南蒙古巴音苏赫图矿区英安岩地球化学特征及其构造意义

摘要

A set of volcanic rocks were recently found in Palaeozoic strata in the Bayinsukhtuore district of South Mongolia.The Rb-Sr isotopic age of the rocks is 409±19Ma,which is consistent with the chronological age(Silurian-Devonian)obtained from previous work.The SiO2content of dacite is 63.23%~73.36%,while the Al2O3content is 10.25%~15.56%.The dacite is LREE enrichment, HREE loss,and has strong Eu negative anomaly.Based on petrology and geochemistry,it is held that this set of volcanic rocks evolved in the same period and came from the same source.The lithologic character of the rocks is mainly of dellenite-dacite type,with little change.According to the Kalium analysis,the volcanic rocks were derived from the potassium calc-alkaline series.It is suggested that the melted mantle source area was reformed by the subducting plate.During this process the original magma was generated from the liquid mantle of the subducting zone.At the same time crustal contamination happened.The magmatic evolution would be the main reason for generating the combined set of the volcanic rocks.It also implies that this set of volcanic rocks was formed in the island arc during the closing of pal-Asia,which indicates that the closing of pal-Asia was later than the formation of this set of volcanic rocks.%在南蒙古巴音苏赫图矿区古生代地层中新发现一套火山岩,其Rb-Sr同位素年龄为409±19Ma,与前人划分的地层年代(志留纪—泥盆纪)较吻合.英安岩的SiO2含量为63.23%~73.36 %,Al2O3含量为10.25%~15.56%.英安岩轻稀土元素富集,重稀土元素亏损,具有较强的负Eu异常.岩石学和地球化学综合研究表明,该套火山岩基本同期同源演化,且岩性变化较小,以流纹岩-英安岩类为主.根据钾含量,该套火山岩为中钾钙碱性系列.英安岩落在靠近盆岭构造域的位置,显示出略亏损的特征,与原始地幔PM端元接近.推测该套火山岩形成于古亚洲洋闭合过程中的岛弧环境,表明火山岩形成时古亚洲洋可能还没有闭合.