Heterogeneous mantle beneath the Neo-Tethys Ocean revealed by ultramafic rocks from the Xiugugabu Ophiolite in the Yarlung-Tsangpo Suture Zone, southwestern Tibet

Ultramafic rocks of the Yarlung-Tsangpo ophiolites (lherzolites, harzburgites, dunites and pyroxenites) have been interpreted as lithospheric mantle relics of the Cretaceous Neo-Tethys Ocean (NTO). Studying these rocks could provide key constraints on the NTO’s evolution and invaluable information on sub-ridge mantle heterogeneity. The Xiugugabu ophiolite crops out in the western segment of the Yarlung-Tsangpo Suture Zone (YTSZ) and is dominantly composed of mantle components with minor crustal material. Here we report a comprehensive field, petrological and geochemical study of the ultramafic rocks from this ophiolitic massif. The lherzolites and harzburgites are less refractory than the Xigaze ultramafic rocks in the middle part of the YTSZ. Petrology and simple models are consistent with 10–20% fractional melting from an anhydrous, depleted mantle source, within the spinel and garnet stability fields. Part of dunites (Dunite-II) and pyroxenites formed during pervasive melt impregnation into host peridotites and record the modal and geochemical effects of post-melting melt–rock interactions. The discrepancy of light rare earth element (LREE) contents between whole rocks and their two pyroxene components in peridotites indicates that they have trapped interstitial melts along grain boundaries at a post-melting stage. These melts were potentially derived from shallow sources in spinel domains. Bulk rock osmium isotopic compositions suggest that the pyroxenites and part of the dunites are highly radiogenic. These enriched Os signatures are likely imposed by percolating melts derived from mantle sources hybridized by ancient recycled crustal materials. When comparing with results from previous studies of other YTSZ peridotites with ultra-depleted nature and unradiogenic Os isotopic compositions, we conclude that the sub-ridge mantle beneath the Neo-Tethyan Ocean was heterogeneous, with not only unradiogenic but also highly radiogenic mantle domains.