The Research On The Key Problems Of Tethyan Arc-basin Tetconic Evolution In Xigaze

The Yarlung Zangbo suture zone, separating the Lhasa block to the north and Indian sub-continent to the south, is marked by ophiolites, subduction zone complex, and volcanic arc terranes. Preveous modle for Xigze Tethyan evolution have controversial problemes on whether the intra-arc and the information about Sunducted Ocean preserved in Xigaze Tethys sutre zone. Detailed studies for them based on field structural geology, petrology and geochemistry have allowed us to characterize the Xigaze segment of YZS and understand its implications for tectonic history before India-Asian collision. SHRIMP zircon U-Pb dating for the west end of Tiding ophiolite yields a 128±2 Ma age for gabbros. Geochemical features of ophiolites (i.e. the Ta, Nb and/or Ti (TNT) negative anomaly, depletion of LREE) infer their supra-subduction-zone (SSZ) environment.The oceanic volcanic basalts and diabase associated with the Rinbung ophiolite, east end of the segment, are similar to oceanic island alkalic basalt series (OIAB) with respect to many aspects such as enrichment of Ti, P and total alkalis, the enrichment distribution patterns of LREE; and a decreasing trend of the normative value of incompatible elements to primitive mantle with their incompatibility in the spider diagram. Significantly, Sr-Nd isotopic ratios of the Rinbung OIAB display an evolution trend from DM to EMⅡwith no crustal contamination and and similar to the OIAB of Keguelen(the NinetyEast Ridge hotspots of Indian ocean). Therefore, the Rinbung OIAB probably represents the product from the hotspots in Tethys, it is believed that Indian mantle have inherited some characteristics of Tethyan ocean.The adakites are also fistly found in this study, and they are a suite of High magnesium tonalites intruding the Rinbung mantle peridotites with higher MgO and Mg#, Sr or Sr/Y, strongly depletion of HREE and Y, probably produced by the reaction between Tethyan subduction slab melting and sub-arc mantle and mark an intra-oceanic subduction.Based on the previous study, an intra-oceanic subduction with a subduction polarity directed to the north does exist in Xigaze Tethyan Ocean. The subduction of Tethys Ocean would like to subduct at least from the middle Jurassic so that they can produce Yeba-arc along the southernmost margin of Lhasa continental block. During the J3-K1 period, the subduction along the southern margin of Lhasa block continued to form a suite of ADRs (include rare adakites) in Sangri group. And then a local extension took place in the region of intra-Eastern Tethys when the India plate drifted to the north but suffered the resistance from the Lhasa block, contemporarily, the intra-oceanic subduction are triggered, subsequently, magmas derived from a partial melting of mantle wedge which suffered from the modification by subducted continental marginal clastics and oceanic sediments upwelled along the oceanic ridges, forming several small marginal basins during 120～128 Ma, and thus lead to the so-called SSZ-type oceanic crust. Adakties, the typical fingerprint of intra-oceanic subduction, intruded the Rinbung mantle peridotites.With the continued subduction of the India plate to the north along the southern margin of Lhasa block, forming Linzizong volcanic rocks, the intra-oceanic subduction system was gradually extinct and the ophiolite was tectonically emplaced onto the southern margin of the Lhasa block at～80～90 Ma. During～45～50Ma, the Gangdese granitic batholiths were emplaced into the southern margin of the Eurasia continent and mark the collision of India-Asia plate. In the Tertiary, mountain frontal molasses deposited between the Yarlung Zangbo ophiolitic melanges and the Gangdese granitic batholiths, and the suture zone together with adjacent tectonic units were modified by thrusting and extension to the present tectonic configuration.