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

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 400 km2, 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) = 0.22-0.96 Ga, 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.