内蒙古狼山地区晚太古代变质杂岩特征探讨

THE FEATURES OF LATE ARCHEAN METAMORPHIC COMPLEX IN THE MOUNT LANGSHAN AREA,INNER MONGOLIA

狼山位于内蒙古中部巴彦淖尔盟,在狼山主峰出露一套角闪岩相变质杂岩,它主要由角闪斜长片麻岩、黑云斜长片麻岩、斜长角闪岩、角闪岩、黑云变粒岩、黑云石英片岩组成,局部夹磁铁石英岩。其原岩以中、基性火山岩为主,顶部则为变粘土质碎屑岩。这套变质杂岩,至少经历了两期混合岩化作用,它与狼山南、北坡中元古代狼山群有明显的差异,作者对其中黑云斜长片麻岩测得sm—Nd等时线年龄为2735 Ma,证明其时代应为晚太古代。从变质杂岩地层层序、岩石化学、微量元素特征来看,它与绿岩带有许多相似之处。狼山主峰变质杂岩是该区的结晶基底,中元古代狼山群中的层控多金属矿床,就是在该基底上发育的裂陷槽中形成的。结晶基底自身的岩石中,Cu、Pb、Zn、Au等元素含量均较高,亦具有一定的找矿前景。

There occurs a suite of amphibolite-facies metamorphic complex in the main peak of Mount Langshan. It is mainly composed of hornblende-plagioclase gneiss, biotite-plagioclase gneiss, plagioclase amphibolite, amphibolite, biotite granulitite and biotite-quartz schist, locally intercalated with itabirite. Its protolith consists mainly of intermediate-basic volcanic rocks and at least underwent two phases of migmatization. It is notably different from the Middle Proterozoic Langshan Group on the northern and southern slopes of Mount Langshan. The biotite-plagioclase gneiss has been measured to have a Sm-Nd isochron age of 2735 Ma, which proves it to be of Late Archean age. It has many similarities to the greenstone belt in respect to the sequence of metamorphic complex, petrochemistry and trace element characters. It shows higher contents of Cu, Pb, Zn and A. The stratabound polymetallic deposits in the Langshan Group were just formed in the taphrogenic trough developing on the crystalline basement composed of this kind of complex.