内蒙古东乌珠穆沁旗早古生代辉长闪长岩年代学和地球化学特征及地质意义  

Geochronology and geochemistry of Early Paleozoic gabbroic diorites in East Ujimqin Banner of Inner Mongolia and their geological significance

作　　者：杨泽黎 王树庆[1] 胡晓佳[1] 辛后田[1] 李承东[1] YANG Ze-li;WANG Shu-qing;HU Xiao-jia;XIN Hou-tian;LI Cheng-dong(Tianjin Institute of Geology and Mineral Resources, Tianjin 300170, China)

机构地区：[1]天津地质矿产研究所,天津300170

出　　处：《岩石矿物学杂志》2018年第3期349-365,共17页Acta Petrologica et Mineralogica

基　　金：中国地质调查局地质矿产调查项目(DD20160041)~~

摘　　要：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Si O2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。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 magmatic 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 SiO_2（51. 60% ~ 54. 28%）,high Al_2O_3,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（^（87） Sr/^（86） Sr）_i= 0. 704 5 ~ 0. 704 7 and 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.

关 键 词：辉长闪长岩 早古生代 兴蒙造山带 古亚洲洋 东乌珠穆沁旗 

分 类 号：P588.122[天文地球—岩石学] P59[天文地球—地质学]