Petrogenesis Of The Nyemo Intrusive Complex In The Middle Segment Of The Gangdese Belt,Tibet,and Its Implications For Tectonic Evolution

The Gangdese magmatic belt has recorded the whole process of the Neo-Tethys subduction and the subsequent India-Eurasia collision,which is of great significance for revealing the formation and evolution of the Tibetan Plateau and exploring the process of the growth and reconstruction of continental crust.The Nyemo intrusive complex is located in the middle segjment of the southern margin of the Gangdese belt,and its formation can be seen as a microcosm of the Gangdese magmatic rocks.For this purpose,the dissertation takes the Nyemo intrusive complex as an object,and conducts an integrated study including detailed field investigations,petrography,zircon U-Pb ages and Hf isotopes,and whole-rock elemental and Sr-Nd isotopic compositions.The aims of this dissertation are(1)to precisely define the formation ages of rocks in the Nyemo intrusive complex.(2)to constrain the petrogenesis of this complex andits implication for the formation of continental crust,and(3)to develop an integrated tectono-magmatism mode for the formation and evolution of the Nyemo intrusive complex,which provides further petrologic evidence in comprehensively understanding the tectonic evolution of the central Gangdese magmatic belt.The Nyemo intrusive complex formed by a multi-stage magmatic events.Zircon LA-ICP-MS U-Pb dating yields ages of 192 Ma and 87-86 Ma for the Early Jurassic deformed granite and Late Cretaceous tonalite,respectively.The Eocene gabbronorite,pyroxene diorite and tonalite have the similar ages of 50-48 Ma,forming a typical gabbro-granite complex closely associated in space and time.The Oligocene biotite monzogranite and its mafic microgranular enclaves(MMEs)also have the same age of 30 Ma.Combined with the published data for the Miocene monzogranite with ages of 15-13 Ma,the diagenetic ages of the Nyemo intrusive complex are from the Early Jurassic(?192Ma)to Miocene(?13Ma),which is basically consistent with the age span of the Gangdese magmatic rocks.The Early Jurassic deformed granite in Nyemo is rich in silicon,sub-alkaline and metaluminous,enriched with light rare earth(LREEs)and large ion lithophile elements(LILEs),and depleted in high field strength elements(HFSE),showing the geochemical characteristics of arc magmatic rocks.It has depleted Sr-Nd-Hf isotopic compositions showing its origination from the partial melting of juvenile crust.The generation of this deformed granite indicates that the subduction of Neo-Tethys ocean slab should be prior to the Early Jurassic.The Late Cretaceous tonalite is relatively silicon-poor,magnesium-rich(Mg#= 48.6-59.6),sub-alkaline and metaluminous,having a typical cha racteristic of I-type granite.This Late Cretaceous tonalite also has the geochemical characteristics of arc magmatic rocks and the depleted Sr-Nd-Hf isotopic compositions,showing its derivation from the partial melting of juvenile lower crust.In addition,some of these samples exhibit the characteristics of adakitic rocks,indicating a thickened crust of the southern margin of the Gangdese belt in the Late Cretaceous.The Eocene gabbronorite,pyroxene diorite and tonalite show a relatively linear variation in the elemental compositions.They all have the geochemical characteristics of arc magmatic rocks and the depleted Sr-Nd-Hf isotopic compositions.The comprehensive analysis shows that this gabbro-granite complex formed under the slab break-off of the Neo-Tethyan oceanic slab.The gabbronorite originated from the partial melting of a depleted mantle metasomatized by fluids released from the subducted slab.The tonalite was generated by partial melting of juvenile lower crust.The pyroxene diorite was likely produced by the mixing between the gabbronorite magma and tonalite magma.The Oligocene biotite monzogranite is characterized by high-K calc-alkaline,metaluminous,relatively low magnesium,and high Sr/Y ratio,showing the characteristics of typical Adakitic rocks.MMEs in these host biotite monzogranite are silicon-poor,iron-and magnesium-rich,and show similar primitive mantle-normalized trace element spidergrams and chondrite-normalized REE distribution patterns to those of the host biotite monzogranite.The biotite monzogranite and MMEs also have similar whole-rock Sr-Nd and zircon Hf isotopic compositions,but they are relatively less radiogenic compared with the previous three-stage magmatic rocks.The comprehensive analysis shows that the Oligocene biotite monzonitic granite were mainly originated from the partial melting of thickened juvenile lower crust,and the MMEs were produced by the mixing between mantle-derived basaltic magmas with the input of enriched components from Indian crust and adakitic magmas parental to the host granitic rocks.The combined petrological,geochemical,and isotopic data suggest that the formation of the Nyemo intrusive complex was mainly controlled by the continuous northward subduction of the Neo-Tethyan oceanic slab from the Early Jurassic to Late Cretaceous,by the break-off of the Neo-Tethyan oceanic slab during the Eocene,and by the geodynamic of India-Eurasia continental collision during the Oligocene.Magma mixing played a key role in the whole process for the formation of the Nyemo intrusive complex and,more generally,for the entire Gangdese magmatic rocks.It is not merely the principal way for forming the Gangdese magmatic rocks,but an important way to contribute the growth of continental crust in the Tibetan Plateau.