内蒙古白乃庙地区绿片岩和花岗闪长斑岩的钕和锶同位素研究

Nd-and Sr-Isotope Study on Greenschist and Granodiorite of the Bainaimiao District,Inner Mongolia,China Nie Fengjun Pei Rongfu

白乃庙群主要分布在内蒙古白乃庙－谷那乌苏一带。这套岩层与毗邻的侵入体不仅是白乃庙－温都尔庙构造岩浆带的重要组成部分，而且赋存有一系列铜（金）和金矿床（点）。长期以来，由于缺乏可信的年龄数据，一般认为：白乃庙群绿片岩和侵入岩体均属奥陶－志留纪￣［１］。据最新资料，白乃庙群绿片岩锆石铀－铅年龄为１１３０±１６Ｍａ￣［２］属中元古代。查明绿片岩和侵入岩体的物质来源和形成机制，对于了解华北板块北缘内蒙古中南段的构造。岩浆演化，确定岩浆活动与金属成矿作用关系具有重要意义。笔者对白乃庙群绿片岩和花岗闪长斑岩进行了Ｎｄ和Ｓｒ同位素研究。并且对它们的形成时代和成因提出了一些新的看法。

Sm-Nd AND Rb-Sr isotope data for greenschist of the Bainaimiao Group,bainaimiao granodioriteand Zhuandayingzhi hornblende-plagioclase gneiss,Inner Mongolia,China are presented in the paper.Seven whole rock samples of the greenschist give a Sm-Nd isochron age of 1107±28Ma(2δ)withε_(Nd)(t)value of+7.1±1.1(2d),which is in agreement with a previously reported U-Pb zircon age of1130+16Ma(2δ).Eight whole rock samples of the granodiorite, which intruded into the Bainaimiaogreenschist give a Sm-Nd isochron age of 440±40Ma(2δ)with ε_(Nd)(t)value of-3.2±1.7 (2δ).This confirms assumption that the granodiorite was formed during Caledonian orogeny.The Rb-Sr isotopic system has been strongly distubed.This is consistent with petrographic andtectonic studies,which indicate that all greenschist,granodiorite and hornblende-plagioclase gneisshave undergone in tensive metamorphism and tectonism.The initial ratios of the Nd isotope signaturessuggested that the greenschist of the Bainaimiao Group was originally derived from a depleted mantlesource.The ε_(Nd)(t)value of the Bainaimiao granodiorite indicates that it may have been formed by par-tial melting of the mixed mantle-derived greenschist and curst-related hornbled-plagioclase gneiss.