Early Cretaceous metadiabases from the Qiongjie ophiolitic mélanges in the eastern Yarlung Zangbo suture zone, southern Xizang

The Qiongjie Complexes (Qiongjie ophiolitic mélanges) in the eastern Yarlung Zangbo suture zone, southern Xizang are delineated as a newly defined geological mapping unit of 1:50 000 regional geological survey, roughly corresponding to part of the Nangxian ophiolitic mélanges in the 1:250 000 geological map. These ophiolitic mélanges consist dominantly of matrix and blocks. The ophiolitic fragments in the blocks are composed of metagabbro, metadiabase and metabasalt. The present paper gives a detailed geochemical study of the metadiabases and reports a LA-ICP-MS zircon U-Pb age of ca. 111 Ma for the crystalline age of magmatic rocks. The metadiabases have low SiO₂ contents ranging from 46.06% to 49.27%, high FeO^T contents (an average of 11.71%), high TiO₂ contents (an average of 1.30%), high Mg^# values (an average of 59.39), and slightly differentiated light/heavy rare earth elements (LREE/HREE=1.83-2.05,(La/Yb)_N=1.35). In the chondritenormalized REE diagram, the metadiabases in the Qiongjie ophiolitic mélanges display the smooth chondritenormalized REE distribution patterns without Eu anomalies (δEu=0.81-1.02), enrichment in high field strength elements Nb, Ta, U and Th, and depletion in large ion lithophile elements Ba and Sr. Geochemically, the metadiabases exhibit the geochemical signatures of normal mid-oceanic ridge basalts, and represent the products of the 10%-20% partial melting of spinel-iherzolite, without crustal contamination during the ascending of the magmas. Tectonically, the metadiabases in the study area are interpreted to be formed in the typical mid-oceanic ridge tectonic setting. It is concluded that the ophiolites in the eastern Yarlung Zangbo suture zone were initiated during the middle Late Triassic to the late Early Cretaceous.