Subduction Of The Meso-Tethys Ocean And Its Magmatic Response

In recent years,significant progress has been made in the study of the subduction evolution history of the Meso-Tethys Ocean.In central Tibet,in addition to the Bangong-Nujiang suture zone,which is considered to represent the main remnants of the Meso-Tethys Ocean Basin,the Shiquanhe-Namtso ophiolitic mélange belt developed in the south has also attracted more and more attention of scholars.There are still many arguments about the Mesozoic evolution history of the ocean basin represented by the Shiquanhe-Namtso ophiolitic mélange belt,especially the subduction history during the Jurassic period.Exploring the Jurassic subduction history of Shiquanhe-Namtso ophiolitic mélange belt is of great significance for further understanding the Mesozoic evolution of the Meso-Tethys Ocean.However,the reports of island arc magmatism are not rich,and further work needs to be carried out urgently.On the basis of field petrology and laboratory petrography,this dissertation studies the Jurassic intermediate acid intrusions in the middle and western section of Shiquanhe-Namtso ophiolitic mélange belt by means of isotopic chronology,whole rock geochemistry and isotopic geochemistry.The following are the advances that have been made in the research:(1)In the Shiquanhe and Lhaguotso areas,a large area of Early and Middle Jurassic intermediate acid intrusions have been identified for the first time,including Shiquanhe granodiorite(184 ? 183 Ma),Shiquanhe tonalite(170 Ma)and Lhaguotso plagiogranite (185?181 Ma).They intrude into ophiolite suite gabbro or are trapped by ophiolite mélange.The results of whole rock geochemical and isotopic tests show that its formation and evolution are closely related to the partial melting of subducted oceanic crust plate,metasomatic mantle wedge and subducted sediments.(2)Many Late Jurassic intermediate acid intrusions closely associated with ophiolite suite have been identified in the Shiquanhe,Lhaguotso and Asa areas,including granodiorite(?163Ma)and plagiogranite(163?162 Ma)in Shiquanhe area,dioritetonalite-diorite porphyrite composite rock mass(165?159Ma)and plagioclase granite(163 Ma)in Lhaguotso area,diorite(161 Ma)and plagiogranite(161 Ma)in Asa area.The results of whole rock geochemistry and isotopic test indicate that the formation of these rocks is also closely related to a subduction zone system.Their source materials include subducted oceanic plate melts,overlying metasomatism mantle wedge melts,subducted sediment melts and fluids from subducted oceanic plate dehydration.These materials interacted with each other in different proportions and eventually formed intermediate-acid intrusive rocks with different geochemical and mineral compositions.(3)By comparing the newly defined Jurassic intrusive series in central Tibet with the oceanic plagiogranites in ophiolite suites developed worldwide,this study has found that they,which are called “forearc granites” in this dissertation,are different from the frational type-,shear type-,subduction type-and obduction type-oceanic plagiogranites proposed by previous scholars,but instead similar to the oceanic plagiogranites developed in the forearc region under the control of subduction of heated young oceanic slabs.The dissertation makes a detailed analysis of the magmatic evolution model of the forearc granites,and it is suggested that this model can well explain the origin of the Jurassic intermediate-acid intrusions in the Shiquanhe-Lhaguotso-Asa area in central Tibet.(4)By collecting,collating and comparing the characteristics of Jurassic ophiolite,magmatic rocks and sedimentary strata developed in Shiquanhe-Namtso ophiolitic mélange belt and Bangong-Nujiang suture zone,the study has constructed a Jurassic subduction evolution model of the Meso-Tethys Ocean,including Jurassic intra-oceanic subduction within Shiquanhe-Namtso ophiolitic melange belt,and Early Jurassic intra-oceanic subduction,Late Jurassic ridge subduction,Late Jurassic ocean-continent subduction within Bangong-Nujiang suture zone.The ocean basins represented by the two ophiolitic melange belts formed the main ocean basin of the Meso-Tethys Ocean together in the Jurassic period.