Tectonic Evolution of the Western Yarlung Zangbo Ophiolitic Belt, Tibet: Implications from the Petrology, Mineralogy, and Geochemistry of the Peridotites

Abstract

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.