内蒙古达茂旗北部奥陶纪埃达克岩类的识别及其意义

内蒙古达茂旗北部奥陶纪侵入岩分布于槽台边界断裂(乌兰布拉格—哥舍深大断裂)以北,岩石类型主要有闪长岩、石英闪长岩、英云闪长岩和花岗闪长岩。该套侵入岩具高w(Sr)、低w(Y)和w(Yb)的特征,具高的w(Sr)/w(Y)和w(La)/w(Yb)比值;轻稀土富集,重稀土亏损,具正铕异常或微弱的负铕异常;微量元素蛛网图中具明显的Sr峰和Nb、Ti低谷,与典型的埃达克岩(adakite)特征一致。通过与典型adakite、太古代TTD(G)及岛弧和大陆边缘弧ADR的对比,表明本区奥陶纪侵入岩以具轻、重稀土强烈分馏型而与岛弧和大陆边缘弧ADR区别明显,而与adakite和TTD(G)相似,同时又以具明显的Sr正异常而区别于太古代的TTD(G)。地球物理、重力及航磁资料显示槽台边界断裂两侧有完全不同的地壳结构。北侧可能是在晚古生代早期拼贴到华北陆块的一个小微地块,拼贴过程中二者之间的加里东洋壳向微地块之下俯冲消减,俯冲板片在榴辉岩相或角闪岩相—榴辉岩相过渡相发生部分熔融形成adakite岩浆,熔融残留物主要为石榴石。

THE IDENTIFICATION OF ORDOVICIAN ADAKITES AND ITS SIGNIFICATION IN NORTHERN DAMAO,INNER MONGOLIA

Ordovician intrusive rocks of Damao in Inner Mongolia only distribute at the north of the rupture between platform and geosycline (the rupture from Wulanbulage to Geshe), and are diorite, quartz diorite, tonalite and granodiorite. Those intrusive rocks have the characteristics of high Sr, low Y and Yb content,high Sr/Y and La/Yb ratio,enrichment in LREE, relative depletion in HREE and absence of Eu negative anomaly. Mutle elements spidergram shows enrichment of Sr, relative depletion of Nb and Ti. All of those indicate that Ordovician intrusive rocks are similar to adakites. Through the contrast among adakite, Archean TTD (G),island arc and continental arc ADR, Ordovician intrusive rocks in this region have strong REE fraction model and Sr positive anomaly, and thus distinguish with island arc and continental arc ADR and Archean TTD (G), and are similar to adakite. The data of physical geography, gravity and aerial magnetometer show that there are different crustal structurs between platform and geosycline.The north of rupture may be a small block that joined the North China Platform during early stage of Neopaleozoic. The adakites in this region might relate to the subduction of ocean plate to the small block, and were the result of partial melting of subduction ocean plate in eclogite facies or transitional phase between eclogite facies and amphibolitic facies.The relict is mainly garnet.