The water-poor and reduced mantle beneath the Neo-Tethys Ocean: Tectonic setting of the western Yarlung-Tsangpo ophiolites

Abstract

Water contents and deformation behaviors of minerals in mantle peridotites are diagnostic features to distinguish the physical conditions between mid-ocean ridges and subduction zones, which could serve as effective indicators for discriminating the tectonic setting of ophiolites. In this study, we report a systematic dataset of petrology, geochemistry, water contents and deformation behaviors for the mantle peridotites from the Cuobuzha ophiolite in the western Yarlung-Tsangpo suture zone (YTSZ), southern Tibet, to provide new insights into the long-standing debate for the tectonic setting of ophiolites therein. Geochemical data revealed that the Cuobuzha mantle peridotites experienced low to moderate degrees of partial melting (∼10–15%), although some harzburgites have been intensively modified by late-stage melt-rock interactions. The Cuobuzha peridotites, on the other hand, are characterized by low orthopyroxene water contents (46–95 wt ppm in average), clear olivine crystallographic preferred orientations of A- and E-types, and low oxygen fugacity (logfO2, QFM −2.55 to −0.81, where QFM means quartz-magnetite-fayalite buffer), thus indicative of the occurrence of high-temperature, water-depleted, and reduced mantle in this ophiolite. The features above of the Cuobuzha mantle peridotites are quite similar to those of abyssal peridotites at modern mid-ocean ridges and systematically different from the relatively refractory, water-rich, and oxidized characteristics typical of forearc peridotites in suprasubduction zones. Based on the data from this study and a compiled dataset for other ophiolites from the western YTSZ, we revealed that mantle peridotites with a water-poor and reduced nature are common in these ophiolites, thus providing strong evidence supporting for their origination from a mid-ocean ridge of a mature ocean basin. From the perspective of mantle peridotites, the ophiolites in the western YTSZ now geographically distributed in two branches (i.e., the northern and southern sub-belts) might have formed in a single ocean basin of the Neo-Tethys and were dismembered by late-stage tectonic events to their current locations.