Geochemical Characteristics And Tectonic Significance Of The Zedong Ophiolite, Yarlung-Zangbo Suture Zone

The Yarlung-Zangbo surture zone marks the Neo-Tethyan remnants existed between the Indian plate and the Eurasian plate. The suture zone can be divided into three parts, namely, the western, middle and eastern segments. Zedong suture located within the eastern segment, comprise a suit of melange made of Zedong ophiolite and island arc volcanic assemblage. Although the ophiolite has undergone complicated tectonic movements, its sequence is basically completed, composed mainly of peridotite, diabase and gabbro dyke swarms and pillowed and massive basalts.The Sm-Nd isochron age of basalts in Zedong ophiolite is 175±20Ma, indicating that the ophiolite formed in the mid-Jurassic. The eNd(t) values of basalts range in +7.0-+7.3, suggesting a strong depleted mantle source and no crustal contamination.The mantle peridotite mostly consists of harzburgite, showing low Ti, Al, high Mg, and the "U" type REE patterns, and belonging to the residual mantle peridotite. The gabbro and diabase all have LREE-depleted patterns and positive Eu anomaly. The rece elements abundance is low, exhibiting the obvious depletion of Nb, Ta, Zr, Hf and enrichment of Cs, Rb, Sr, Ba, K. According to the geochemical properties, the crustal lava of Zedong ophiolite can be divided into two groups: basalt I (close to N-MORB) and basalt II (is of IAT). Two groups of rocks all indicate smooth LREE-depleted REE patterns and similar distributions of race elements, and the element contents of basalt I are higher. Basalt II underwent the stronger subduction than ophiolite I, exhibiting obvious Nb and Ta depletion.Zedong ophiolite shows the compound geochemistry characterisitics of island arc and MORB, belonging to the SSZ-type ophiolite. Considering the geological setting, we conclude it formed in the fore-arc basin above the subduction zone.The subducted-type adakites in the Zedong segment of the Yarlungzangbo suture zone were discovered and identified for the first time. Zedong adakites were produced by the partial melting of young and hot subducted oceanic crust in amphibolite-eclogite transition zone and by going through the mantle wedge. This work presents new evidence for the intra-Tethyan subduction and the previous suggestion about the existence of intra-oceanic island arc within Tethys.Based on the former studies, a north directed intra-ocean subduction system once existed in Zedong Tethyan ocean, and the Zedong ophiolite and island arc are all the production of intra-oceanic subduction. In mid-Jurassic, there was a ductile nappe shear zone developing in Neo-Tethyan ocean, and at the same time, the bidirectional compressing of Indian plate and Lhasa block on the oceanic lithosphere induced the intra-oceanic subduction. The reinforce of slab subduction resulted in the local extension of oceanic plate above the subduction zone and the formation of spreading ridges. Plenty of H₂O and LILE entered the depleted mantle wedge, led to the partial melting. Along the spreading ridges, the magma extruded on the sea floor and formed the new SSZ-type oceanic crust and basin(175Ma). Meanwhile, the old oceanic crust started to subduct toward the Lhasa block, and produced the Yeba-arc at the south edge of Gangdese. The Tethyan double subduction situation came into being. During this period, the partial melting of young and hot subducted oceanic crust at 75-85km produced he adaktitic magma. The subduction also led to the formation of the Zedong intra-oceanic island arc in the north, which is active at 156Ma.At late Cretaceous, the Neo-Tethyan ocean basin stopped to spread and turned to be the single subduction situation, and then the intra-oceanic subduction system obducted onto the Indian plate in the south. Because of the northward compressing of Indian plate, the subduction along the south edge of Lhasa block was stronger. The volcanic magma activities were extensive, producing abundant intermediate-acid calcium alkaline island arc igneous rocks. In Eocene, the Indian plate collided with the Eurasian plate, the Neo-Tethys closed and died out, and the Tibetan plateau started to uplift. The suture and the adjacent terranes were reconstructed to be the present situation by the late tectonic movements.