于远山, 张海, 王富明, 王兴明, 刘卜桢, 秦烽焰, 何海. 藏东日扎山一带马拉松多组流纹岩年龄、地球化学特征及其地质意义[J]. 地质通报, 2019, 38(5): 697-710.
    引用本文: 于远山, 张海, 王富明, 王兴明, 刘卜桢, 秦烽焰, 何海. 藏东日扎山一带马拉松多组流纹岩年龄、地球化学特征及其地质意义[J]. 地质通报, 2019, 38(5): 697-710.
    YU Yuanshan, ZHANG Hai, WANG Fuming, WANG Xingming, LIU Buzhen, QIN Fengyan, HE Hai. Age and geochemical characteristics of Malasongduo Formation rhyolite in Riza Mountain, east Tibet, and its geological significance[J]. Geological Bulletin of China, 2019, 38(5): 697-710.
    Citation: YU Yuanshan, ZHANG Hai, WANG Fuming, WANG Xingming, LIU Buzhen, QIN Fengyan, HE Hai. Age and geochemical characteristics of Malasongduo Formation rhyolite in Riza Mountain, east Tibet, and its geological significance[J]. Geological Bulletin of China, 2019, 38(5): 697-710.

    藏东日扎山一带马拉松多组流纹岩年龄、地球化学特征及其地质意义

    Age and geochemical characteristics of Malasongduo Formation rhyolite in Riza Mountain, east Tibet, and its geological significance

    • 摘要: 为确定藏东日扎山一带马拉松多组流纹岩的形成时代及成岩构造环境,探讨古特提斯洋闭合时间,应用LA-ICP-MS方法对其进行锆石U-Pb精确定年,并开展岩石学和地球化学研究。结果表明,该区流纹岩岩浆锆石206Pb/238U年龄加权平均值为244±1.2Ma,较精确地限定了马拉松多组流纹岩的形成时代,为中三叠世早期。岩相学及地球化学研究结果显示,马拉松多组流纹岩具高硅(SiO2=72.72%~76.88%),富碱((K2O+Na2O)=6.64%~7.41%,K2O>Na2O),过铝质(Al2O3=11.76%~13.03%,A/CNK=1.17~1.31)特征;岩石富集大离子亲石元素K、Rb,高场强元素Th、U,而亏损大离子亲石元素Sr和Ba,高场强元素Nb、Ta、P、Zr、Hf、Ti等;稀土元素配分模式表现出轻稀土元素富集(LREE/HREE=1.93~2.89),轻稀土元素分馏程度稍高于重稀土元素的右倾V字形分布模式,具有明显的负Eu异常(δEu=0.36~0.41);其成因可能与幔源岩浆的底侵和加厚地壳的减薄有关,是幔源基性岩浆底侵导致地壳物质脱水发生部分熔融的产物,即主要是陆壳和硅铝质源岩部分熔融形成的,岩浆在上升过程中经历了结晶分异作用。其化学性质具有S型向A型流纹岩演化的趋势,以及同碰撞弧火山岩与碰撞后A型流纹岩的双重特性。综合研究认为,研究区在早中三叠世处于弧-陆碰撞后活动大陆边缘短暂的后造山伸展构造环境,古特提斯洋(金沙江洋)在此之前已经闭合。

       

      Abstract: In order to determine the formation age and tectonic environment of east Tibetan Malasongduo Formation rhyolite and provide new evidence for studying ancient Tethys closing time, the authors applied the LA-ICP-MS method for the precise zircon U-Pb dating and studied petrology and geochemistry. The results show that the weighted average 206Pb/238U age of rhyolite magma in this area is 244 ±1.2Ma (MSWD=0.59), which accurately defines the formation age of the Malasongduo Formation rhyolite, i.e., Early-Middle Triassic. Petrographic and geochemical studies show that Malasongduo Formation rhyolite has high silica (SiO2=72.72%~76.88%) and alkali (ALK (K2O+Na2O)=6.64%~7.41%, K2O>Na2O), and exhibits peraluminous nature (Al2O3=11.76%~13.03%, A/CNK=1.17~1.31); the rocks are enriched in large ion lithophile elements K and Rb, and high field strength elements Th and U, but depleted in large ion lithophile elements Sr and Ba, and high field strength elements such as Nb, Ta, P, Zr, Hf and Ti; REE patterns show that LREE is enriched (LREE/HREE=1.93~2.89), light rare earth elements (LREE) is slightly higher than the degree of fractionation of heavy rare earth elements (HREE), and there exist right-inclined V-shaped distribution and obvious negative Eu anomalies (Eu=0.36~0.41). The genesis of the rocks may be related to the underplating of the mantle derived magma and the thinning of the thickening crust. They were formed by the mantle derived basic magma underplating resulting in dehydration of crustal material and partial melting. They were mainly formed by partial melting of continental crust and aluminosilicate source rocks, and the magma underwent crystallization differentiation during the process of rising. The chemical properties have the evolution trend of S type to A type rhyolite, and have the dual characteristics of the syn-collisional arc volcanic rocks and the post collisional A type rhyolite. According to the comprehensive study, the study area was in a short post orogenic extensional tectonic environment on the post collisional active continental margin in the Early-Middle Triassic period, and the ancient Tethys Ocean (Jinshajiang Ocean) had been closed before that.

       

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