雅鲁藏布江缝合带西段错不扎地幔橄榄岩铂族元素特征

Platinum-Group Element Features of Cuobuzha Peridotite from Western Yarlung Zangbo Suture Zone

对位于雅鲁藏布江缝合带西段北亚带错不扎地幔橄榄岩铂族元素地球化学特征进行研究,旨在对其岩石成因和构造环境进行限定。错不扎地幔橄榄岩以方辉橄榄岩和含单辉方辉橄榄岩为主。含单辉方辉橄榄岩具有较高的铂族元素(PGEs)含量,w(PGEs)=22.31×10^(-9)~36.12×10^(-9),平均值为29.60×10^(-9),高于原始地幔含量;而方辉橄榄岩的w(PGEs)处于17.34×10^(-9)~25.18×10^(-9)之间,平均值为21.08×10^(-9),略低于原始地幔值。相对岩浆成因超基性岩,两岩类皆强烈富Os和Ir,为部分熔融后残余的地幔体。含单辉-方辉橄榄岩及部分方辉橄榄岩含较高的Pd,(Pd/Ir)m比值远大于1,具左倾型PGE组成模式,而且两者都具有低于原始地幔的Cu/Pd比值。综合研究表明错不扎方辉橄榄岩是熔融程度较低的含单辉-方辉橄榄岩继续部分熔融后的残余;两者在部分熔融过程中可能存在部分赋存Pd的硫化物未完全溶解进入硅酸盐熔体,此后又受到俯冲带岩浆渗滤形成的富硫化物熔体的改造。

For the origin and tectonic setting, feature of Platinum-group elements（PGEs） from Cuobuzha peridotites in western Yarlung-Zangbo suture zone was reported in this study. The studied peridotite mainly consists of Cpx-harzburgite and harzburgite. The Cpx-harzburgite is characterized by higher PGEs content（22.31×10^-9-36.12×10^-9） than primitive mantle, and with a average of 29.60×10^-9, whereas, the harzburgite has lower PGEs content（17.34×10^-9-25.18×10^-9） than the primitive mantle. Compared with magmatic ultramafic rocks, two types of rocks are characterized by higher Os and Ir, implying that they are residual mantle rocks after partial melting and melt extraction. In addition, the Cpx-harzburgites and some harzburgites are characterized by high Pd content,（Pd/Ir）m ratio, low Cu/Pd ratio, and left-leaning PGE model. Based on the above discussion, we suggest that the harzburgite are the residual mantle rocks resulted from higher degree of partial melting of the cpx-harzburgite. There may be residual sulfides which have not been totally melted into the silicate melt. At the same time, melt supersaturated in sulfur probably added to the peridotites by melt-rock reaction in the suprasubduction zone.