Petrogenesis Of The Chabu Composite Granitic Pluton In The Middle Segment Of The Gangdese Belt,Tibet,And Its Tectonic Implications

The formation of the Gangdese magmatic rock belt is closely related to the Neo-Tethys subduction,the collision and post-collision events of the India-Eurasia continent,which is of great significance to discuss the growth of continent crust and to reserach the tectonic evolution of the convergent plate margins.In this dissertation,we select the Chabu composite granitic pluton outcropped in the middle segment of Gangdese belt as an example,and conduct an integrated study including petrology,zircon U-Pb dating and Hf isotopic analyses,and whole-rock elemental and Sr-Nd isotopic measurement.Based on these works,this research has precisely dated the rock-forming ages,systematically ascertained the geological-geochemical characteristics,and comprehensively revealed the petrogenesis for each stage rocks within the pluton.Besides,this paper also discussed their implications for tectonic evolution and crustal accretion,and finally proposed an integrated tectono-magmatic model to decipher the formation of the Chabu composite granitic pluton.This research can enhance the understanding of the evolution of the Gangdese belt.The Chabu composite granitic pluton,which is tectonically lies to the middle segment of the Gangdese belt and geographically situated in the north of Xietongmen County,is a complex intrusion formed by two stages of magmatism of Eocene?52?50Ma?and Miocene?16?13Ma?.The main part of this pluton consists of the Eocene granitoids which can be further grouped into two phases,i.e.,a hornblende biotite monzogranite porphyry phase cropped out in the north part of this pluton around Jiaduobule,and a biotite monzogranite phase in south and central parts of this pluton around the Chabu village.The Miocene granitoids include hypabyssal biotite monzogranite and subvolcanic dacite porphyry,which are locally outcropped as stocks or knobs within the pluton.The Eocene granites from the Chabu pluton are chemically subalkaline,metaluminous,and show typical I-type granite signature.They are enriched in large ion lithophile elements and depleted in high field strength elements,showing the geochemical characteristics of arc magmatic rocks.Isotopically,these rocks have enriched Sr-Nd isotopic compositions((⁸⁷Sr/⁸⁶Sr)_i=0.7065,e_Nd?t?=-3.83),with zircon?_Hf?t?values varying from negative to positive?=-3.33?3.42?.The integrated geological and geochemical data indicate that the Eocene granites were likely originated from partial melting of mafic lower curst,and have underwent a magma mixing process between the juvenile crustal-melted magma and the mantle-derived magma.Besides,a small amount of Indian continental crust materials were also involved in their genesis.The Eocene biotite monzogranite has suffered a higher degree of differentiation than the hornblende biotite monzogranite porphyry.Both of them are likely generated by fractional crystallization from the same magma.The Miocene granitoids are chemically high-K calc-alkaline and metaluminous-weakly peraluminous,and enriched in light rare earth elements and large ion lithophile elements?LILEs,e.g.,Rb,Th and U?,and depleted in high field strength elements?HFSEs,e.g.,Nb,Ta and Ti?.They also have low Cr,Ni concentrations,and show indistinct Eu negative anomalies.These rocks have high Sr,low Y and Yb abundances,with high Sr/Y ratios and?La/Yb?_Nratios,suggesting that they can be grouped into adakitic rocks.The Sr-Nd isotopic compositions of the Miocene rocks((⁸⁷Sr/⁸⁶Sr)_i=0.7063?0.7076,?_Nd?t?=-4.2?-7.3)are more enriched than that of the Eocene rocks,and they also have zircon?_Hf?t?values varying from negative to positive?=-5.99?7.78?,indicating that more Indian crustal materials have contributed to their genesis.The integrated petrology,and elemental and isotopic compositions suggest that the Chabu adakitic rocks were likely produced by mixing between magmas from partial melting of the thickened mafic juvenile crust and components from the subducted ancient Indian continental crust.Based on a comprehensively analysis of the tectonic background,it is shown that the Eocene granitoids were formed under a tectonic regime of the break-off of the Neo-Tethyan oceanic slab,and the Miocene adakitic rocks were generated under a post-collisional extensional setting after the India-Eurasia continental collision.In conjunction with the research results of magmatic rocks with different ages,it is inferred that the Gangdese magmatic belt has experienced multi-stage events of underplating by mantle-derived magmas.The multi-stage underplating of mantle-derived magmas is not only the main mechanism leading to the formation of the Gangdese batholith,but also an important way resulting in continental accretion in the Gangdese belt.