The northern subbelt in the western segment of the Yarlung Zangbo suture zone, Tibet, China, includes the Dajiweng, Kazhan, Baer, Cuobuzha, Jianabeng, and Zhalai ophiolitic massifs. These ophiolites are strongly dismembered, typically 1–2 km wide and 10–20 km long, and composed chiefly of peridotites with minor volcanic and siliceous sedimentary rocks. No cumulates have been observed in the northern ophiolitic belt. Harzburgites of the Dajiweng and Zhalai ophiolites have prominent light rare earth element (LREE)–enriched (U-shaped or spoon-shaped) chondrite-normalized rare earth element (REE) patterns. Such patterns have generally been interpreted as the result of modification by suprasubduction zone (SSZ) melts/fluids. However, the abundance of peridotites sampled from mid-ocean ridge with similar LREE-enriched REE patterns suggest that this feature is not unique to SSZ peridotites. The U-shaped REE patterns of the Dajiweng harzburgites, combined with their low heavy rare earth element (HREE) contents and their mineral chemistry, indicate that these rocks most likely have been modified by SSZ melts (e.g., boninitic melts) in a forearc setting. In contrast, the Zhalai harzburgites, which also have U-shaped REE patterns but are characterized by high HREE contents, high Al₂O₃/SiO₂ ratios, low MgO/SiO₂ ratios, and relatively fertile mineral compositions, most likely have been refertilized in a mid-ocean ridge setting. The Zhalai, Kazhan, Baer, and Cuobuzha peridotites are similar to abyssal and back-arc peridotites in mineral chemistry and whole-rock geochemistry. Combining the mafic intrusions from Jianabeng, Baer, and Cuobuzha massifs, we propose that the ophiolites in the northern belt of the western segments have been trapped in an intraoceanic forearc–arc–back-arc system. According to the zircon U-Pb age of mafic intrusions, the geochemical characteristics of both mafic and ultramafic rocks, a detrital zircon study of Zhongba terrane, and the klippen structure of ophiolitic massif in the southern belt, we conclude that the northern and southern ophiolitic belts were developed in the same intraoceanic subduction system.

The Pozanti-Karsanti ophiolite is situated in the eastern segment of the Tauride belt. This ophiolite is composed of mantle peridotites, ultramafic to mafic layered cumulates, hornblende gabbros, plagiogranite with minor sheeted dikes and pillow lavas. Metamorphic sole and the Aladag mélange were accreted beneath the base of the Pozanti-Karsanti ophiolite. Gabbroic and doleritic dikes intrude the Pozanti-Karsanti ophiolite and the metamorphic sole around Late Cretaceous, as determined by the zircon U-Pb ages (86.9 ± 3.1 Ma) of the dikes, but no dikes have been observed in the underlying Aladag mélange. The Pozanti-Karsanti dikes show light rare earth element–depleted chondrite-normalized rare earth element patterns and high field strength element–depleted (Nb, Ta, and Ti) but large ion lithophile element–enriched (Rb, Ba, and U) primitive mantle–normalized trace element patterns. These dikes have Th/Nb ratios and oxygen fugacity higher than those of mid-ocean ridge basalt (MORB) but lower than island arc basalt. The Pozanti-Karsanti mafic dikes have (⁸⁷Sr/⁸⁸Sr)_t ratios of 0.70433–0.70489 and εNd(t) of +1.8 to +2.4, indicating a depleted mantle source and mixing of crustal or sedimentary components. Combining the characteristics of both trace elements and radiogenic isotopes, we conclude that these dikes are derived from Sp or Sp + Grt facies (Sp > Grt) depleted MORB mantle, with some addition of a subduction component in a forearc setting. On the basis of previous studies and our new work on the Pozanti-Karsanti ophiolite, we conclude that this ophiolite formed during the subduction initiation of an intraoceanic subduction zone in the Late Cretaceous.