内蒙古西乌旗罕乌拉地区双峰式侵入体年代学、地球化学特征及其对古亚洲洋闭合时限的制约

Geochronological and Geochemical Features of Bimodal Intrusive Rocks in the Hanwula Area of Xiwu Banner,Inner Mongolia:Constraints on Closure of the Paleo-Asian Ocean

中亚造山带南缘二叠纪岩浆岩的成因及形成环境一直存在争议。本文以西乌旗罕乌拉地区发育的伊和绍荣复式岩体为研究对象,开展了野外地质、岩石学、锆石U-Pb同位素年代学、地球化学研究。结果表明:伊和绍荣复式岩体由辉长岩、辉石闪长岩、碱长花岗岩组成,地球化学特征显示SiO_2含量为44.01%～77.60%,在55.91%～75.97%之间存在一明显的间断,显示出双峰式岩浆岩的特征。辉长岩和碱长花岗岩的形成年龄分别为280.1±1.3 Ma和279.8±1.2 Ma,为早二叠世侵入岩。辉长岩为一套低钾拉斑系列-钙碱性系列岩石,SiO_2含量为44.01%～52.63%,Na_2O+K_2O为2.78%～5.33%(Na_2O>K_2O),Al_2O_3为15.11%～24.43%,TiO2为0.55%～3.59%,Mg#介于23～60之间;稀土元素总量较低,具有一致的平坦型配分型式,具Eu正异常;岩石相对富集Ba、K、Sr、Eu等大离子亲石元素,而亏损Nb、Zr、Hf等高场强元素,反映了俯冲带构造背景特征。碱长花岗岩具有较高的SiO_2、K_2O+Na_2O、FeO^T/MgO,低Al_2O_3、CaO和MgO;稀土元素总量较低,具有一致的海鸥型配分曲线;微量元素总体含量较高,具有一致的配分曲线,明显富集K、Th、U、Rb等大离子亲石元素,强烈亏损Ba、Nb、Ta、Sr、P和Ti,具有A型花岗岩地球化学特征,为高温低压下长英质地壳部分熔融的产物。结合岩体地质学特征及区域地质资料,提出伊和绍荣复式岩体最可能形成于弧后扩张环境,是早二叠世古亚洲洋闭合前洋壳俯冲消减作用的产物。

There remains controversy about petrogenesis and tectonic environments of the Permian magmatic rocks in the southern margin of the Central Asia Orogenic Belt（CAOB）.This study carried out analyses of petrology,zircon U-Pb isotopic geochronology and geochemistry of the Yiheshaorong complex massif in the Hanwula area of Xiwu Banner,Inner Mongolia.The results show that the Yiheshaorong complex massif comprises gabbro,pyroxene diorite and alkali-feldspar granite.Their geochemical features show SiO2 content of 44.01%～77.60%,which exhibits a gap between 55.91% and 75.97%,displaying agood bimodal geochemical distribution.LA-ICP-MS U-Pb dating yielded the ages of 280.1±1.3 Ma and 279.8±1.2 Ma for gabbro and alkali-feldspar granite,respectively,suggesting they are early Permian intrusive rocks.The gabbros are a suite of low-K tholeiitic-calc alkaline rocks,with SiO2=44.01%～52.63%,Na2O＋K2O=2.78%～5.33%（Na2O〉K2O）,Al2O3=15.11%～24.43%,TiO2=0.55%～3.59% and Mg#=23～60.Low total REEs and flat REE distribution pattern,positive Eu anomalies,enrichment of LILE（such as Ba,K,Sr,and Eu）,and depletion of HFSE（like Nb,Zr,and Hf）imply that this gabbros were formed in a subduction setting.The alkali-feldspar granites are characterized by high SiO2,K2O＋Na2O,FeO^T/MgO,low Al2O3,CaO and MgO;relatively low total REEs and seagull-type distribution pattern;high total amount of trace elements,characterized evidently by enrichment of LILE（such as K,Th,U and Rb,and depletion of Ba,Nb,Ta,Sr,P and Ti）.All these features are consistent with features of A-type granite.Therefore,the granites should be the product of partial melting of felsic crust in low pressure,high temperature condition.Together with their petrological and geological features,it can be concluded that the Yiheshaorong complex massif was most probably formed in a back-arc spreading basin triggered by oceanic crust subduction before closure of the Paleoasian Ocean in the Early Permian.