Petrological and Os isotopic constraints on the origin of the Dongbo peridotite massif, Yarlung Zangbo Suture Zone, Western Tibet

• Published in: Journal of Asian earth sciences Vol. 110; pp. 72 - 84
• Main Authors: Niu, Xiaolu, Yang, Jingsui, Dilek, Yildirim, Xu, Jifeng, Li, Jie, Chen, Songyong, Feng, Guangying, Liu, Fei, Xiong, Fahui, Liu, Zhao
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.2015
• Subjects: Dongbo peridotite massif; Neotethys; Residual mantle rocks; Re–Os isotopes; Yarlung Zangbo Suture Zone; Residual mantle rocks; Re–Os isotopes; Neotethys; Yarlung Zangbo Suture Zone; Dongbo peridotite massif
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2014.09.036

•The Dongbo peridotites were residues of mantle rocks after variable degrees of basaltic magma extraction at mid-ocean-ridge environment.•The Dongbo peridotites were then involved in an arc environment and modified by percolating subduction-ralted melts.•The Neo-Tethys Oceanic mantle from which the Dongbo ophiolite was derived was heterogeneous in Os isotopes. The Dongbo peridotite massif in the western part of the Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet represents a mantle thrust sheet of more than 400km2, tectonically overlying a Cretaceous mélange. Spinel-harzburgite with a porphyroclastic texture is the main rock type, accompanied by subordinate dunite. Olivine (Fo90.3–91.3), orthopyroxene (En87–90), clinopyroxene (En49–51Wo46–51Fs0–4) and spinel [Mg#=100Mg/(Mg+Fe)]=41.1–73.1; Cr#=(100Cr/(Cr+Al)=23.6–87.3] are the major minerals. The peridotites are characterized by “spoon-shaped”, chondrite-normalized REE patterns with (La/Yb)CN=0.09–0.40, and (Gd/Yb)CN=0.23–0.95. The patterns show relative uniform depletion from Lu to Nd, followed by slight enrichment from Nd to La. These rocks represent residues of mantle rocks after variable degrees of basaltic magma extraction beneath a seafloor spreading environment, followed by modification by percolating, suprasubduction-related melts. The Dongbo peridotites have variable, unradiogenic Os isotopic compositions with 187Os/188Os=0.1235–0.1282 and rhenium-depletion ages (TRD)=0.22–0.96Ga, implying heterogeneity of Os isotopes in the mantle beneath Neotethys. A corollary to this interpretation is that the convecting upper mantle is heterogeneous in Os isotopes.