大别山北部榴辉岩的Pb同位素特征

Pb ISOTOPIC CHARACTERISTICS OF THE ECLOGITES FROM THE NORTHERN DABIE MOUNTAINS

大别山北部榴辉岩的全岩 Pb同位素研究结果表明 ,Pb同位素比值 N(2 0 6 Pb) /N (2 0 4 Pb) =16 .773～ 18.339,N (2 0 7Pb) /N (2 0 4 Pb) =15 .346～ 15 .5 16 ,N (2 0 8Pb) /N (2 0 4 Pb) =37.133～ 38.346 ,平均值分别为 17.6 5 ,15 .4 5和 37.81)明显接近于下地壳成分 N(2 0 6 Pb) /N(2 0 4 Pb) ,N(2 0 7Pb) /N(2 0 4 Pb) ,N(2 0 8Pb) /N(2 0 4 Pb)值分别为 17.6 2 ,15 .35和 38.75 ) ,同时 ,北部榴辉岩的 U /Pb比 (0 .0 0 2～ 0 .0 0 9)及 U质量分数 (0 .0 4× 10 - 6 ～ 0 .2 3× 10 - 6 )稍低于下地壳平均值 ,而 Pb质量分数 (6 .72× 10 - 6～ 2 6 .6 4× 10 - 6 )则接近于或略高于地壳平均值 ,但是 ,U/Pb比则明显偏离上地壳平均值、也不同于大别山南部榴辉岩及有关岩石 (较低的 Pb质量分数和较高的 U/Pb比 )。此外 ,考虑到研究区榴辉岩的岩石地球化学及 Sr,Nd同位素年代学方面研究已证明它们大多数为印支期扬子陆壳俯冲变质成因。由此进一步表明 ,大别山北部榴辉岩的原岩大多数应来自于扬子大陆下地壳。这对正确认识大别山碰撞造山带的构造格局和演化具有重要意义。

The whole rock Pb isotopic compositions of the eclogites from the northern Dabie Mountains were reported.Their ~ 206Pb/ ~ 204Pb, ~ 207Pb/ ~ 204Pb and ~ 208Pb/ ~ 204Pb ratios are 16.773~18.339,15.346~15.516 and 37.133~38.346 with average values of 17.65,15.45 and 37.81 respectively,which are close to the compositions of lower crust( ~ 206Pb/ ~ 204Pb, ~ 207Pb/ ~ 204Pb and ~ 208Pb/ ~ 204Pb are 17.62,15.35 and 38.75).In addition,the metamorphic petrology,geochemistry and Sr-Nd isotopic chronology of the eclogites suggest that they were resulted from the subduction of the Yangtze continental crust under ultrahigh-pressure metamorphism of 2.5 GPa.The result provided important data for the recognition of tectonic framework and evolution of the Dabie Mountains.