Genesis and tectonic setting of the Dongqiao peridotites in the central segment of the Bangong Co-Nujiang Suture Zone

Located in the central segment of the Bangong Co-Nujiang Suture Zone, the Dongqiao ophiolite is divided into east and west massifs. The research on the origin and tectonic setting of the ophiolite in the Bangong Co-Nujiang suture zone can provide key clues for revealing the tectonic evolution of the Neo-Tethys and the collision between the Indian plate and the Eurasian plate. The area of the west rock massif is relatively large, consisting of mantle peridotites, pillow-like basalts, gabbros, diabases, etc., while the area of the east massif is small and contains only part of mantle peridotites, with the faults being in contact between different units. Based on petrology, mineralogy and geochemistry of the peridotites in Dongqiao ophiolite, the authors have reached some conclusions:(1) The Dongqiao peridotites are dominated by harzburgites, with minor proportion of dunites (about 15%). The podiform chromitites mainly show banded and disseminated structures, and occur in a thick layer of far-extending dunites. (2) The clinopyroxene content of Dongqiao peridotites is less than 3%. The mineral and the whole-rock geochemistry show that Dongqiao peridotites were formed after 22%-28% degree of partial melting of the spinel-phase mantle source, higher than the partial melting of abyssal peridotites (10%-22%). (3) The Cr^# values of spinels in the Dongqiao peridotites are higher than 60. The chondritenormalized REE patterns of the Dongqiao peridotites display enrichments in LREE. In the primitive mantle-normalized spider diagrams, the Dongqiao peridotites exhibit slightly positive Rb, U, Zr, Sr anomalies and slightly negative Hf, Nb anomalies. These geochemical characteristics jointly indicate the selective enrichments of LREE and some trace elements according to interaction between the residual mantle and the fluid/melt above the subduction zone. Based on the mineral composition and the whole-rock geochemical characteristics of Dongqiao peridotites, the authors hold that the Dongqiao peridotites were formed in the dilated environment of the mid-ocean ridge, and then were influenced by intra-ocean subduction, resulting in highly partially melted peridotites interacting with the fluid/melt above the suprasubduction zone.