Petrology, Geochemistry And Tectonic Setting Of The Luobusa Ophiolite, Southern Xizang (Tibet)

The Luobusa ophiolite exist in the eastern segment of Yalu Zangbu(Yarlung Zangbo) suture zone, whose rock units are complete and which has many characteristics in itself, such as the major rock unit is mantle peridotite and the crust rock is complicated;there exist obduction-type granitoid rocks in the upper level of cumulate;diabases as veins crop out in the mantle harzburgite.Zircons were picked out from diabases in The Luobusa ophiolite, whose ²⁰⁶Pb/²³⁸U apparent ages SHRIMP U-Pb dating show very complex, but a majority of which distribute between the 148.3Ma and 174.2Ma. A precise zircon SHRIMP U-Pb concordia age obtained was 162.9 ± 2.8Ma. This age was interpreted as formation time of The Luobusa ophiolite. In addition, two different age data had been obtained in the precedure of analysis, which demonstrate that it is possible during the formation of the neo-tethys, that there are some older zircons coming from an older oceanic crust or pelagic sediment which have been caught when the mantle magmas upwell beneath mid-oceanic ridges and prove further that The Luobusa ophiolite has emplaced in late Cretaceous.The crust lavas of The Luobusa ophiolite belong to transition type between alkaline basalt and andesite/basalt, but diabases fall into the range of subalkaline basalt. Geochemical characteristics of these rocks suggest The Luobusa ophiolite derived from a very depleted mantla source with the characteristic of N-MORB, but may undergo the contamination of crust-derived materials. All the geochemical characteristics indicate that The Luobusa ophiolite belongs to arc-type ophiolite (SSZ). That is, its formation background is interarc basin of supra-subduction zone. The crust lavas and diabases in The Luobusa ophiolite have big eNd(t) which resemble N-MORB. It was found that the mantle source of The Luobusa ophiolite may be a depleted mantle reservoir contaminated or mixed by EM II from Sr, Ndand Pb isotopic data. The mantle source has Dupal isotopic anomaly and the Dupal anomaly of mantle source results from the mix of DMM and EM II mantle end-member.All the results of the Sr-Nd-Pb isotope analysed of Tethyan ophiolites show that they have the isotopic characteristics similar to those of Indian Ocean and reveal the Dupal anomaly. The large-scale Dupal isotopic anomaly may derive from one or some plumes of south hemisphere. These plumes with Dupal composition have existed 350Ma and more, and may be the cause which results in the splitting of Gondwana continent and opening of paleo-Tethyan and neo-Tethyan ocean early and late.In late Permian to early Triassic, the Lhasa block separated form Indian plate. The Tethyan ocean began to form between the Indian plate and the Lhasa block. The Tethyan ocean had been large in the late Triassic. With the spread of mid-ocean ridge and excursion northward of Indian plate, a inter-mantle-type thrusted ductile shear zone had been developed in the Tethys which results in the subduction between oceanic crusts. In mid-Jurassic, the magmas in the mantle wedge of supra-subduction zone convected intensively. Upwelling of mantle magmas cause northern older oceanic crust spreading and forming new oceanic crust. Later on, Zedong island arc was produced in the north of new oceanic crust. At one time, the Tethyan oceanic crust to the south of Lhasa block began to downthrust toward the Lhasa block. The island-arc magmas spewed at the southern edge of the Lhasa block and gave birth to the Sangri arc in the late Jurassic to early Cretaceous. But later the Sangri arc took place disassembly due to the tectonic event. The downthrusting between ocean crusts in Tethys had been completed in late Cretaceous. Both old and new oceanic crust emplaced on the stratum of Langjiexue Formation. In Tertiary, all the downthrusting has been finished and the collision between Indian plate and Eurasian plate began.