白银厂矿田石英钠长斑岩Sm-Nd,Rb-Sr同位素特征及意义

THE CHARACTERISTICS OF THE QUARTZ ALBITOPHYRES FROM BAIYIN MINE FIELDS ASCERTAINED BY ISOTOPES OF Sm-Nd AND Rb-Sr

甘肃省白银厂矿田含矿火山岩中的石英钠长斑岩 ,对于该区火山岩的定年及形成物源区特征的研究具有十分重要的意义。作者在前人同位素分析数据稀疏的区域内 ,于现采区井下采集了 4个新鲜无蚀变的石英钠长斑岩 ,进行了 Sm- Nd,Rb- Sr同位素分析。结果表明 ,该区石英钠长斑岩形成时代有二期 ,第一期成岩年龄为 5 0 2 .5 0～5 39.46 Ma,形成时代为中晚寒武世 ;第二期成岩年龄为 395 .80～ 40 2 .97Ma,形成时代为早泥盆世 ,依据石英钠长斑岩的 Sm- Nd模式年龄 ,并结合次火山岩的形成时代与该区火山岩同期或稍晚的关系 ,可确定该区火山岩的形成时代为中寒武世。同位素特征还表明 ,该区火山岩物质来源于经过一定富集作用的地幔 ,且地幔源区发生过部分熔融。

Based on the previous studies of the volcanic rocks, the authors sampled 4 fresh quartz albitophyres from the well producing areas in Xiao Tieshan Mine to make up for the location not yet studied by previous workers. The isotopic analyses of the samples are made. The results, together with previous information, show that the formation of the quartz albitophyres has 2 episodes-the first at 502.5~539.46 Ma BP and the second at 395.8~402.97 Ma. According to the Sm\|Nd isotopic model age for the quartz albitophyres and the formation of the subvolcanic rocks synchronogenic with or a little later than the volcanic rocks in this area, it is well defined that the volcanic rocks were formed in the Mid\|Cambrian period. Their characteristics indicate that the volcanic rocks could be generated from the variably enriched mantle source.