滇西地区上地幔铅同位素组成的确定及其应用

滇西地区基性 超基性岩、碱性岩中的长石、石榴子石、橄榄石等单矿物及羊拉铜矿区二叠世海相玄武岩的铅同位素组成可以代表原始地幔的铅同位素组成 ,由此确定的该区上地幔铅同位素组成变化范围为　2 0 6Pb/2 0 4Pb =1 7.877～1 8.5 0 6,平均 1 8.1 0 8,2 0 7Pb/2 0 4Pb=1 5 .470～1 5 .5 87,平均 1 5 .479,2 0 8Pb/2 0 4Pb=3 7.797～ 3 8.5 67,平均 3 8.1 77。研究发现 ,由于受地壳物质的混染和成岩后U、Th衰变产生的放射成因铅的影响 ,这些幔源岩石全岩铅同位素组成已不能代表原始上地幔的铅同位素组成 ;金顶铅锌矿床的铅并非上地幔来源 ,而是相当于下地壳来源的铅与兰坪盆地沉积岩铅二者的混合铅。

DETERMINATION AND APPLICATION OF THE UPPER MANTAL LEAD COMPOSITION IN WESTERN YUNNAN

Based on the minerals plagioclase, garnet and olivine from basic-ultrabase rocks,alkaline rocks, and basalts from the Yangla copper deposit in western Yunnan, the lead isotopic composition of the mantle in the region studied was determined. The HT10.5,0.25] ~ 206Pb/~ 204Pb values vary from 17.877 to 18.506 with an average of 18.108,~ 207Pb/~ 204Pb ratios from 15.470 to 15.587 with an average of 15.479, and HT10.5,0.25] ~ 208Pb/~ 204Pb values from 37.797 to 38.567 with and average of 38.177. Because of the contamination of crustal lead and the influence of radioactive lead derived from decay of U and Th after crystallization of the rock, the lead isotopic composition of the whole rock is different from that of the upper mantle. The mantle lead isotopic background determined thereby is of significance in constraining the origin of magmatic rocks and the lead source of ore deposits in the region. By the way, the authors discovered that lead in the Jinding Pb-Zn deposit is not the upper mantle lead, it may be derived from the lower crust and sedimentary rocks in the Lanping Basin.