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

Abstract: 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)N being 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.