【作者】 林金辉；

【导师】 孙传敏；

【作者基本信息】 成都理工大学 ， 矿物学 岩石学 矿床学， 2003， 博士

【摘要】 青藏高原的形成和演化经历了一个漫长的历程，而新生代以来的50Ma又是一个有着特殊地质意义的时期，藏北高原新生代火山岩正是在这一特定时期内形成的。其中高钾钙碱性系列火山岩(high-k calc-alkaline volcanic rock series)是一类重要且具有特殊成因指示意义的火山岩。 藏北高原新生代高钾钙碱性系列火山岩主体分布于昆仑山脉主脊的南侧和羌塘微板块北部，作为板块俯冲、碰撞及高原形成、演化过程中深部地质作用事件的浅部响应，是探索新生代以来高原岩石圈物质组成、壳幔结构及其相互作用方式以及高原隆升机制的“重要窗口”和“地质探针”，长期以来一直受到地学界的极大关注。 由于藏北高原自然地理条件极端恶劣、高寒缺氧、交通异常困难，平均海拔在5000m以上，是“世界屋脊”的核心，素有“生命禁区”和“地球第三极”之称。迄今，国内外有关青藏高原地质学、岩石学和大地构造学的著述不少，但大多偏重于高原的南部或东部，北部很少，尤其是关于藏北高原广泛分布的新生代高钾钙碱性火山岩的区域地质调查和系统研究尚属空白。作者历经五个春秋的艰苦拼搏，通过大量的野外地质科学考察和地质填图，夯实了野外第一手资料，将火山活动的研究置于高原形成和演化的总体框架之内，视为新生代高原地质历史过程中一个重大的地质事件响应，应用区域地质学、岩石学、矿物学、同位素年代学、地球化学等学科的新研究方法和手段，从时空分布、岩相特征、岩石和矿物化学、微量元素和稀土元素地球化学、同位素年代学和同位素地球化学等方面对藏北高原新生代高钾钙碱性火山岩进行系统研究，取得了如下认识和进展。 1．查明了藏北高原新生代高钾钙碱性火山岩的分布、产状、喷发类型和地层层序。其主体分布于东经87°～91°、北纬33°40′～35°范围内，出露面积约6000km~2，集中分布在祖尔肯乌拉山、多格错仁—太平湖、错尼—乱青山、永波湖、跃进拉和美日切错等地区，其中以祖尔肯乌拉山地区出露面积最大，约为2500km~2，厚度10m～424m；火山岩产状平缓，主要以熔岩台地和熔岩被方式产出，呈面状展布，为典型的陆相中心式喷发；岩相类型主要为爆发相和喷溢相，火山岩地层层序自下而上可划分为火山角砾岩段、橄榄玄粗岩段、安粗岩段和粗面岩段等四个岩性段，其中安粗岩段和粗面岩段构成了藏北高原新生代高钾钙碱性火山岩的主体。 2．K-Ar、Ar-Ar同位素年代学系统测试结果分析表明，藏北高原新生代高钾钙碱性火山岩年龄介于45Ma～35Ma之间，特别是通过Ar-Ar同位素年龄的精确限定，其喷发时代集中分布于40Ma左右，可大致视为同期火山活动。不仅为藏北高原新生代高钾钙碱性火山岩形成时代提供了新的同位素年龄时限，更正了许多学者认为藏北高原新生代高钾钙碱性火山岩主要形成于中新世以后，始新世藏北高原火山活动相对宁静的看法，而且为藏北高原富集n型地慢形成和岩石圈增厚、减薄及隆升等深层次地质作用时限提供了同位素年代学的约束，始新世藏北高原广泛发育的高钾质钙碱性火山熔岩与下伏不同时代的沉积地层呈角度不整合接触，下伏最新陆相沉积地层时代为sl.7Ma~46.2Ma，与野外地质事实相吻合。 3.根据主要造岩矿物组成、含量和TAS化学成分分类图解，藏北高原新生代高钾钙碱性火山岩以中性岩类占绝对优势，主要为一套安粗岩一粗面岩岩石组合，其次为安山岩一英安岩岩石组合，相当部分火山岩具有过渡性质。 4.藏北高原新生代高钾钙碱性火山岩化学成分较稳定，510:含量集中分布于55%~65%之间，A1203含量13.03%~17.77%，TIO:平均值0.51%，全碱(KZO例贬。)含量为5.470/005.84%，K20/N aZo比值0.57一1 .62，显示出富碱的特征，KZO含量峰值为2.5%、口4.0%，属典型的高钾钙碱性岩系，不存在拉斑玄武岩系列。CIPw标准矿物组合主要为Qz+o什Ab+An+Hy，大部分属于510:过饱和的正常岩石类型。 5.对藏北高原新生代高钾钙碱性火山岩中最重要的造岩矿物及麻粒岩包体中变质矿物进行了系统的电子探针矿物化学分析，为探讨火山岩熔体的生成和岩浆演化提供了大量的信息。 6.藏北高原新生代高钾钙碱性火山岩强烈富集大离子亲石元素K、Rb、sr、Ba、Th、Pb等，亏损相容元素C。、Ni和Cr;具有类似的不相容元素分布型式，高场强元素Ta、Nb、Zr、H么Ti、P等相对其它大离子亲石元素为负异常，出现Th、Ce、Sm三个峰值和Nb、P、Ti三个亏损槽。火山岩稀土总量高，平均364.10xl。一，LREE强烈富集，且有相似的稀土元素配分型式，均属强烈右倾轻稀土富集型，基本不显示或具有微弱的负铺异常。Sr、Nd、Pb同位素成分系统测试分析表明，藏北高原新生代高钾钙碱性火山岩具有相对高的“75护6sr和低的’4加‘144Nd值及高的Pb同位素组成，且变化范围很窄。“，s沪65:比值介于0.707101一0.707998之l’ed，明显地高于均匀储集库的(s，s沪6sr)皿现代值0.7045;’43N出，44Nd比值为0.512379司.512480，明显低于球粒陨石均一储集库现代值0.512638，限制了其源区具有壳一慢物质混合的性质。 7.岩石地球化学和同位素地球化学提示了藏北高原新生代高钾钙碱性火山岩不是直接更多还原

【Abstract】 The formation and evolution of the Tibetan Plateau, which occupies a very important position in contemporary geological research field, has gone through a very long geological time and the last 50 Ma is thought as the most important period with specific geological significances, during which the Cenozoic high-k calc-alkaline volcanic rocks occurred in the Northern Tibetan Plateau.The main part of the Cenozoic high-k calc-alkaline volcanic rock series distributes at the south margin of the prime back of the Kunlun mountain and at the Northern part of the Qiangtang mini-plate. As the respond in the shallow position of such deep geological events as the subduction and collision of the plates and the formation and evolution of the Tibetan Plateau, the Cenozoic high-k calc-alkaline volcanic rocks are thought as the important window and geological probe for researching the material composition of the lithosphere, the structure and interaction of the crust-mantle in the plateau and the mechanism of the plateau uplift. The rocks have been followed with great interest by geologists for a long time.Although some publications on geology, petrology and architectonics in Qinghai Tibetan Plateau have been involved, most of writings concentrate on researching field in Eastern and Southern Tibet. Due to such rough conditions as extremely cold, less oxygen, bad transport in this area with the elevation more than 5000 m, systematic study and regional geologic survey on the widely distributed Cenozoic high-k calc-alkaline volcanic rocks in Northern Tibetan Plateau have not been reported. The writer has worked for more than 5 years and got magnanimous first-hand field geological data in this area. In this dissertation, the volcanism study is put into the general framework of the formation and evolution of the whole plateau and the volcanism is thought as an important geologic event. Based on the systematic study of regional geology, petrology, petrography, mineralogy, isotopic geochronology and geochemistry, the following conclusions about the Cenozoic high-k calc-alkaline volcanic rocks in the Northern Tibetan Plateau are obtained.1. The distribution, occurrence, eruption type and strata sequence of the Cenozoic high-k calc-alkaline volcanic rocks were cleared through investigation. The main parts of the rocks distribute in the range of E 87~91, N3340’ -35, with an outcrop area about 6000 km2. The high-k calc-alkaline rocks are mainly seen at Znerkenwula mountain region, Duogecuoren-Taiping lake region, Cuoni-Luanqing mountain, Yaojinla region and Meiriqiecuo region. In the Zuerkenwula region, the area of the volcavic rocks is the biggest (about 2500 km2) and the thickness is from 10 to 424 m. The volcanic rocks in this region are with gentle inclinations,exposed mainly as lava plateau and lava sheet. Typical continental central eruption can be found and two facies, explosion and extrusion-overflow facies can be mainly distinguished. Upwards, 4 lithological segments(volcanic breccia segment, olive basaltic trachyandesite segment, latite seqment and trachyte segment) can be divided in the volcanic sequence, among with the trachyte and latite segments can be commonly seen in the Cenozoic high-k calc-alkaline volcanic rock area in the Northern Tibetan Plateau.2. The K-Ar, Ar-Ar isotopic chronology measurement result shows that the ages of the high-k calc-alkaline rocks are from 45 to 35 Ma. The Ar-Ar isotopic age exactly prescribes the limit to the volcanic eruption at about 40 Ma and it can be regarded as synchronous volcanic movement. It not only provides with new ages of the Cenozoic high-k calc-alkaline volcanic rocks and makes corrections of the theories as many researchers believe that the high-k calc-alkaline volcanic rocks mainly occurred after the Miocene and during the Eocene there was no obvious volcanic movements but provides isotopic chronology binding to time limit of the formation of enriched EII mantle and the thickening, thinning and uplifting of the lithosphere in the Northern Tibetan Plateau. The Eocene hi更多还原