藏南罗布莎蛇绿岩成因:壳层熔岩的Sr-Nd-Pb同位素制约

ORIGIN OF THE LUOBUSA OPHIOLITE,SOUTHERN TIBET:Sr-Nd-Pb ISOTOPIC CONSTRAINTS ON CRUST LAVAS

罗布莎蛇绿岩是雅鲁藏布江蛇绿岩带东段出露较好，也是研究程度较高的（蛇绿岩片之一，其壳层熔岩的Sr，Nd，Pb同位素组成具有较高的ISr值，较低的Isd值，大的εNd（f）值和富集放射性成因铅的特征。ISr值偏高可能是存在富集型地幔源区和蛇绿岩形成过程中经受了地幔流体的交代作用引起的。Nd同位素及Sr-Nd同位素图解表明罗布莎蛇绿岩壳层玄武岩源区有N-MORB的特征，又有EMⅡ型富集地幔组分的贡献。Pb-Pb图解上样品均投影在NHRL上方，ASr为47．4～73．0（平均为58．4），△7／4Pb为0．69～12．20（平均为5．87），A8／4Pb为12．78～58．39（平均为35．23），表明存在Dupal同位素异常。根据Sr-Nd，PhPb和Nd-Pb同位素相关特征，判明罗布莎蛇绿岩壳层熔岩是由具亏损地幔端员（DMM）性质的岩浆和具EMⅡ型富集地幔端员性质的岩浆在源区混合形成的。

The Luobusa ophiolite is one of well-outcroped and well studied rock in the eastern segment of Yarlung Zangbo ophiolite belt. The Sr Nd Pb isotopic compositions of crust lavas are characterized by higher initial Sr isotopic ratios,lower Nd isotopic ratios,large εNa（t） and enrichment in radiogenic lead. The reasons for higher initial Sr isotopic ratios are probably that there is enriched mantle endmember and the ophiolite underwent metasomatism of fluid from the melting of terrestrial matter in the course of formation. Nd isotope and Sr Nd isotope demonstrate that the source of crust lavas in Luobusa ophiolite has not only the characteristics of N-MORB,but al so the contribution of EMⅡ enriched mantle endmember. On the Pb Pb diagram,the analytical spots are all plotted above NHRL,ASr=47.4-73.0 （58.4 in average）,△7/4Pb=0. 69-12. 20 （5.87 in average）,A8/4Pb= 12.78-58.39 （35.23 in average）,indicating that there is Dupal isotopic anomaly. The Sr-Nd,Pb-Pb and Nd-Pb isotopic relationships of the crust basalts suggest a mixing source of magmas with the properties of the depleted mid-ocean ridge basalt mantle endmember （DMM） and EMⅡ enriched mantle endmember in Luobusa ophiolite.