B isotopes of Carboniferous‐Permian volcanic rocks in the Tuha basin mirror a transition from subduction to intraplate setting in Central Asian Orogenic Belt
B isotopes of Carboniferous‐Permian volcanic rocks in the Tuha basin mirror a transition from subduction to intraplate setting in Central Asian Orogenic Belt
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DOI：
10.1002/2016jb013288
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发表时间：
2016-11
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通讯作者：
Hai‐Quan Liu;Yigang Xu;G. Wei;Jing‐Xian Wei;Fan Yang;Xuan‐Yu Chen;Liang-mei-zi Liu;Xun Wei
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作者：
Hai‐Quan Liu;Yigang Xu;G. Wei;Jing‐Xian Wei;Fan Yang;Xuan‐Yu Chen;Liang-mei-zi Liu;Xun Wei
关于中亚造山带的二叠纪构造背景，特别是二叠纪岩浆活动的成因，一直存在争议。为此，对西南吐哈盆地大南湖火山岩进行了地球化学研究。长石分离物的40Ar/39Ar分析表明，安山岩的石炭纪年龄为321.2±9.8 Ma，玄武岩的二叠纪年龄为278.9±4.2 Ma。安山岩和玄武岩的微量元素和Sr - Nd - Pb同位素组成在地球化学上都类似于俯冲相关岩浆，但它们在岩石成因上可能有所不同。安山岩为典型的埃达克岩。它们的Cr和Ni含量高，重稀土元素含量低，B/Nb比值与δ11B值呈正相关，表明它们是由板块衍生熔体/流体与地幔楔体相互作用产生的。二叠系玄武岩可分为碱玄武岩和拉斑玄武岩两个亚群。高Ba/La和Ba/Zr比值表明，碱性玄武岩的来源是蚀变洋壳流体/熔体的交代作用。相比之下，考虑到拉斑玄武岩具有较高的Th/Zr和Th/Ce比值，可以推断出由沉积物衍生流体/熔体交代的无挥发源。二叠系玄武岩的B/Nb与δ11B呈负相关，表明弧特征是由以前的俯冲事件所施加的，没有直接的板块流体/熔体参与其岩石成因。这些地球化学特征与区域地质记录共同表明，CAOB西南地区从晚石炭世的俯冲相关环境向早二叠世的板内环境转变。
Controversies remain as to the Permian tectonic setting in the Central Asian Orogenic Belt (CAOB), in particular, regarding the triggering of Permian magmatism. To address this issue, we carried out a geochemical study on Dananhu volcanics from the Tuha basin, southwestern CAOB. 40Ar/39Ar analyses for feldspar separates yield a Carboniferous age of 321.2 ± 9.8 Ma for the andesites and a Permian age of 278.9 ± 4.2 Ma for the basalts. Both the andesites and basalts geochemically resemble subduction‐related magmas in aspects of their trace element and Sr‐Nd‐Pb isotopic composition, but they may differ in petrogenesis. The andesites are typical adakites. Their high Cr and Ni contents, and low heavy rare earth element contents, and positive correlation between B/Nb ratios and δ11B values suggest that they were generated by interaction between slab‐derived melts/fluids and the mantle wedge. The Permian basalts are subdivided into two subgroups: alkali and tholeiitic basalts. High Ba/La and Ba/Zr ratios indicate that the source of the alkali basalts has been metasomatized by fluids/melts derived from altered oceanic crust. In contrast, a volatile‐free source metasomatized by sediment‐derived fluids/melts is inferred for the tholeiitic basalts given their high Th/Zr and Th/Ce ratios. Negative correlation between B/Nb and δ11B for the Permian basalts imply that the arc signatures were imposed by previous subduction events, and no direct slab‐derived fluids/melts participated in their petrogenesis. These geochemical characteristics alongside regional geologic records collectively suggest a tectonic shift in the southwestern CAOB from subduction‐related setting during the Late Carboniferous to an intraplate setting during the Early Permian.