| 阿拉善北部杭乌拉地区早二叠世火山岩地球化学、Nd-Hf同位素特征及其构造意义 |
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| 引用本文： | 李润武,张鑫,石强,陈万峰,安屹,黄尧燊,刘懿馨,王金荣.阿拉善北部杭乌拉地区早二叠世火山岩地球化学、Nd-Hf同位素特征及其构造意义[J].地质通报,2020(5):647-658. |
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| 作者姓名： | 李润武 张鑫 石强 陈万峰 安屹 黄尧燊 刘懿馨 王金荣 |
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| 作者单位： | 兰州大学地质科学与矿产资源学院/甘肃省西部矿产资源重点实验室;宁夏大学资源环境学院;中国科学院广州地球化学研究所 |
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| 基金项目： | 中国地质调查局项目《中国北方(天山-北山-阴山以北)中段石炭纪—二叠纪构造演化研究》(编号:121201011000161111-01);中央高校基本科研业务费专项资金(编号:Lzujbky-2018-it20)。 |
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| 摘 要： | 位于阿拉善恩格尔乌苏蛇绿岩带北侧的珠斯楞-杭乌拉构造带,分布较完整的早二叠世方山口组火山岩,对解析阿拉善地块晚古生代晚期的构造演化具有重要意义。方山口组以灰绿色安山岩、安山质角砾熔岩为主,局部含集块。安山岩SiO 2含量为54.23%~60.81%,Al2O 3为13.67%~15.96%,MgO为4.26%~5.76%,Mg^#值为42~50;Na2O含量(2.21%~3.48%)与K2O(2.51%~3.49%)相当,具较高的TFe2O 3(7.98%~11.24%),低的CaO(2.52%~3.36%)和TiO 2(0.75%~1.32%),为过铝质钙碱性安山岩;安山岩在稀土元素球粒陨石标准化配分图上,轻稀土元素略富集,(La/Yb)N=3.73~9.86,具负Eu异常(δEu=0.52~0.75);在微量元素原始地幔标准化蛛网图上,富集Rb、K,亏损Nb、Ta、Ti等;变化范围非常大的εNd(t)(-4.3^-14.2)、εHf(t)(-19.8^+1.4)。上述特征表明,安山岩属于壳幔物质混合的产物,形成于板内裂谷环境。结合区域地质背景研究资料,进一步指出,恩格尔乌苏蛇绿岩带所代表的大洋在晚石炭世之前已经闭合;晚石炭世-早二叠世北山-阿拉善-内蒙古中部地区已进入板内构造岩浆-沉积阶段。
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| 关 键 词： | 方山口组 火山岩 早二叠世 阿拉善地区 古亚洲大洋 |
Geochemistry and Nd-Hf isotopes of the Early Permian volcanic rocks in Hangwula of northern Alxa area and their tectonic significance |
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| LI Runwu,ZHANG Xin,SHI Qiang,CHEN Wanfeng,AN Yi,HUANG Yaoshen,LIU Yixin,WANG Jinrong.Geochemistry and Nd-Hf isotopes of the Early Permian volcanic rocks in Hangwula of northern Alxa area and their tectonic significance[J].Geologcal Bulletin OF China,2020(5):647-658. |
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| Authors: | LI Runwu ZHANG Xin SHI Qiang CHEN Wanfeng AN Yi HUANG Yaoshen LIU Yixin WANG Jinrong |
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| Institution: | (Key Laboratory of Mineral Resources in Western China/School of Earth Sciences,Lanzhou University,Lanzhou 730000,Gansu,China;College of Resources and Environmental Science,Ningxia University,Yinchuan 750021,Ningxia,China;Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,Guangdong,China) |
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| Abstract: | Volcanic rocks of Fangshankou Formation are widely distributed in Hangwula area on the northern side of Engger Us ophiolitic belt in the Alxa area.These rocks are very essential for interpreting tectonic evolution of the southern part of Central Asian orogenic belt.Early Permian rocks in this stratum mainly consist of grayish green andesite and andesitic breccia lava,partly with agglomerate.The andesite has SiO2 content of 54.23%~60.81%,Al2O3 content of 13.67%~15.96%,MgO content of 4.26%~5.76%,and Mg^# of 42~50 with the content of Na2 O(2.21%~3.48%)less than that of K2 O(2.51%~3.49%).These rocks also have high TFe2O3(7.98%~11.24%)and low CaO(2.52%~3.36%)as well as TiO2(0.75%~1.32%).They should be peraluminous calc-alkaline rocks.Andesite samples are enriched in light rare earth elements(LREE)and depleted in heavy rare earth elements(HREE).The differentiation between LREE and HREE is not obvious with(La/Yb)N equal to 3.73~9.86.Samples show negative Eu anomalies withδEu being 0.52~0.75.The chondrite-normalized REE patterns are similar to E-MORB.The samples of andesite are enriched in such elements as Rb and K and depleted in such elements as Nb,Ta and Ti.What`s more,they haveεNd(t)=-4.3^-14.2 andεHf(t)=-19.8^+1.4.These characteristics suggest that andesites were sourced from mixing of the mantle and the crust and formed in intraplate rift.Considering a lot of geological materials related to this area,the authors conclude that the branch of the PAO(Paleo-Asian Ocean)represented by Engger Us ophiolitic belt had been closed before Late Carboniferous,and that,during Late Carboniferous to Early Permian,magmatic activities in the study area occurred in an intraplate extensional setting. |
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| Keywords: | Fangshankou Formation volcanic rocks Early Permian Alxa area Paleo-Asian Ocean |
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