶年龄；在地球化学组成上,花岗岩类富集轻稀土(HREEs)和大离子亲石元素(LILEs),强烈亏损高场强元素Mb、Ta、P等,样品显示出岛弧型花岗岩的特征。此外,分析样品还表现为准铝质I型花岗岩类的特征,主要位于钙碱性到高钾钙碱性系列区域。锆石的Lu-Hf同位素分析结果显示,多数样品的εH(t)以正值为主,少数锆石为负的εH(t)值,结合地球化学成分判别图解,花岗岩类主要是新生地壳的部分熔融一由变基性到变英云闪长质岩石的部分熔融以及变砂岩的部分熔融,在此过程中,底侵的地幔物质也参与了花岗质岩石的形成和演化。在Sr-Yb图解上始新世的样品表现为碰撞期花岗岩类的地球化学特征,因此也暗示了印度-亚洲在始新世早期时已经完全碰撞。始新世冈底斯地区的岩浆作用可能是由于新特提斯洋板片俯冲到欧亚板片之下,由于印度-亚洲大陆的碰撞,致使新特提斯洋板片俯冲角度变陡,并且发生了板片断离(slab break off),引发了始新世冈底斯地区强烈的岩浆-火山作用。4.通过野外详细的地质调查、构造解析以及室内薄片鉴定和EBSD组构分析,确定了谢通门-曲水韧性剪切带的构造属性和演化特征。谢通门-曲水韧性剪切带主要由强变形域和弱变形组成,包括面理化花岗岩、糜棱岩化花岗岩、初糜棱岩、糜棱岩组成,局部地区可见千糜岩。几何学和运动学研究结果显示谢通门-曲水韧性剪切带具有一致的运动学特征,表现为以向北的正向滑覆为主。此外,运动学涡度显示(Wk0.71)该剪切带为典型的简单剪切为主的一般剪切,为伸展减薄型剪切带,平均减薄量为20%。剪切带的EBSD组构分析显示了谢通门-曲水韧性剪切带主要滑移系以柱面a、菱面a和底面a滑移为主,变形主要以中低温作用为主(550～300℃)。5.谢通门-曲水韧性剪切带中的黑云母、绢云母40Ar-39Ar年代学以及锆石U-Pb年代学共同限定了剪切带活动的时限。综合年代学结果表明：谢通门-曲水韧性剪切带形成于中新世早期21～24Ma之间。6.经过详细的野外地质调查、室内显微薄片鉴定、EBSD组构分析等研究认为色甫-鸡公韧性剪切带以右行走滑为主。长石-石英变形矿物对以及石英EBSD组构研究结果显示,鸡公-曲水韧性剪切带发生变形的温度为500-550℃C。其次,通过对剪切带中发育的同构造长英质脉体的锆石U-Pb年代学分析,获得了色甫-鸡公韧性剪切带活动的时限35～38Ma,即始新世晚期。7.通过对冈底斯中段11件磷灰石样品的裂变径迹分析,获得了中段南缘的构造隆升剥蚀史。研究结果显示：冈底斯中段的隆升是多阶段性的,渐新世以前的抬升、剥蚀和印度-亚洲板块的碰撞有关,23Ma以来的快速隆升和冈底斯地区逆冲断裂的活动有关,并且在快速隆升阶段也存在短暂的构造平静期。10Ma以来是冈底斯中段南缘地区又一次快速的隆升期,该次隆升和雅鲁藏布江的快速下切侵蚀有关。第四纪以来在构造活动和气候变化以及地表径流的联合作用下,共同塑造了现今的冈底斯。8.综合结果表明：冈底斯岩浆带经历了一个长期的演化过程,从晚三叠纪开始俯冲消减(230Ma),直到60～55Ma时印度-亚洲大陆的碰撞。始新世印度亚洲的碰撞导致了逆冲断裂和大型走滑断层的形成,并导致了新生代冈底斯岩基及邻区的第一次整体的隆升剥蚀。23Ma以来,由于藏南地区地壳侧向增厚变化不均匀以及南北向应力挤压的松弛,导致了拉萨地体发生东西向的崩塌和南北向的伸展,此时大型正断层以及拆离构造应用而生,同时加厚的地壳发生部分熔融而形成了具有特殊地球化学属性的花岗质斑岩,此时也是冈底斯地区成矿大爆发时期。
[Abstract]:The magmatic belt is located in the Lhasa terrane, is the magma evolution and Neo Tethyan ocean India Asia continental collision, the Gangdese magmatic belt just unfolding research, but some basic problems still exist controversy (such as late three to Early Triassic Middle Jurassic - polarity, Neo Tethys evolution of the Gangdise magmatic belt formation the ductile shear zone of the era, dynamic mechanism and the Middle Cenozoic uplift and erosion pattern and so on), which have a direct impact on the formation and evolution of the understanding of the new Tethys domain structure and the Qinghai Tibet Plateau, and the Gangdese metallogenic belt of the resource evaluation. This paper chooses the Gangdese magmatic belt in the South area as the research object. To carry out detailed geological survey, first found in Shigatse and Lhasa area and QUXU Namling determined the intrusive rock in Late Triassic period, were carried out. With the characteristics of petrology, mineralogy, geochemistry, zircon Lu-Hf geochronology and isotope analysis; in the early Middle Jurassic granitoids and volcano rock was carried out research on petrology, geochronology and Lu-Hf isotope; the central Gangdise high altitude Eocene granites of Geochemistry and geochronology of the mid Gangdise Xietongmen; - QUXU ductile shear zone and the area of color - Fu QUXU Jigongshan ductile shear zone of detailed geological survey, the research of optical microscope, EBSD, mica and 40Ar-39Ar zircon U-Pb dating; on the basis of previous research on mid Gangdise selective sampling, added the new apatite fission track data; comprehensive analysis the above research data, combined with the previous research, the regional geological features, trying to establish and study the southern central Gangdise tectonic evolution .1. card Namling Zi Xiang mylonitic granite formed in 206-212Ma, formed in the continental margin, epsilon Hf zircon (T) values from 8.95 to 12.91, a stage model of age 326-486Ma; the QUXU flash gabbro emplacement and crystallization age of slave Pt. -210Ma, epsilon Hf zircon (1) value from 9.56 to 14.75, a model age of 256 ~ 459Ma; Qushui granite Dagar emplacement and crystallization age of 225-230Ma, with epsilon Hf positive (T) value (13.91 to 15.54); geochronology, petrology, mineralogy, geochemistry and zircon Lu-Hf isotope show that late Triassic granite, northward subduction of bojite and the Neo Tethys ocean, the partial melting of hornblende gabbro and subduction fluid metasomatism of mantle wedge, granitic rocks and juvenile crust partial melting of the mantle, and played in the formation of granitic rocks in There is an important role. The Late Triassic magmatism that Neo Tethys subduction should not later than 230Ma.2. on the southern margin of the middle Shigatse area and the NUMA Nimu area diorite, granodiorite, granite and geochronology, geochemistry of volcano rock, a comprehensive study of mineral petrology and zircon Lu-Hf isotopes. The conclusions are as follows: Granite and volcano rock formed mainly in 169 ~ 191Ma, as a result of early Middle Jurassic late magmatism; in chemical composition, granite and volcano rock enrichment light rare + (LREEs) and large ion lithophile elements (LILEs), strong depletion of Nb, Ta. Field elements, geochemistry with arc type granite; granite sub alkaline, belong to calc alkaline I type granite; epsilon Hf samples (T) value is 10.10-15.44, with obvious loss of zircon Hf Isotopic composition; comprehensive study shows that the early Middle Jurassic and Late Triassic should have the same tectonic environment, namely Zangnan Gangdise area (active continental margin) should belong to the tectonic background of Neo Tethys subduction, granitic rocks are mainly formed from partial melting of.3. middle lower crust of middle Eocene Gangdise granite class (granodiorite to granite porphyry) of zircon U-Pb geochronology, geochemical and Lu-Hf isotopic analysis. Analysis results show that the granite zircon LA-ICP-MS U-Pb dating results for 40-55Ma, which represent the age of Granite Emplacement and crystallization age; in geochemical composition of granitoids, enrichment of LREE (HREEs) and large ion lithophile elements (LILEs), field strength elements Mb, Ta, P etc., the samples show characteristics of island arc granite. In addition, the analysis also shows the sample For the characteristics of quasi aluminous I type granite, mainly located in the calc alkaline to high potassium calc alkaline series. Lu-Hf isotope analysis results of zircon shows that most samples of the epsilon H (T) with positive and negative minority zircon as epsilon H (T) value, combined with the geochemical composition of granitoids mainly discrimination is the partial melting of juvenile crust partial melting of a mafic to tonalitic rock partial melting and sandstone, in this process, the mantle underplating is also involved in the formation and evolution of granitic rocks in Eocene. The Sr-Yb diagram of the sample showed the geochemical characteristics of syncollisional granite class, so it suggests that India - Asia in the early Eocene has completely collision magmatism. The Eocene Gangdise area may be due to the Neo Tethys subduction to Eurasian plate, the India Asia continental collision Hit, resulting in the Neo Tethys subduction angle becomes steeper, and the occurrence of slab break off (slab break off), triggered a strong magmatic Eocene in Gangdise volcano.4. through detailed field geological survey, structural analysis, thin section and EBSD fabric analysis, determine the xaitongmoin - Qushui ductile shear zone the tectonic setting and evolution. QUXU ductile shear mainly by strong deformation and weak deformation domain composed of Xietongmen zone, including foliated granite, mylonitize granite, protomylonite, mylonites, local area visible phyllonite. The geometry and kinematics of ductile shear belt - xaitongmoin QUXU display kinematic characteristics consistent with that is characterized by positive sliding northward. In addition, the kinematic vorticity (Wk0.71) showed that the shear zone is a typical general shear simple shear dominated, extensional thinning Shear zone, the average reduction of 20%. shear band of EBSD fabric analysis shows the xaitongmoin - Qushui ductile shear zones mainly slip in cylindrical a, rhombohedral A and bottom a slip, deformation mainly in low temperature effects (550 to 300 DEG C).5. - xaitongmoin QUXU ductile shear zone in the dark mica, mica and 40Ar-39Ar geochronology of zircon U-Pb geochronology together define the shear activity time. The results showed that the comprehensive chronology xaitongmoin - QUXU ductile shear zone formed in the early Miocene between 21 ~ 24Ma.6. through detailed geological survey, indoor insufficient sheet identification, EBSD fabric analysis of color - Fu chicken with dextral strike slip ductile shear zone. The deformation of feldspar quartz minerals and quartz EBSD fabric research results show that the ductile shear deformation with chicken - QUXU temperature of 500-550 C. followed by the shear zone Analysis of zircon U-Pb chronology isomorphism development in the making of felsic veins, the color of Fu - Jigongshan ductile shear zone activity time of 35 ~ 38Ma, the late Eocene.7. through the analysis of 11 samples of mid Gangdise apatite fission track, the southern margin of the middle of the tectonic uplift and denudation history. The research results show that: the middle uplift is the stage, before the Oligocene uplift, denudation and collision of India Asia plate, 23Ma since the rapid uplift of the Gangdise area and the thrust of the activities, and in the stage of rapid uplift since there are short quiet tectonic period is.10Ma and a southern central Gangdise area rapid uplift stage, the uplift and the Brahmaputra rapid downcutting. In Quaternary tectonic activity and climate change and surface runoff under the combined effect of mold together The results show that the.8. integrated Gangdise magmatic belt has undergone a long evolution process, from the Late Triassic to the subduction (230Ma), until 60 to the India Asia continental collision 55Ma. India Asia collision resulted in Eocene thrust faults and large strike slip faults, and caused the new Daigang Gangdese batholith and its adjacent areas, the first overall uplift since.23Ma, due to the relaxation of Zangnan regional crustal thickening and uneven lateral changes of North-South stress extrusion, resulted in the Lhasa terrane occurred east-west and North-South extensional collapse, the large normal fault and detachment structure application and at the same time. Partial melting of thickened crust formed has special geochemical properties of granitic porphyry mineralization, this is also the Gangdise area outbreak period.

【学位授予单位】：中国地质科学院
【学位级别】：博士
【学位授予年份】：2016
【分类号】：P542

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