拉萨地块晚三叠纪-晚侏罗纪岩浆-构造演化

发布时间：2018-03-11 01:33

  本文选题：青藏高原　切入点：拉萨地块　出处：《中国科学院广州地球化学研究所》2017年博士论文　论文类型：学位论文

【摘要】：近年来,拉萨地块新生代时期的高原隆升历史和成矿作用研究已经取得了一系列突破性进展,但对中生代(尤其是晚三叠纪-晚侏罗纪)所经历的大洋俯冲和陆/弧-陆碰撞过程,目前还存在很大争议。重塑拉萨地块中生代构造演化历史不仅是研究新生代碰撞作用的前提,更重要的是可以查明拉萨地块发育的中生代时期的大型-超大型矿床形成的动力学背景。基于以上目的,本学位论文选取拉萨地块南部和北部中生代侏罗纪代表性岩浆岩和蛇绿岩为研究对象,取得了如下的研究进展:(1)位于拉萨地块北部的达如错火山岩(~162 Ma)是班怒带中段首次发现的侏罗纪火山岩,其地球化学组成具有高镁安山岩的特征。此外,达如错火山岩还具有高的Th含量和(La/Sm)N比值,低的Ba/Th和Sr/Y比值以及富集的SrNd-Hf同位素组成,特别是其εNd(t)值与区域沉积物的εNd(t)值的一致,表明它们是沉积物融体与地幔楔反应的产物,形成于高温低压的环境。野外考察表明,达如错火山岩是藏北湖蛇绿混杂岩的一部份,并不能作为班怒洋南向俯冲的证据,结合区域上岩浆-构造分布特征,其形成与班怒洋分支向北的初始俯冲有关。(2)首次确定位于拉萨地块北部的永珠蛇绿岩中席状岩墙群的锆石年龄为~150 Ma,表明拉萨地块内部在晚侏罗纪时期存在海底扩张。永珠镁铁质岩石地球化学成分具有以E-MORB为主,P-和N-MORB次之,不具有Nb-Ta负异常等特征,暗示它们形成于洋中脊环境。永珠蛇绿岩中还含有斜方辉石为主的斜方辉石岩和二辉辉石岩,全岩和矿物成分表明它们源区是一个高度亏损的地幔,由玻安岩类似的岩石堆晶形成。Re-Os同位素研究表明辉石岩源于经历过部分熔融后的残余软流圈地幔,而橄榄岩则源于古老的大陆岩石圈地幔,但它们是大陆裂解时同时拉出至地表的。结合区域地质、橄榄岩、沉积等多方面的证据,我们认为永珠蛇绿岩代表了拉萨地块大陆岩石圈裂解形成的初始洋盆,而不是前人所认为的弧后盆地。(3)日喀则东嘎地区的角闪岩,闪长岩等形成于早侏罗纪(~180 Ma),闪长岩是源于流体交代地幔楔产生的富H2O岩浆结晶分异形成,而角闪岩是这些富H2O岩浆分异过程中由角闪石为主的矿物堆晶形成。由于它们与区域上同时期的长英质岩石有着相似的Sr-Nd同位素组成,结晶分异在这些长英质岩石的形成过程中起着十分重要的作用。尽管南拉萨地块存在大规模的早侏罗纪岩浆作用,但拉萨市以西只有侵入岩出露,而以东则既有火山岩也有侵入岩出露。地球化学组成对比表明拉萨市以西的侵入岩比拉萨市以东的火山岩和侵入岩源区具有更高的H2O含量,可能和新特提斯洋北向俯冲差异有关。(4)结合前人研究,我们提出了一个新的拉萨地块晚三叠纪-晚侏罗纪时期的构造演化模式:新特提斯洋在晚三叠纪-早侏罗纪开始向北俯冲,在南拉萨地块形成了以叶巴组和桑日群为代表的火山岩及同时期的侵入岩;与此同时,班公-怒江洋盆向北俯冲到羌塘地块之下,但很可能并不存在南向俯冲;由于大洋俯冲的远程效应,拉萨中部大陆岩石圈开始伸展和裂解,形成了狮泉河和拉果错等蛇绿岩。晚侏罗纪时期,新特提斯洋洋壳转为平板俯冲,以及班公-怒江洋持续的向北俯冲,进一步促进了拉萨中部地块岩石圈伸展和裂解,形成了以永珠蛇绿岩为代表的初始洋盆。
[Abstract]:In recent years, the Lhasa block Cenozoic uplift history research and mineralization has made a series of breakthroughs, but the Mesozoic (especially late Triassic to late Jurassic) oceanic subduction and continental / arc continental collision process, there are a lot of controversy. The Lhasa Mesozoic tectonic block remodeling history is not only the premise of Cenozoic collision, more important is to identify dynamic background of formation of large and super large deposits of Lhasa land development of the Mesozoic era. Based on the above purpose, this paper selects the Lhasa southern and Northern representative plots of the Mesozoic Jurassic ophiolite and magmatite as the research object, made research progress as follows: (1) is located in the northern part of the Lhasa block was wrong as volcano rock (~162 Ma) is a class of Middle Jurassic volcano rock anger first discovered, the geochemical composition. Have the characteristics of high Mg andesite. In addition, as if the wrong volcano rock also has a high content of Th and (La/Sm) N ratio, low Ba/Th and Sr/Y ratios and enriched SrNd-Hf isotopic composition, especially the epsilon Nd (T) value epsilon Nd and regional sediment (T) values, indicating that they is a product of sediment melt and mantle wedge formed in the reaction, high temperature and low pressure environment. The field survey, as if the wrong volcano rock is a part of Tibetan Lake OPHIOLITIC MELANGE, and cannot be used as evidence of the southward subduction of Bannu ocean, combined with regional magmatic tectonic characteristics, its formation and Bannu the initial subduction northward branch. (2) for the first time to determine the zircon age of sheeted dike swarms Yunzhug ophiolite is located in the northern part of the Lhasa block in ~150 Ma, showed that Lhasa block within the seafloor spreading in the late Jurassic period. Yunzhug mafic rocks have geochemical composition based on E-MORB Lord, P- and N-MORB, with Nb-Ta negative anomalies, suggesting that they form a ridge Yu Yang. Yunzhug ophiolite also contains orthopyroxene mainly orthorhombic pyroxenite and two are Shiyan, whole rock and mineral compositions show that they source region is a highly depleted mantle, from boninite similar rocks cumulate rock formation of residual.Re-Os isotope studies indicate that the pyroxenite derived from experienced partial melting of the asthenospheric mantle peridotite, and is derived from the ancient continental lithospheric mantle, but they are at the same time, cracking out to the surface. Combined with regional geology, sedimentary peridotite, etc. the evidence, we consider the initial ocean basin Yunzhug ophiolite represents the Lhasa massif of continental lithosphere cracking formation, rather than previously thought back arc basin. (3) Shigatse Dongga area of amphibolite, diorite formed in early Jurassic (~180 Ma), flash Long rock is derived from magmatic crystallization of H2O rich fluid metasomatized mantle wedge generated differentiation, and they are dominated by amphibole mineral these H2O rich magmatic differentiation during cumulate formation. Because of their area of the same period and felsic rocks have similar Sr-Nd isotopic composition, crystallization and differentiation in the formation process of these felsic rocks plays a very important role. Despite the massive early Jurassic magmatism in South Lhasa block, but only the west of the city of Lhasa invaded Iwade and East Lu, both volcano rocks have invaded Iwade Lu. Geochemical contrast group showed that the intrusive rocks west of the city of Lhasa with H2O the content of higher than the source rock in the East of the city of Lhasa volcano rocks and intrusive, and Neo Tethys northward subduction related differences. (4) based on previous research, we propose a new Lhasa block Late Triassic to late Jurassic During the period of tectonic evolution pattern: Ji Neo Tethys northward subduction began in the Late Triassic and early Jurassic, in South Lhasa block formed by the Yeba SANGRI group and as the representative of the volcano rocks and intrusive rocks of the same period; at the same time, the Bangong Lake Nu River basin subducting northward beneath the block, but probably not there is a southward subduction; because the remote effect of oceanic subduction, continental lithosphere in central Lhasa began to stretch and cracking, and formed the Shiquanhe Lagkor co ophiolite. Such as Jurassic period, Neo Tethys ocean crust to plate subduction, and the class public - Nu River Yang continued to further promote the northward subduction of lithospheric extension and central Lhasa block cracking, formed the initial ocean basin to the yonzhug ophiolite represented.

【学位授予单位】：中国科学院广州地球化学研究所
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P542
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本文编号：1596016