西藏达巴—休古嘎布绿岩带铂族元素的分布特征

Distribution characteristics of platinum-group elements in the Daba-Xiugugabu ophiolite zone,Tibet

区内各岩体的地幔橄榄岩中PGE含量及分布型式有所差异。相对于原始地幔,拉昂错和东坡岩体的PGE含量没有显著异常,标准化分布曲线大致平坦,PPGE与IPGE之间分馏不明显,总体特征与中国日喀则地区以及阿尔卑斯和东地中海地区的地幔橄榄岩相似。而当穷岩体的PPGE含量变化范围较大,出现IPGE与PPGE强分馏。单斜辉石岩脉的PGE含量及分布型式与地幔橄榄岩不同:Os、Ir、Ru、Rh亏损而Pt、Pd富集,模式曲线向左倾斜,可能反映其较高程度部分熔融作用和熔体分离作用的产物。铬铁矿石的PGE含量较高,而且变化幅度大(19.7×10^(-9)～5526×10^(-9)),各样品∑PGE平均值(729.33×10^(-9))是原始地幔的25.6倍。与原始地幔相比,Os、Ir、Ru、Rh总体上表现为较强富集,Pt、pd富集与亏损并存。其标准化模式曲线较分散,曲线向右倾斜,IPGE与PPGE分馏较强,多数样品呈现Pt负异常。这些特点与中国罗布莎、希腊、南斯拉夫以及塞浦路斯等世界各地蛇绿岩中的铬铁矿床类似。铬铁矿石的PGE含量与相应样品的BMS含量无相关性,PGE不是主要赋存于BMS内。推测PGE主要以PGM的形式存在,具体矿物名称有待进一步发现与研究。

Platinum-group element （PGE） contents and distribution patterns of different mantle peridotites from ophiolite masses （blocks） are to some extent different. Relative to things of the primitive mantle, PGE in Langcuo and Dongpo masses don＇t show distinct anomalies. The curves of primitive mantle-normalized distribution patterns are approximately even, and there exists no strong fractionation between PPGE and IPGE. On the whole, the PGE characteristics are similar to those of mantle peridotites from Xigaze, Alps and Eastern Mediterranean regions. However, PPGE contents of Dangqiong masses vary in a wide range and don＇t show a strong fractionation between PPGE and IPGE. PGE contents and distribution patterns of clinopyroxenite dykes differ greatly from those of the mantle peridotite：Os, Ir, Ru and Rh are depleted whereas Pt and Pd are enriched. The pattern curve declines to the left. These characteristics suggest that clinopyroxenite dykes were probably formed by the high degree of partial melting as well as melting segregation. PGE contents of chromitite are high and vary in a very wide range of 19.7-5526 ppb. The average ∑PGE （729.33ppb） is equal to 25.6 times its value in primitive mantle. Os, Ir, Ru and Rh are enriched but Pt and Pd have experienced both enrichment and depletion as compared with their values in the primitive mantle. The pattern curves are scattered and decline to the right. There exists a strong fractionation between IPGE and PPGE, and Pt in most of samples assumes negative anomalies. These characteristics are similar to those of chromitite in ophiolites from China＇s Luobusha, Greece, Yugoslavia, Cyprus and some other regions in the world. PGE contents of chromitite are not related to base metal sulfides （BMS） contents in the same samples. PGE don＇t mainly exist in BMS. It is thought that PGE mostly occur in platinum-group element minerals （PGM）, which remain to be investigated and found in future.