内蒙古东乌珠穆沁旗晚奥陶世辉长岩地球化学特征及其地质意义

Geochemical Features and Significance of Late Ordovician Gabbros in East Ujimqin Banner,Inner Mongolia

内蒙古东乌珠穆沁旗朝不楞以西及巴润布尔嘎斯台地区出露一套辉长岩。LA-MC-ICPMS锆石U-Pb测年结果显示该辉长岩体侵位于450～461Ma,属于晚奥陶世。样品的Si O_2、K_2O、TiO、MgO含量低,σ=0.10～2.77,总体属低钾拉斑玄武质系列。随着Mg O含量的增加,主要氧化物如Si O_2、CaO、Fe_2O_3含量呈上升趋势,Al2O3、Na2O呈下降趋势。根据稀土和微量元素特征,可将辉长岩分为三类。第一类REE含量最高,具有微弱的Eu正异常,(La/Yb)_N=19.99～20.3,稀土配分曲线明显右倾,富集LILE(Rb、Sr、Ba),相对亏损Nb、Ta。第二类REE含量减少,具有明显的Eu负异常,(La/Yb)_N=4.88～9.68。第三类REE含量最低,具有明显的Eu负异常,(La/Yb)_N=1.33～2.03。后两类辉长岩均亏损Nb、Sr、Ba,富集Rb、Th、U、Zr、Hf,稀土配分曲线弱右倾,岩石形成过程中可能受到陆壳物质混染。三类辉长岩整体兼具MORB型及岛弧玄武岩特征,可能是晚奥陶世古亚洲洋向北俯冲过程中弧后盆地的产物。

Central Asian Orogenic Belt （CAOB） is a giant accretionary orogeny between Siberia and Tarim—North China cratons, and its formation is closely tied to the evolution of the Paleo Asian Ocean （PAO） documented by multi stage magmatic activities in Paleozoic. A bi direction subduction model of the PAO has been proposed to account for the formation of two orogens in its eastern segment which is called Xing’an Mongolia Orogenic Belt （XMOB） （Xiao Wenjiao et al., 2003; Jian Ping et al., 2008; Xu Bei et al., 2015）. Abundant data of southward subduction related Ordovician igneous rocks have been reported along the northern margin of the North China Craton （Chen Bin et al., 2001＆; Liu Dunyi et al., 2003＆; Jian Ping et al., 2008）, however, report on the data of the magmatism related to the northward subduction beneath the Siberia Craton is scarce. We present new data of late Ordovician gabbros from East Ujimqin Banner to exemplify the northward subduction of the PAO. Zircon U-Pb LA-MC-ICP-MS dating was applied to constrain the emplacement of the gabbros, and geochemical analysis is performed on major, rear earth and trace elements to discuss their petrogenesis and tectonic setting.  Gabbroic plutons are exposed to the west of Chaobuleng and Barunbuergasitai in East Ujimqin Banner. Zircon U-Pb LA-MC-ICP-MS dating yielded ages ranging from 450 Ma to 461Ma, which constrained the emplacement time to Late Ordovician. Gabbroic samples have low SiO2, K2O, TiO2 and MgO, σ = 0.10～2.77, belonging to low K tholeiite series. Major elements such as SiO2, CaO and Fe2O3 show positive linear correlation with the increasing MgO content, while Al2O3 and Na2O present negative linear relationship, suggesting fractional crystallization probably contributed to the formation of gabbros. Regarding the characteristics of major and trace elements, the gabbros can be classified into three groups. Gabbros of the first group have highest REE contents, with slight positive Eu anomaly, （La/Yb）N=19.99～20.3, enriched in LILE （Rb, Sr and Ba）, relatively depleted in HFSE （Nb and Ta） and showing characteristics of subduction related island arc or continental margin arc basalts. REE abundance of samples from the second group decrease a little, with slight Eu negative anomaly, （La/Yb）N=4.88～9.68. The third group has lowest REE contents, with striking Eu negative anomaly, （La/Yb）N=1.33～2.03. Gabbros of the second and the third groups are depleted in Nb, Sr and Ba, but enriched in Rb, Th, U, Zr and Hf to different extent, with less fractionated REE pattern, suggesting impact from continental crust contamination. Comprehensively, late Ordovician gabbros from East Ujimqin Banner show both hallmarks of island arc basalts and MORB, which indicates a subduction related extensional setting. Combined with the characteristics of lower Ordovician strata which consist of Duobaoshan and Wubinaobao formations, a back arc basin can be identified in East Ujimqin Banner. Regional geological evidences have testified a northward subduction beneath the south margin of the Siberia Craton occurred along south of Sonid Zuoqi—Xilin Hot （Li Chengdong et al., 2012＆; Xu Bei et al., 2015）. The northward subduction probably lead to the break off a subducting oceanic plate to give rise to the formation of a back arc basin in East Ujimqin Banner succeeded by emplacement of gabbros during 450～460 Ma. The existence of back arc basin argued for the northward subduction of the PAO and testified that the south margin of the Siberia Craton is an active continental margin during the late Ordovician.