【摘要】：贛東北地處揚子板塊與華夏板塊的交接部位,在大地構造上屬于揚子板塊邊緣的江南造山帶。贛東北蛇綠混雜巖位于江紹斷層的西南部,沿著江西省東北部邊緣的樟樹墩到茅橋出露,呈東北向延伸。本文在充分收集整理前人研究成果的基礎上,對贛東北蛇綠混雜巖中輝長巖展開了野外地質勘查、室內的巖石礦物學、年代學和全巖地球化學研究工作,目的是查明輝長巖的地球化學特征、形成時代,并試圖探討贛東北蛇綠混雜巖的構造演化歷史。論文取得了如下認識:(1)贛東北蛇綠混雜巖帶中的輝長巖呈灰綠色,中—粗粒結構,塊狀構造,礦物顆粒自形。輝長巖的主要礦物為單斜輝石和斜長石,其中單斜輝石顆粒內部會有小的新鮮的或者弱蝕變的輝石和長石包體。斜長石蝕變較為嚴重,但部分蝕變的斜長石仍可見到聚片雙晶結構;(2)礦物化學研究表明,輝長巖所含斜長石的化學成分變化很小,具有高Al2O3、CaO,低Na2O和K2O的特征,為鈣長石質斜長石。單斜輝石為透輝石,其中Na2O和Al2O3含量較低,TiO2含量為0.03%~0.59%,平均為0.34%,較高的Si和Ti含量的特征,可以初步判斷輝長巖形成環(huán)境可能為擴張的洋中脊;(3)贛東北蛇綠混雜巖帶內樟樹墩輝長巖的鋯石SHRIMP U-Pb測年結果表明,輝長巖的207Pb/206Pb加權平均年齡為1029±13Ma,屬于中元古代晚期,說明當時贛東北地區(qū)還處在洋盆的初始擴張階段;(4)地球化學研究表明,輝長巖屬于亞堿性輝長巖,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。輝長巖的Mg#值為73~83之間,Mg#較低,SiO2、K2O、Na2O、CaO、Al2O3的含量高于原始地幔的含量,而MgO較原始地幔低,說明了其地幔虧損程度低。稀土含量介于3.91×10-6至7.41×10-6之間,稀土配分曲線為平坦型,輕稀土弱虧損,輕重稀土分餾并不明顯,LREE/HREE=0.74~1.17,(La/Yb)N為0.18~0.49,Eu表現為明顯的正異常,表明輝長巖來自于虧損地幔。輝長巖中大離子親石元素(LILE)Sr、K、Rb、Ba等元素富集,高場強元素(HFSE)Ce、P、Zr、Hf、Sm、Ti、Y、Yb虧損,TNT負異常。通過以上輝長巖的礦物學、年代學和地球化學的結果,本文認為贛東北蛇綠混雜巖的形成可能經歷了多個期次的構造演化過程,輝長巖可能代表了古華南洋的初始擴張過程,礦物組成上斜長石為鈣長石質斜長石,An均大于95;單斜輝石為透輝石且含有較高的Si和Ti,地球化學上也具有弱虧損地幔的源區(qū)特征,稀土元素具有MORB型分布型式。后續(xù)古華南洋發(fā)生洋內俯沖過程中,俯沖過程中釋放的流體交代了俯沖帶的地幔楔,從而影響了輝長巖的微量元素組成,形成大離子親石元素(LILE)Sr、K、Rb、Ba等元素富集,高場強元素(HFSE)Ce、P、Zr、Hf、Sm、Ti、Y、Yb虧損。因此認為贛東北蛇綠混雜巖形成于大洋中脊環(huán)境,后續(xù)又迭加了俯沖帶流體改造,并受到了陸源物質的混染的影響。
[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 鹵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.
【學位授予單位】：東華理工大學
【學位級別】：碩士
【學位授予年份】：2017
【分類號】：P597.3;P588.124

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