塔里木大火成岩省瓦吉里塔格霞石岩铂族元素地球化学特征

作为塔里木大火成岩省形成最晚的火成岩,新疆巴楚瓦吉里塔格霞石岩的岩浆源区性质的确定对于揭示塔里木大火成岩省的深部地质过程具有重要的约束作用。对瓦吉里塔格霞石岩的铂族元素地球化学特征进行了研究,铂族元素(PGE)分析结果显示,原始地幔标准化的PGE呈正斜率型分布,且Pd/Ir值高于原始地幔比值,说明霞石岩的铂族元素发生了分异。霞石岩全岩的PGE与Mg O呈正相关,Pd/Ir、Cu/Pd与Mg O则呈负相关,说明PGE的分异主要受到橄榄石的结晶分异作用控制,也是其Cu/Pd值极高及岩浆S饱和的因素之一,同时Cu/Pd值说明霞石岩岩浆为硫饱和岩浆,但是没有因素导致岩浆S过饱和进而发生硫化物的熔离。与其他大火成岩省岩石相比,瓦吉里塔格霞石岩极度亏损PGE,SCSS(硫承载量)计算结果表明母岩浆在形成之初就发生S过饱和,主要是地幔低程度部分熔融造成的,据此认为地幔源区的部分熔融程度在塔里木大火成岩省Cu-Ni硫化物铂族元素矿床形成过程中起着至关重要的作用。

Geochemical characteristics of platinum group elements in Wajilitag nephelinite from Tarim Large Igenous Province, Northwest China

This paper reports the geochemical characteristics of platinum-group elements (PGEs) in nephelinite lavas from Wajilitag complexes of Bachu County in Xinjiang. The magma source and the magma evolution of nephelinite are discussed. The values of such platinum-group elements as Os, Ir, Ru, Rh, Pt and Pd in nephelinite were determined by ICP-MS. They display a positive slope in the mantle-normalized pattern. The fact that Pd/Ir values are higher than those of the primitive mantle suggests the fractionation of PGE in nephelinite. And positive correlations for PGE versus MgO, negative correlations for Pd/Ir and Cu/Pd versus MgO suggest that the PGE differentiation was probably controlled by crystallization differentiation of olivine. It is also one of the factors that cause the high Cu/Pd ratio and magma S saturation. The Cu/Pd ratio indicates that the magma is sulfur saturated magma, but there is no factor that leads to the S-oversaturation of magma and sulfide liquation. Sulfur capacity calculation shows that, compared with other rocks of large igneous province, Wajilitag nephelinite is extremely depleted in PGE due to the S-oversaturation of parent magma, with the main reason being the low degree partial melting of the mantle. It is considered that the partial melting degree of mantle source played an important role in the formation of Cu-Ni sulfide PGE deposits in the Tarim Large Igneous Province.