The Composition,Geochemical Characteristics And Tectonic Significance Of The Intra-oceanic Island Blocks In The Mulbubei Accretionary Complex In Gaize County,Tibet

The study area is located in the hinterland of Qiangtang,northern Tibet,tectonically located on the midwest of the Longmuco-Shuanghu suture zone.The accretionary complexes in the area are mainly exposed by the matrix and block structure.Both the block and the matrix accepted strong deformation characteristics,all have the strong foliation and the same direction,the long axis has been basically oriented.Both the matrix and the blocks develop two stages of structural deformation.The first stage is dominated by simple shear action,involving a large number of fluids,manifested as bedding quartz vein folds;the second stage is dominated by pure shear action.The structure formed by shear action is transformed to form a large number of crenulation cleavages.The detailed field observations of the accretionary complex in the Red Ridge Mountain-Xiangtao Lake area combined with the results of laboratory data have determined that the basaltic blocks in the accretionary complex are composed of gabbro and basalt.No complete ophiolitic rock cover combination has been found,and it is mostly modified by the metamorphism of the middle-low greenschist facies.By comparison,the REE distribution curve of basalt is shown to be left-leaning,and LREE is more depleted than HREE,showing a similar distribution pattern to that of N-MORB.Trace elements also have a similar distribution pattern as N-MORB.U,Ta,Zr,and Hf have positive anomalies of varying degrees,Th and Nb have obvious negative anomalies,and Ti has weak negative anomalies,and high field strength elements(HFSE).This characteristic may be related to the fluidization of the subduction zone.The REE distribution curve of the gabbro(diabase)shows a right-dip type.Compared with HREE,the LREE is more enriched,and the REE distribution pattern is similar with the OIB-type basalt.Trace elements also have a similar distribution pattern to OIB,and Th,U,Nb,Ta,Zr,Hf,and Ti all show positive anomalies to varying degrees,indicating that they originate from enriched mantle source regions.In the tectonic-environmental discrimination map,the basalts in the basalt masses of the accretionary complex show features of both NMORB(normal mid-ocean ridge basalt)and IAB(island arc basalt).Combined with regional tectonic settings,it is believed that the basalt masses in the accretionary complex of the study area are formed in the stretching environment above the subduction zone.The gabbro(diabase)in the basic rock mass are OIB-type basalts formed in the oceanic island environment and are the product of collision and extrusion of late convergence plates.Together with the ophiolitic rocks,they form the basalt masses of accretionary complex.