Geogenesis and Characteristics of the Western Part of the Yarlung Zangbo Ophiolites, Tibet

The tectonic setting and relationship between the Daba-Xiugugabu (Southern belt) and Dajiweng-Saga (Northern belt) ophiolitic sub-belts divided from Saga in the western part of the Yarlung Zangbo suture zone are the subject of debate. This paper imposes some restrictions on the genesis and relationship between the southern and northern sub-belts based on the construction of geological sections across from the Gangdise island arc, Southern sub-belt, Zhada-Zhongba terrane, Northern sub-belt and Tethys Himalayan formation, comparative analyses of rocks association, geochronology and geochemistry of the mafic dikes and the mineral compositions of their wall rock harzburgites in two sub-belts. The mafic dikes from the Cuobuzha, Jianabeng and Baer ophiolites in the northern belt are characterized by high Si, Al contents and low Ti, K, P contents. They plot as members of the calc-alkaline basalt series and in-situ analyses of the zircon grains yielded ages of 125 similar to 128 Ma. However, the mafic dikes from the Dongbo, Purang and Xiugugabu ophiolites in the southern belt are characterized by high Mg, Ti contents and low Si, K contents, so belonging to a low potassium tholeiite series, and the zircon grains yielded ages 120 similar to 130Ma. The Southern and Northern ophiolite sub-belts are interpreted as having formed in a similr fore-arc tectonic setting of the supra-subduction zone, because both mafic dikes share similar occurrences, the zircon dating gives overlapping ages (120 similar to 130Ma), show N-MORB type REE patterns normalized by chondrite and noticeable Nb, Ta and Ti negative anomalies in N-MORB normalized spider diagrams, which indicate that they have a fore-arc basin or back arc basin affinity in a supra-subduction environment. The wall rocks harzburgites of mafic dikes have geochemical compositions of both fore-arc and abyssal peridotites. Combined with the absence of any island arc, but showing a southward tectonic emplacement direction in the southern belt, detrital zircon geochronology and Hf isotope detain Permian elastic rocks suggest that, during the Paleozoic period, the Zhada-Zhongba terrane was a part of the Qiang-tang-Greater India-Tethyan Himalaya system. Those lines of evidence indicate that both ophiolitic sub-belts probably represent the same Neo-Tethys ocean lithosphere in the same tectonic setting.