雅鲁藏布江蛇绿岩带西段错不扎地幔橄榄岩组成特征及岩石成因

Petrology, geochemistry and genesis of the Cuobuzha peridotite in the western Yarlung Zangbo suture zone

错不扎蛇绿岩位于雅鲁藏布江缝合带西段北亚带,岩体呈北西-南东走向带状产出,主要由地幔橄榄岩和辉长岩脉组成。地幔橄榄岩主体为方辉橄榄岩,详细的矿物学及岩石地球化学研究表明,错不扎方辉橄榄岩中橄榄石为镁橄榄石,斜方辉石主要为顽火辉石,而单斜辉石主要为顽透辉石和透辉石,铬尖晶石具有高Al和高Mg（Mg#=60~70）特征。稀土配分图解显示其具有轻稀土亏损而重稀土富集的左倾型亏损地幔源区特征,（La/Yb）N=0.11~0.60,模拟结果显示其为经历了15%~20%部分熔融后的残余,与快速扩张大洋中脊环境下形成的深海橄榄岩的熔融程度（10%~22%）较为一致。此外,错不扎方辉橄榄岩轻稀土含量明显高于部分熔融模型中LREE的含量,而且,在微量元素原始地幔标准化图解中富集大离子亲石元素Rb、Sr和高场强元素Ta、Hf和Ti,这一特征指示错不扎方辉橄榄岩在大洋中脊环境形成后又受到后期俯冲带熔/流体的改造。结合南北两带不同蛇绿岩体构造环境的对比,笔者认为雅鲁藏布江西段南北两带蛇绿岩体具有相似的形成环境,两者在地理位置以及产状方面的差别可能是受到构造侵位的影响。

The Yarlung Zangbo suture zone（YZSZ）, which separates Eurasia in the north from the Indian plate in the south, can be divided into eastern, central and western segments. The western part is further divided into Dajiweng- Saga ophiolite zone in the north and Daba-Xiugugabu ophiolite zone in the south. The studied Cuobuzha ophiolite is located in the north part of the western YZSZ. The mantle peridotites mainly consists of harzburgites and associatd dikes or veins of gabbro in the massif. The characteristics of the mineral show that the Fo values of the olivine in mantle peridotites are high, and all of the olivines are forsterites; in addition, the orthopyroxenes are mainly enstatites, the clinopyroxenes are endiopsides and diopsides. Chrome spinels are characterized by high Al（Cr#=22-39） and high Mg#（60-70）. The harzburgite in Cuobuzha has similar distribution patterns of rare earth elements, characterized by their slight depletion of LREE and weak enrichment of HREE with（La/Yb）Nbeing 0.11-0.60.Fractional melting modeling indicates that harazburgites may be the residues from 15%-22% melting. However, the LREE values of the Cuobuzha peridotites are higher than the calculated LREE values. In combination with their enrichment of large ion lithophile elements（LILE, e.g., Rb and Sr） and high field strength elements（HFSE, e.g., Ta, Hf and Ti）, the authors hold that these rocks originated from partial melting of mantle source and was later modified by fluids and melts in a suprasubduction zone mantle wedge.Comparative study of the western YZSZ suggests that the tectonic setting of the two ophiolitic subbelts are similar to each other, and the tectonic emplacement probably resulted in the differences between them.