商丹构造带东段辉长岩年代学和地球化学研究

发布时间：2018-04-08 10:19

  本文选题：东秦岭　切入点：商丹构造带　出处：《西北大学》2017年硕士论文

【摘要】：商丹构造带作为华北板块和扬子板块的分界线,是商丹洋发生俯冲、消减到闭合过程的产物。商丹构造带主体是一套绿片岩相至低角闪岩相的火山-沉积岩系,如蛇绿混杂岩、变质火山岩、变质碎屑岩、大理岩等。商丹构造带中还发育一系列酸性至超基性的侵入岩,包括辉长岩、闪长岩和花岗岩等,其中有很多呈透镜状线性展布的基性-超基性侵入岩体,这些侵入岩大都保存有完好的岩浆岩结构,成岩时代为早古生代。已有的研究对这些岩体的成因、演化有着不同的认识,且对丹凤地区基性-超基性侵入岩缺乏相应的研究。这些侵入岩的成岩时代、构造背景及岩浆成因对探讨北秦岭构造演化有重要的地质意义。本文在系统总结前人研究的基础上,选取东秦岭丹凤地区的古城沟辉长岩体作为研究对象,开展了详细的野外地质学、岩石学、地球化学和地质年代学等方面的研究,得到以下几点认识:(1)丹凤地区的古城沟辉长岩体规模较小,呈透镜状平行于商丹断裂带展布,与丹凤岩群火山岩呈侵入接触关系。岩体以辉长岩为主,也有少量辉长闪长岩和闪长岩。从岩体边缘向中心,岩石的基性程度变高,岩石的矿物颗粒变粗。古城沟辉长岩体中心部位变形较弱,边部变形程度较强,发育透入性构造面理,为辉长质初糜棱岩。(2)LA-ICP-MS锆石U-Pb定年结果表明,古城沟辉长岩体的加权平均年龄为425.1±1.7Ma(MSWD=0.10)和424.3±1.3Ma(MSWD=0.16),表明古城沟辉长岩形成于志留纪文洛克世。(3)古城沟辉长岩体具有富Na低K的钙碱性-亚碱性特征,稀土元素为右倾配分模式,LREE/HREE=8.03-11.95,LaN/YbN=9.82-17.64,δEu为0.93-1.31,Eu异常不明显或弱正异常。大离子亲石元素Cs、K、Sr、Ba明显富集,高场强元素U、Th、Nb、Ta、Hf、Zr相对亏损,Th/Ta值为6.00-13.67。古城沟辉长岩不属于商丹构造带内早古生代的蛇绿混杂岩岩石单元,其地球化学特征表明古城沟辉长岩体形成于活动大陆边缘环境。辉长岩岩浆来源于板块俯冲消减作用引起的地幔楔形区部分熔融,除了消减组分的加入,岩浆在上升的过程中还受到了地壳混染作用的影响。(4)结合古城沟辉长岩体和其它北秦岭基性-超基性侵入岩已有的研究成果,建立了北秦岭从晋宁期晚期到加里东期的构造演化过程:600Ma左右商丹洋处于成熟期;约515Ma开始,商丹洋洋壳开始向北俯冲;约440Ma前后,进入深层次俯冲阶段,部分地方发生了初始板块碰撞;约422Ma,商丹洋闭合,进入陆-陆碰撞阶段。
[Abstract]:The Shangdan tectonic belt as the dividing line between North China plate and the Yangtze plate, is taking Dan Yang subduction, subduction to the product of the closing process of the Shangdan tectonic belt. The main body is a set of greenschist to low amphibolite facies volcano sedimentary rocks, such as OPHIOLITIC MELANGE, metamorphic volcano rocks, metamorphic clastic rocks marble, etc. the Shangdan tectonic belt has developed a series of acidic to ultrabasic intrusive rocks, consists of gabbro, diorite and granite, which has many basic and lenticular linear distribution of ultrabasic intrusive rocks, these intrusive rocks mostly preserved intact structure magmatite, diagenesis the early Palaeozoic era. The existing research on the causes of these rock evolution, there are different understandings, and the lack of corresponding research - based ultrabasic intrusive rocks in this area. These intrusive rock diagenetic age, tectonic background and magmatic origin of North Qinling Mountains tectonic evolution There are important geological significance. Based on the previous research summary system, taking the ancient city of East Qinling Mountains ditch gabbro Danfeng area as the research object, carried out detailed field geology, petrology, geochemistry and geochronology and so on, are the following: (1) the ancient city ditch gabbro body size small Danfeng area, lenticular parallel to the Shangdan fault zone, showing intrusive contact relationship with the Danfeng Group volcano rock. The rock with gabbro, with small gabbro diorite and diorite rock. From the edge to the center, the rock basic degree becomes higher, the mineral particles of rock thicker. Gu Cheng Gou gabbro center weak deformation, edge deformation degree is strong, the development of penetrative foliation, as gabbroic protomylonite. (2) LA-ICP-MS zircon U-Pb dating results show that the weighted average of the ancient city ditch gabbro Age is 425.1 + 424.3 + 1.7Ma (MSWD=0.10) and 1.3Ma (MSWD=0.16), show the ancient city of gabbro formed in the Silurian Gou Wen Rock (3). The City Ditch gabbro body with calc alkaline - rich Na low K sub alkaline characteristics of rare earth elements for the distribution pattern of LREE, LREE/HREE=8.03-11.95, LaN/YbN=9.82-17.64, Delta Eu is 0.93-1.31. Eu anomaly is not obvious or weak positive anomaly. Large ion lithophile elements Cs, K, Sr, Ba are obviously enriched in high field strength elements U, Th, Nb, Ta, Hf, Zr relative loss, Th/Ta value of the ancient city of 6.00-13.67. ditch gabbro does not belong to the Shangdan tectonic belt in the early Paleozoic OPHIOLITIC MELANGE rock unit, the geochemical characteristics of the ancient city ditch gabbro formed in active continental margin environment. Gabbro magma originated from plate subduction caused by partial melting in the mantle wedge, by addition of components, processes of magma on the rise in the crust has been mixed Influence of dyeing effect. (4) combined with the ancient city of Qinling Mountains north ditch gabbro body and other mafic ultramafic intrusive rocks research has been established in northern Qinling Mountains, from Jinning to the late Caledonian period tectonic evolution process: 600Ma's Dan Yang is in mature period; about 515Ma, Shangdan ocean crust the northward subduction; about 440Ma before and after entering the deep subduction stage, the initial collision parts; about 422Ma, Shang Dan ocean closed, into the continental collision stage.

【学位授予单位】：西北大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P59;P588.124
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本文编号：1721177