The Wulagai early Paleozoic gabbroic diorite pluton is outcropped in the East Ujimqin Banner of Inner Mongolia.It is located on the northern margin of the Xing-Meng Orogenic Belt (XMOB) and was previously referred as part of the Erenhot-East Ujimqin Paleozoic magrnatic belt.This study reports zircon U-Pb age,elemental and isotopic geochemical data of the pluton,so as to reveal its petrogenesis and implications for the evolution of the XMOB.Zircon U-Pb dating yielded an weighted mean age of 499.6 ± 1.2 Ma,indicating that it is the earliest intrusive pluton in Erenhot-East Ujimqin Paleozoic magmatic belt.Geochemically,the Wulagai pluton shows moderate SiO2(51.60％ ～54.28％),high Al2O3,depletion of magnesium and iron,and low alkali and potassium,thus belonging to calc alkali rocks.The gabbroic diorites are also enriched in LILE,depleted in HFSE,and exhibit gradual right-oblique chondrite-normalized REE patterns with unconspicuous positive europium anomalies (δEu =1.03 ～ 1.34).All the samples display depleted isotopic compositions with the data (87Sr/86Sr) i =0.704 5 ～～0.704 7and εNd(t) =+ 2.71 ～ + 4.17,but the εHf(t) values of zircons (+ 10.8 ～ + 18.7) are much higher relative to the εNd(t) and show a Nd-Hf decoupling feature.Integrated geochemical,geochronological and Sr-Nd-Hf isotopic data suggest that the Wulagai gabbroic diorites were generated by the subduction of Paleo-Asian Ocean along the Sunid-Xilinhot island arc in early Paleozoic,and were formed at the initial stage of the subduction.The rocks originated from a mantle wedge which was mainly modified by the slab-derived melt prior to fluids,and there were also indistinct sedimentary materials in the source.The back-arc extension and opening of Hegenshan Ocean possibly led to the separation of Wulagai pluton from the Sunid-Xilinhot island arc.Along with the closure of Paleo-Asian Ocean,the pluton was ultimately isolated from the subduction zone by Hegenshan ophiolite complex.%东乌旗乌拉盖地区出露一套早古生代辉长闪长岩,为兴蒙造山带北缘二连-东乌旗早古生代岩浆岩带的组成部分.对该岩体进行了锆石U-Pb年龄、全岩地球化学及Sr-Nd-Hf同位素分析,探讨了岩石成因及其对兴蒙造山带北缘早古生代构造演化的启示.锆石LA-ICP-MS U-Pb定年显示岩体年龄为499.6±1.2 Ma,为二连-东乌旗地区出露的最古老侵入体.岩体具有中等的SiO2含量(51.60％～54.28％),富铝,贫铁、镁,全碱及钾含量较低,属钙碱性岩浆系列;富大离子亲石元素,亏损高场强元素,具有弱的Eu正异常(δEu=1.03 ～ 1.34),稀土元素配分型式呈平缓的右倾型.辉长闪长岩同位素组成比较亏损,(87 Sr/86 Sr)i=0.704 5 ～0.704 7,εNd(t)=+2.71 ～ +4.17,εHf(t)相对εNd(t)明显偏高,为+10.8～ +18.7,存在Nd-Hf同位素解耦现象.年代学、岩石地球化学以及Sr-Nd-Hf同位素综合分析表明,乌拉盖辉长闪长岩是早古生代古亚洲洋沿苏左旗-锡林浩特一线向北俯冲的产物,岩体形成于俯冲作用的初始阶段,源区为受到俯冲物质交代的地幔楔,交代物质以板片熔体为主,流体交代为辅,无明显沉积物加入,后期由于弧后拉张、贺根山洋盆打开与主体岛弧带脱离,最终形成了与俯冲带彼此分隔的格局.
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