Petrological and Re‐Os Isotopic Constraints on the Origin and Tectonic Setting of the Cuobuzha Peridotite, Yarlung Zangbo Suture Zone, SW Tibet, China

Abstract

The Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet includes the remnants of Neotethyan oceanic lithosphere and marks a major suture between the Indian Plate to the south and the Lhasa Terrane of Tibet to the north (Dupuis et al., 2005; Yang et al., 2011). In the western part of the YZSZ, the Northern and the Southern sub‐belts form two sub‐parallel zones of mafic – ultramafic rock assemblages with overlapping crystallization ages (Xiong et al., 2011; Hebért et al., 2012; Liu et al., 2015). The upper mantle section of the Cuobuzha ophiolite in the northern sub‐belt of the Yarlung–Zangbo Suture Zone (YZSZ) in SW Tibet comprises mainly clinopyroxene (cpx)–rich and depleted harzburgites. Spinels in the cpx‐harzburgites show lower Cr# values (12.6–15.1) than the spinels in the harzburgites (26.1–34.5), and the cpx‐harzburgites display higher heavy rare earth element concentrations than the depleted harzburgites. The harzburgites have subchondritic Os isotopic compositions (0.11624–0.11699), whereas the cpx‐harzburgites have suprachondritic 187Os/188Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb). The cpx‐harzburgites plot in a Re vs. Al2O3 diagram as a result of subsequent addition of Re following the last partial melting event that occurred during mid‐ocean ridge melt evolution processes (Uysal et al., 2015).Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid‐ocean ridge type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx‐harzburgites formed from low‐degree partial melting (∼5%) of a primitive mantle source, and they were subsequently modified by melt–rock interactions in a mid‐ocean ridge environment. The depleted harzburgites, on the other hand, were produced by re‐melting of the cpx‐harzburgites, which later interacted with MORB‐ or island arc tholeiite (IAT)‐like melts (Fig. 1) possibly in a trench‐distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neotethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.