Abstract: GangdiseTengchong Block is one of most significant components of EastTethyan orogenic belt and therefore is the key to explore the evolution of Tethys. An outcrop of Santaishan ophiolites tectonically located in the GaoligongRuili fault zone and exposed as part of an accretionary complex, consist mainly of dunites and harzburgites that are dominated by Mgperidotites (Fo=9093), enstatite (En=8992) and Crspinel (Cr# =5478). Highly depleted modal, mineral and bulk rock compositions of the dunites and harzburgites indicate both rocks were residues from high degree partial melting (25%35%) in the spinel stability field. These rocks, lacking typical MORBtype chondritenormalized Rare Earth Element (REE) patterns, display a pattern of slightly enriched LREE and weak depletion of HREE. These characteristics, combined with the rocks' mineral and wholerock compositions, which lie between those of abyssal and forearc peridotites, imply that the selective enrichments of LREE and Pb were the result of meltrock interactions. This study suggests a twostage model for the genesis of peridotites, i.e., these rocks originated from a MORB upper mantle source and were subsequently trapped to form part of a mantle wedge above a subduction zone. Comparative observations on the ophiolitic massifs along the entire YZSZ (Yarlung Zongbo Suture Zone) further suggest that the tectonic setting of the Santaishan ophiolites is similar to YZSZ.

Key words: Yarlung Zongbo subduction complex, western Yunnan, mantle peridotites, spinel, ophiolites zone