赣东北蛇绿混杂岩中辉长岩锆石SHRIMP U-Pb定年及地球化学特征

发布时间：2018-07-24 20:01

【摘要】：赣东北地处扬子板块与华夏板块的交接部位,在大地构造上属于扬子板块边缘的江南造山带。赣东北蛇绿混杂岩位于江绍断层的西南部,沿着江西省东北部边缘的樟树墩到茅桥出露,呈东北向延伸。本文在充分收集整理前人研究成果的基础上,对赣东北蛇绿混杂岩中辉长岩展开了野外地质勘查、室内的岩石矿物学、年代学和全岩地球化学研究工作,目的是查明辉长岩的地球化学特征、形成时代,并试图探讨赣东北蛇绿混杂岩的构造演化历史。论文取得了如下认识:(1)赣东北蛇绿混杂岩带中的辉长岩呈灰绿色,中—粗粒结构,块状构造,矿物颗粒自形。辉长岩的主要矿物为单斜辉石和斜长石,其中单斜辉石颗粒内部会有小的新鲜的或者弱蚀变的辉石和长石包体。斜长石蚀变较为严重,但部分蚀变的斜长石仍可见到聚片双晶结构;(2)矿物化学研究表明,辉长岩所含斜长石的化学成分变化很小,具有高Al2O3、CaO,低Na2O和K2O的特征,为钙长石质斜长石。单斜辉石为透辉石,其中Na2O和Al2O3含量较低,TiO2含量为0.03%~0.59%,平均为0.34%,较高的Si和Ti含量的特征,可以初步判断辉长岩形成环境可能为扩张的洋中脊;(3)赣东北蛇绿混杂岩带内樟树墩辉长岩的锆石SHRIMP U-Pb测年结果表明,辉长岩的207Pb/206Pb加权平均年龄为1029±13Ma,属于中元古代晚期,说明当时赣东北地区还处在洋盆的初始扩张阶段;(4)地球化学研究表明,辉长岩属于亚碱性辉长岩,具有低K2O(0.09%~2.62%)、TiO2(0.14%~0.25%),高Na2O(0.58%~2.46%)、Al2O3(14.51%~16.72%)等特征。辉长岩的Mg#值为73~83之间,Mg#较低,SiO2、K2O、Na2O、CaO、Al2O3的含量高于原始地幔的含量,而MgO较原始地幔低,说明了其地幔亏损程度低。稀土含量介于3.91×10-6至7.41×10-6之间,稀土配分曲线为平坦型,轻稀土弱亏损,轻重稀土分馏并不明显,LREE/HREE=0.74~1.17,(La/Yb)N为0.18~0.49,Eu表现为明显的正异常,表明辉长岩来自于亏损地幔。辉长岩中大离子亲石元素(LILE)Sr、K、Rb、Ba等元素富集,高场强元素(HFSE)Ce、P、Zr、Hf、Sm、Ti、Y、Yb亏损,TNT负异常。通过以上辉长岩的矿物学、年代学和地球化学的结果,本文认为赣东北蛇绿混杂岩的形成可能经历了多个期次的构造演化过程,辉长岩可能代表了古华南洋的初始扩张过程,矿物组成上斜长石为钙长石质斜长石,An均大于95;单斜辉石为透辉石且含有较高的Si和Ti,地球化学上也具有弱亏损地幔的源区特征,稀土元素具有MORB型分布型式。后续古华南洋发生洋内俯冲过程中,俯冲过程中释放的流体交代了俯冲带的地幔楔,从而影响了辉长岩的微量元素组成,形成大离子亲石元素(LILE)Sr、K、Rb、Ba等元素富集,高场强元素(HFSE)Ce、P、Zr、Hf、Sm、Ti、Y、Yb亏损。因此认为赣东北蛇绿混杂岩形成于大洋中脊环境,后续又迭加了俯冲带流体改造,并受到了陆源物质的混染的影响。
[Abstract]:Northeast Jiangxi is located at the junction of Yangtze plate and Huaxia plate, and belongs to the Jiangnan orogenic belt on the margin of Yangtze plate. The ophiolite melange is located in the southwest of Jiangshao fault. It is exposed from Zhangshu pier to Maoqiao along the northeast edge of Jiangxi Province and extends northeast. On the basis of fully collecting and arranging the previous research results, this paper has carried out field geological exploration, indoor petromineralogy, geochronology and whole rock geochemistry of gabbro in the ophiolite melange in northeastern Jiangxi Province. The purpose of this paper is to find out the geochemical characteristics and formation age of gabbro and try to discuss the history of tectonic evolution of ophiolite in northeast Jiangxi. The main results are as follows: (1) the gabbro in the ophiolite melange belt in northeastern Jiangxi is gray-green medium coarse grain structure massive structure and mineral particle shape. The major minerals of gabbro are clinopyroxene and plagioclase, in which there will be small fresh or weakly altered gabbro and feldspar inclusions inside the clinopyroxene grains. The alteration of plagioclase is serious, but the partial alteration of plagioclase can still be found in the polycrystalline double crystal structure. (2) the chemical composition of plagioclase in gabbro has little change and has the characteristics of high Al _ 2O _ 3-CaO, low Na2O and K _ 2O. It is calcium feldspar plagioclase. The clinopyroxene is diopside, in which the content of Na2O and Al2O3 is lower and the content of TIO _ 2 is 0.030.59, with an average of 0.34. (3) the zircon SHRIMP U-Pb dating of the Zhangshu pier gabbro in the ophiolian melange belt of northeast Jiangxi shows that the mean 207Pb/206Pb weighted age of the gabbro is 1029 卤13 Ma, which belongs to the late Middle Proterozoic. The geochemical study shows that the gabbro belongs to subalkaline gabbro, which is characterized by low K _ 2O (0.09% 2.62%), high Na2O (0.58% 2.46%) and low K _ 2O (0.09% 2.62%) and high Na2O (0.58% 2.46%) Al _ 2O _ 3 (14.51%) and so on. The Mg# value of gabbro is between 73 and 83, which indicates that the content of Mg# is lower than that of the original mantle, and the content of MgO is lower than that of the original mantle, which indicates that the depletion degree of the mantle is low. The rare earth contents range from 3.91 脳 10 ~ (-6) to 7.41 脳 10 ~ (-6), the rare earth partition curve is flat, the light rare earth is weak, the light rare earth fractionation is not obvious LREE / HREE _ (0.74) 1.17, the (La/Yb) N is 0.18 ~ 0.49 EU, which indicates that the gabbro comes from the depleted mantle. The large ion lipophilic element, (LILE) Srn, K, K, B, Ba and other elements are enriched in gabbro, and the high field strength element (HFSE) is negative abnormal. Based on the results of mineralogy, geochronology and geochemistry of the pyroxene mentioned above, it is concluded that the formation of the ophiolitic melange in northeastern Jiangxi may have undergone multiple tectonic evolution processes, and that the pyroxene may represent the initial spreading process of the Gu Hua South Ocean. The mineral composition of plagioclase is calcareous plagioclase (n > 95), clinopyroxene is diopside and contains high Si and Ti, geochemistry also has the characteristics of weak depleted mantle source, and rare earth elements have MORB type distribution pattern. The fluid released during the subsequent subduction process in the Southern Ocean of Gu Hua resulted in the metasomatism of the mantle wedge in the subduction zone, which affected the trace element composition of the gabbro and resulted in the enrichment of the large ion lithophile element, such as (LILE) srsrk, RbPBA, and so on. The high field strength element, (HFSE), (HFSE), HfHfU, Sm, TI-YB, is deficient in YYb. Therefore, it is considered that the ophiolite was formed in the mid-oceanic ridge environment in the northeast of Jiangxi Province, and then the subduction zone fluid was superimposed and influenced by the mixed contamination of land-based materials.
【学位授予单位】：东华理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P597.3;P588.124

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