【作者】 周肃；

【导师】 莫宣学；

【作者基本信息】 中国地质大学（北京） ， 矿物—岩石—矿床， 2003， 博士

【摘要】 雅鲁藏布江缝合带及其北面的冈底斯—念青唐古拉构造—岩浆岩带是青藏高原内中—新生代岩浆活动最强烈的地区，经历了从洋盆形成—洋壳俯冲—大陆碰撞—后碰撞事件，以及高原形成演化的种种复杂地质作用，是研究特提斯形成演化及青藏高原隆升过程的理想地区。然而，过去该区的同位素年代学研究工作无论在数量和质量上都显得不足。本论文通过35个样品的⁴⁰Ar-³⁹Ar及19个样品的Sm-Nd同位素测年，获得了关于俯冲—碰撞阶段形成的林子宗火山岩、后碰撞火山岩与花岗岩及雅鲁藏布蛇绿岩的同位素年龄数据，为完善研究区构造—岩浆事件的时间格架提供了新的资料与成果。 根据一条主干剖面9个样品，12次实验系统的⁴⁰Ar-³⁹Ar测年结果，将林周盆地林子宗火山岩的喷发年龄厘定为54—64Ma，其中帕那组：54—56Ma，年波组为57Ma，典中组为61—64Ma。建立了林子宗组火山岩形成的时间框架。 羊应乡火山岩样品中透长石及黑云母的⁴⁰Ar-³⁹Ar定年结果确认该区存在后碰撞火山岩，其喷发年龄在11Ma左右。其地球化学特征与该区52Ma左右的林子宗帕那组火山岩有明显区别。 对邬郁盆地嘎扎村组火山岩剖面的年代学研究表明，该区后碰撞期火山岩的⁴⁰Ar-³⁹Ar年龄为12—14Ma，侵入其中的花岗岩脉的⁴⁰Ar-³⁹Ar年龄为10.84Ma，与冈底斯西段前人研究资料对比，认为冈底斯带后碰撞火山岩时代有自西向东逐渐年轻的趋势。 西藏雅鲁藏布蛇绿岩带典型岩体：罗布莎、昂仁、休古嘎布岩体辉长岩Sm-Nd矿物内部等时线年龄值为177Ma、166Ma、173Ma，日康巴岩体的⁴⁰Ar-³⁹Ar年龄也为180Ma，显示区域上在早—中侏罗世沿该带有较强的岩浆活动，175Ma是雅鲁藏布蛇绿岩带岩浆活动的主体时间。根据Pb、Sr、Nd同位素初始值，认为早-中侏罗世西藏雅鲁藏布江蛇绿岩所代表的特提斯洋具有印度洋MORB型地幔地球化学域特征。 当穹岩体（?）辉辉长岩Sm—Nd矿物内部等时线年龄为373±28Ma，认为反映了晚泥盆世岩浆活动。其εNd（T）=+3.3，认为其时形成环境为洋岛，可能代表了新特提斯蛇绿岩形成时所携带的古老洋壳。 ⁴⁰Ar／³⁹Ar阶段升温年龄图谱显示，100Ma左右罗布莎岩体经历了较强的后期蚀变作用，对其K—Ar体系造成明显影响；77±1Ma休古嘎布岩体围岩中形成玄武质火山集块岩；55Ma日康巴岩体岩石枕状玄武岩受到较强热事件影响；这些⁴⁰Ar-³⁹Ar年龄上的变化显示了印度板块与欧亚板块俯冲—碰撞对该区的强烈影响。 通过对罗布莎岩体辉长岩脉作重点解剖，研究了三个辉长岩岩脉的矿物、全岩^?Ar／³⁹Ar、K-Ar和Sm-Nd年龄，对基性岩定年方法及测年对象的选用进行了初步研究及评价。认为对蛇绿岩这类形成于水下环境，含钾极低的样品，其岩体的形成年龄宜选用对蚀变影响反应不明显的Sm-Nd方法测定；但阶段升温^?Ar-^?Ar年龄图谱常常能反映后期地质作用对同位素体系的影响，与其它测年手段配合使用，可以更多地了解蕴含在岩石中的丰富地质信息，准确地反演其形成、演化的过程。更多还原

【Abstract】 The Gangdese-Nianqing Tanggula tectonic-magmatic belt locating in the north of Yarlung Zangpo suture and the Yarlung Zangpo ophilite suture are an ideal district of studying the formation and development of Tethyan as well as the process of Tibetan plateau uplifting. It is the most widespread magmatic activities within Tibetan plateau during Mesozoic and Cenozoic, and has undergone such accidents as the ocean basin formation, ocean crust subduction, continental collision and post-collision, as well as plateau formation and evolution and all sorts of complicated geological functions. Nevertheless, the geochronology research in this area in the past is inadequate in both quantity and quality. By having conducted 40Ar-39Ar dating of 35 samples and Sm-Nd dating of 19 samples from the Linzizhong volcanic rocks, the post-collision volcanic rocks and a granite dike and the Yarlung Zangpo ophilite, the dissertation provides significant new data and results for optimizing the time framework of tectonic-magmatic events of this region. According to isotopic dating of 9 samples, 12 experiments survey in the main trunk profile, the eruption age of the Linzizhong volcanic rock in the Linzhou basin has been determined as 54-64Ma. among them Pana group 54-56Ma,Nianbo group 57Ma, Dianzhong group 61-64Ma.The 40Ar-39Ar dating of the Sanidine and biotite in the 5 volcanic rock samples from the Yangying Xiang confirmed that it was post-collisionaal volcanism and gave an extrusive age of UMa. Their geochemical characteristics are different from those in Pana fromation of Linzizhong group.Geochronology research on the volcanic rock profile of the Gazha Cun Group in the Wuyii basin indicates that the 40Ar-39Ar age of the post-collision volcanic rock is 12-14Ma. and the granite that cut the volcanic rock is 10.84Ma. Comparing to the data of others, we infer that the age of the post-collision volcanic rock in Gangdese area is younger eastwards.The results of combined internal mineral separates and whole-rock Sm-Nd isochron age for gabbro dikes from: Luobusha, Angren and Xiubugabu in Yarlung Zangpo ophilite zone are 177Ma, 166Ma and 173Ma respectively. The 4lAr-39Ar step heating spectra of Rikangba ophiolite suggest that its formation age is also ISOMa. It indicates that there was a stronger regionally inagmatism during Middle-Early Jurassic, and the time of I75Ma was the main period of the magmatic event of Yarlung Zangpo ophilite. Based on the measurements of Pb. Sr,Nd initial values, we conclude that Tethyan ocean represented by Yarlung Zangpo ophilite possesses the Indian-Ocean-type isotopic affinity.The internal mineral separates and whole-rock Sm-Nd isochron age for gabbro dikes from the Dangqiong ophiolite is 373?8Ma, inferring a magmatic activity during Late Devonian, which eNd(t) = +3.3 suggests that its tectonic setting was probably oceanic island. We deduce that it is a relic old crust that was brought upwards during the formation of Neo-Tethyan ophiolite.The step heating 40Ar-’9Ar spectra show that during 100Ma, Luobusha ophiolite suffered intensely alternation, which affected distinctly the K-Ar system; during 771Ma some basalt formed in the country rock of Xiubugabu ophiolite; during 55Ma, the pillow basalt of Rikangba ophiolite affected by a strong thermal event; All the variety of ages illustrates possible impacts of the subduction-collision between India and Eurasia.Through dissecting mainly the gabbro of Luobusa ophiolite and studying the 40Ar /39Ar. K-Ar and Sm-Nd ages of the mineral separators and rocks of three gabbro rock among them, the method of dating and choosing object for dating on the base rock has been preliminarily evaluated. The author suggests that for those kinds of samples like ophiolite which formed in the underwater environment and had very low potassium content, it should be better to determine their formation age by the Sm-Nd method as it is less sensitive to alteration. However, the step heating 40Ar-39Ar spectra could often reflect the influence of later geological events on isotope更多还原