内蒙古东乌旗地区崇根山岩块超基性岩地球化学特征

崇根山岩块是贺根山蛇绿岩带内超基性岩出露面积最大的岩块,岩性主要以二辉橄榄岩为主,方辉橄榄岩次之以及少量纯橄岩透镜体或团块。岩石学和地球化学特征表明这套超基性岩属于位于蛇绿岩套中下部的地幔橄榄岩序列,其二辉橄榄岩和方辉橄榄岩具有富Al的特征,分异指数很低(DI=0.1~2.79),而固结指数很高(SI=79.9~89.62),代表了原始地幔的残余。稀土元素表现为LREE弱亏损的特征,与阿尔卑斯型辉橄岩相似;纯橄岩稀土含量仅为球粒陨石的1/10,具有"U"型稀土元素分布模式,与阿尔卑斯橄榄岩和SSZ型蛇绿岩地幔橄榄岩相似。崇根山大面积出露的二辉橄榄岩可能代表了洋盆扩张早期的地幔橄榄岩的残余。整体上富Sr贫Ti的现象暗示了后期消减作用对地幔橄榄岩地球化学结构的影响。

Geochemical characteristics of Chonggenshan ultrabasic rocks of Dong Ujimqin in Inner Mongolia

Chonggenshan ultrabasic rock is the largest block of Hegenshan ophiolite belt，which mainly com- posed by lherzolite，harzburgite and a few dunite lenticular body or mass． Characteristics of petrography and geo- chemistry indicate that the ultrabasic rocks belong to the mantle peridotite sequence at lower- middle part of ophiolite suite． Lherzolite and harzburgite are enrichment in Al，with low differentiation index (DI =0. 1 ～2. 79) and high consolidation index (SI =79. 9 ～89. 62)，representing the remnants of primitive mantle． REE showed weak loss of LREE ，which is similar to Alpine lherzolite． REE of dunite is only 0. 1 times of chondrite，displaying irregular “U- shaped”patterns，which is simlilar to Alpine peridotite and mantle peridotite of SSZ type ophiolite． The large area exposed Chonggenshan lherzolite may represent residual mantle peridotite caused by the early expansion of o- cean basin． The phenomenon of rich in Sr，but low in Ti suggested that the effect on the geochemical structures of mantle peridotite by later subduction．