Study On Ophiolite And Its Emplacement Mechanism Of The Midwestern Yarlung-zangbo Ophiolite Belt In Southern Tibet

This paper mainly selects Xigaze-Sangsang-Saga-Zhongba ophiolite areas as the target research areas in the mid-western Yarlung-Zangbo ophiolite belt. The key study areas are Sangsang ophiolite zone, Saga ophiolite zone and Zhongba ophiolite zone. Research object are ophiolites within key research areas, focusing on their petro-geochemistry, geochronology, microstructure, geochronology and mantle rheology. Based on research results, the emplacement mechanism of mid-western Yalung-Zangbo ophiolite belt has been explored. And a reasonable model has been proposed to explain the emplacement mechanism of the mid-western Yalung-Zangbo ophiolite belt.Beginning at Sangsang ophiolite, the western Yarlung-Zangbo ophiolite belt had been divided in two sub-belts. One is southern sub-belt, the other is northern sub-belt. Mantle peridotite, basalt and gabbro within Sangsang ophiolite sequence are depleted in HFSE. Basalt and gabbro are calc-alkaline series, and alkaline basalt has been found widely, belonging to the MORB. Gabbro zircon U-Pb age is 125±1.2Ma. The ophiolite sequence within southern Saga ophiolite had been found. However, northern ophiolite sequence had been recognized. Basic rocks are calc-alkaline low-potassium series. Basalt within Saga ophiolite is widely distributed in the middle of Saga ophiolite, Basalt within the edge of Saga ophiolite is sub alkaline, with the center of this ophiolite belongs to alkaline. Basalt contains the mineral which has been rich in alkali and poor in Si. Diabase zircon U-Pb age is 125.6±2.7Ma. It is inferred that Saga ophiolite in the north of Saga is formed within a supra-subduction zone(SSZ). Mantle peridotite within Zhongba ophiolite had not been found. The edge of Zhongba ophiolite is distributed mainly in calc-alkaline basalt, and the center is distributed in alkaline basalt. The lithologic association of Zhongba ophiolite is regarded as the rock assemblage of typical ocean island basalt. Rima basalt zircon U-Pb age is 129.3±1.0Ma, suggesting to be formed in the fore-arc extensional setting. Bailang garnet pyroxenite in Xigaze belongs to calc-alkaline mafic rocks. Its zircon U-Pb age is 149.0±3.1Ma. It has been derived from mantle and marks that neo-Tethys ocean had been absolutely closed.Fresh mantle peridotite has been selected to make experiments on EBSD to analyze olivine fabric. Based on EBSD analytical results, its olivine fabric belongs to B-type olivine fabric. And The dominant slip system displays as(010)[001], suggesting high pressure and high stress settings. Its fabric is closely related to SSZ, indicating southward thrust. This result is consistent with field observation. EBSD analytical results on orthopyroxene and clinopyroxene within peridotite have the same conclusion.According to research data, the possible kinematic model have been proposed, called motion model of heterogeneous subduction plane. The model can be proved to be reasonable by lineation and foliation measured within the research area. According to basalt research, tectonic setting and dynamic mechanism of ophiolite belt is inferred to suggest mantle rheology mechanism. According to this research, the reasonable and valuable model has been put forward working in the mid-western ophiolite belt.