PETROLOGOCAL AND RE-OS ISOTOPIC CONSTRAINTS ON THE ORIGIN AND TECTONIC SETTING OF THE CUOBUZHA PERIDOTITE, YARLUNG ZANGBO SUTURE ZONE, WESTERN TIBET, CHINA

The Yarlung Zangbo suture zone (YZSZ) in southern Tibet includes the remnants of Neo-Tethyan oceanic lithosphere and marks a major suture between the Indian plate to the south and the Lhasa terrane of Tibet to the north. In the western part of the YZSZ, the northern and the southern subbelts form two subparallel zones of mafic-ultramafic rock assemblages which showing different lithological, geochemical, and structural characteristics with overlapping crystallization ages.

The upper mantle section of the Cuobuzha ophiolite in the northern subbelt of the western YZSZ 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), yielding Re-depletion model ages (T_RD) ages from 1.8 to 1.7 Ga (Table 6), indicating that the Cubuzha mantle underwent at least one ancient melt extraction event ca. 1.8-1.7Ga; whereas the cpx-harzburgites have suprachondritic ¹⁸⁷Os/¹⁸⁸Os ratios (0.12831–0.13125) with higher Re concentrations (0.380–0.575 ppb), indicating subsequent addition of Re following the last partial melting event that occurred during mid-ocean ridge melt evolution processes.

Although these geochemical and isotopic signatures suggest that both peridotite types in the ophiolite represent mid-oceanic ridge–type upper mantle units, their melt evolution trends reflect different mantle processes. The cpx-harzburgites formed from low-degree partial melting of a primitive mantle source, and they were subsequently modified by melt-rock interactions in a mid-oceanic ridge environment. The depleted harzburgites, however, were produced by remelting of the cpx-harzburgites, which later interacted with mid-oceanic ridge basalt– or island-arc tholeiite–like melts, possibly in a trench–distal backarc spreading center. Our new isotopic and geochemical data from the Cuobuzha peridotites confirm that the Neo-Tethyan upper mantle had highly heterogeneous Os isotopic compositions as a result of multiple melt production and melt extraction events during its seafloor spreading evolution.