Magmatic-Tectonic Process Of Qulin Complex Pluton In Gangdese Batholith,Southern Tibet

The Gangdese magmatic belt is located on the southern margin of the Tibet Plateau and has recorded a series of magmatic events.Its tectonic evolution is closely related to the subduction and closure of the Neo-Tethyan Ocean and the Indian-Asian continental collision.In this paper,the host granite porphyry,granodiorite porphyrite dyke and lamproitic dyke of Qulin pluton developed in the post-collision extension environment were taken as the research objects,and systematic field geological investigation petrology,chronology,geochemistry,Sr-Nd isotope and zircon Hf isotope studies were carried out,combined with contemporary literature data from southern Tibet,providing new restrictions on the response of the continental lithosphere?thickening,partial melting and elevation evolution,etc.?of the continen-tal collision orogen.The Qulin pluton is located in the southern margin of the middle section of the Gangdese belt and is an Oligocene-Miocene complex pluton.The main body is coarse-grained granite porphyry,which was cut by the later lamproite dykes and granodiorite porphyry dykes.It is the product of multi-stage magmatism.The zircon U-Pb dating results of granite porphyry are?30 Ma,which are high-K calc-alkaline series,quasi-aluminum,characterized by low Mg O and low Mg^#,and are highly enriched in light rare earth elements?LREE?and Large ion lithophilic element?LILE?,depleted heavy rare earth element?HREE?and high field strength element?HFSE?.In addition,it has the characteristics of high Sr,Sr/Y,high?La/Yb?_N;low Y and Yb,weak Eu negative anomaly.The ages of the two granodiorite prophyrite dykes are 15-14 Ma,and they have a distribution pattern of rare earth and trace elements similar to the host granite.They are also rich in light rare earths and large-ion lithophile elements,and deplete heavy rare earths and high field strength elements.In terms of the whole rock Sr-Nd isotope composition and zircon Hf isotope composition,granite porphyry has the initial ⁸⁷Sr/⁸⁶Sr_?i?=0.706102?0.706201,?_Nd?t?=-0.6?+0.6,zircon?_Hf?t?=+4.9?+7.9;the initial ⁸⁷Sr/⁸⁶Sr_?i?of granodiorite prophyrite dykes is 0.705429?0.75424,?_Nd?t?=-1.4?-0.2,zircon?_Hf?t?=+2.6?+7.6.The zircon U-Pb data of the lamproite indicate that the formation age is 29.7±0.4 Ma.It is characterized by high MgO,K₂O and TiO₂,low Al₂O₃and Na₂O content,and is extremely riched in incompatible trace elements,such as large ion philophile elements?LILE?and light rare earth elements?LREE?.The initial ⁸⁷Sr/⁸⁶Sr_?i?of lamoproite is 0.7141?0.7154with a very low?_Nd?t?value?-5.9 to-12.6?.Combining the high Sr/Y and high K-ultrahigh-K magmatic activities developed in southern Tibet since Oligocene and in the north-south trending rift system,their geochemical characteristics show that:?1?both the granite porphyry and granodiorite porphyrite dykes of the Qulin complex are formed by the partial melting of the thickened lower crust in Southern Lhasa,and the lamproite dykes are generated by the partial melting of the continental lithospheric mantle beneath the Lhasa block,respectivelt;?2?the Qulin high Sr/Y magmatic suite may be related to the"refertilization"of the lower crust due to underplating of mantle-derived basaltic magmas,resulted from long-term magmatic responses to the break-off of Neo-Tethyan slab at?50 Ma;?3?the Qulin lamproitic magmatic rocks could be an indicator as convective thinning of the continental lithospheric mantle and their occurrence might suggest that the initiation of the south-north-south southern rift system may be as early as 30 Ma.