【作者】 李剑波；

【导师】 张长厚； 周志广；

【作者基本信息】 中国地质大学(北京) ， 构造地质学， 2012， 硕士

【摘要】 研究区隶属内蒙古兴安盟，大地构造位置位于西伯利亚与华北板块之间，属于古亚洲洋的范围。关于两大板块的闭合位置及时间长期以来存在争议，其中两大板块最终缝合线的位置主要存在两种认识，一是认为沿贺根山带一线，另一种观点则主张沿索伦山-西拉木伦河缝合线。近年来，更多的学者提出贺根山缝合带为华北板块与西伯利亚板块最终的缝合位置。但因中、新生代底层覆盖或岩体破坏，该缝合线的向东延伸不明。关于贺根山缝合带闭合的时间，有人认为在泥盆纪-石炭纪，也有学者主张在晚二叠世。本文通过对内蒙古东部大石寨地区发现的超基性岩带地质、地球化学和年代学特征研究，以期为解决上述相关问题，提供区域研究资料和成果。在野外地质填图基础上，结合样品测试和铼锇同位素年代学分析，本文对大石寨超基性岩带的野外地质特征、岩石学特征、岩石地球化学特征及同位素年代学特征进行了研究，取得以下主要成果和认识：1，通过野外填图，在内蒙古大石寨地区发现存在构造侵位的超基性岩。他们以断夹块形式产出于二叠纪吴家屯组砂质板岩、粉砂岩及硅质泥岩中。2，超基性岩岩石组合为蛇纹石化辉石橄榄岩、纯橄榄岩及橄榄岩。岩石学特征表现出m/f比值均在9左右，为镁质超镁铁岩，具有较高含量MgO含量和Mg#值，在ACM和AFM判别图解中，均落在超镁铁质堆积岩和变质橄榄岩的过渡部位，表明超基性岩为肢解的残余洋壳部分。3，稀土元素配分模式显示稍微右倾趋势，稀土总量偏低，轻稀土较富集，表明岩石分异程度不强烈，δEu值为0.88～2.93，表明了早期地幔部分熔融的结晶分异作用；微量元素蜘蛛网图显示富集Cr、Ni、Ti元素贫Nb、Rb元素特征，表明来自于亏损的地幔。4，超基性岩Re-Os同位素测年结果表明，其年龄值介于475Ma～313Ma之间，大致反映了初始洋盆的形成时代。5，将本区超基性岩与西部邻区贺根山超基性岩对比发现，他们在岩石学、地球化学及同位素年代学方面存在较高的相似性，据此认为大石寨地区超基性岩带可能为贺根山缝合线向东的延伸部分，从而为解决贺根山缝合带东延问题及厘清其中超基性岩组分的形成时代，提供了新的重要证据。更多还原

【Abstract】 The study area is located in Xing’anmeng, Inner Mongolia,between the NorthChina Craton and the Siberian Craton, and belongs to the Palaeo-Asian Oceanstructural domain. It has been discussed a long time for the age of Paleo-AsianOcean’s closing and the location of suture zone between North China craton andSiberia craton, two scenarios have been proposed for the location: some scholarsthought Hegenshan suture is the two cratons collided location, while the otherscholars thought Suolun mountain-Xilamulun river is the location of the suture. Inrecent years, more and more scholars began to agree Hegenshan suture is the locationof the two cratons collided. However, I t’s eastern part is not well exposed becausecovered by the Cenozoic strata or the rock bodies intrusion. When did the Hegenshansuture formed is also controversial, some scholars proposed it’s in the period fromDevonian to Carboniferous,Others thought it’s in late Permian.In order to solve theabove problems related,This article has done some research about Dashizhaiultrabasic rocks in Inner Mongolia.In this paper, on the basis of field geological mapping, combined with theanalysis of the samples in the laboratory, we explored the field geological,petrological, petro-geochemical and isotopic chronology characteristics of theDashizhai ultrabasic rocks, and made the following main achievements:1,Through the geological mapping, we have found the adjacent rocks of theseultrabasic rocks which emplaced structurally are sandy slate, siltstone and siliceousmudstone, which belongs to Wujiatun formation of Permian age.2,These ultrabasic rocks are composed of serpentinized pyroxene peridotite,dunite and peridotite with m/f about9which belong to magnesia ultramafic rocks,they have high content of MgO and Mg#and all of them locate in the transition zonebetween ultramafic cumulate and metaperidotite in the ACM and AFM figures,suggestting these rocks are remnants of ancient oceanic plate.3,The REE distribution patterns show slightly right-dipping, the total amout ofREE are relatively low and the LREE are slightly enriched, suggestting these rocks’ differentiation are not strong, δEu are between0.88and2.93, indicating fractionalcrystallization of the partial melts of the early mantle. Trace element diagrams showthese rocks are enriched in Cr、Ni、Ti and depleted in Nb、Rb, indicating they camefrom depleted mantle.4,Through the isotopic dating of Re-Os, we achieved their ages between475and313Ma which was the formation age of the ocean basin.5,By the comparition of the petrological, geochemical and isotopic chronologycharacteristics of these ultrabasic rocks with the Hegenshan’ultrabasic rocks in thewest,and they have the very high similarity, we believe that this area is probably theeastward extension of the Hegenshan suture. So this paper provide important newevidence for the problem of the eastern of the Hegenshan suture and clarify the timeof the ultrabasic rocks formation.更多还原