Eocene Magmatic Mixing In The Eastern Of Southern Lhasa Terrane,Tibet

The giant structure-magmatic rock zone developed in Lhasa,southern Tibet,recorded important information about the subduction of the New-Tethys Ocean and subsequent Indian-Asian collision.In this paper,the granites and mafic microgranular enclaves(MMEs)in Qushui area of the eastern part of the southern Lhasa terrain are studied,and the systematic field geological survey,petrology,zircon U-Pb dating and Hf isotope,whole rocks geochemistry and single mineral geochemical analysis are carried out.The nature of magma source area and petrogenesis in this region are analyzed.It provides more basis for discussing the geodynamic constraints of continental collision process after ocean subduction.The exposed rock mass in the study area contains a large number of MMEs.The host rocks obtained in this paper are adamellite-granodiorite,and MMES are dioritic enclaves.Zircon U-Pb dating results show that the age of host granites in Qushui area is 51.19±0.54 Ma,the zircon ?Hf(t)value ranges from 7.62 to 13.32,and the Hf isotope crustal model age varies from 0.3 to 0.6 Ga.The age of dioritic enclaves is 48.78±0.32 Ma,and the zircon?Hf(t)value and Hf isotope crustal model age value are distributed in 10.73¹3.24 and0.3⁰.4Ga,respectively.The host granite have characteristics of high SiO₂(60.73%⁶4.13%),K2O(2.08².29%),A/CNK = 0.79⁰.90,which belong to the partial aluminum high potassium calc-alkaline series of rocks.The dioritic enclaves have characteristics of high Mg O(5.12%⁵.22%),Al2O3(16.10%¹8.18%),Ca O(7.27%⁷.99%),A/CNK=0.73⁰.80,which belong to the partial aluminum calc-alkaline series of rocks.The host granite and dioritic enclaves are enriched in LREE and the HREE is relatively deficient,with weak positive Eu anomaly(?Eu=0.85⁰.91),and enriched in LILE(Rb?Th?U?K)and depleted in HISF(Nb?Ta?Ti).The geochemical characteristic is island arc type.Paleocene-Eocene granites are the main body of the Gangdese batholith.The analysis of the Eocene samples in this paper shows that the magma source area is mainly juvenile crust material,magma mixing occurred during the magma evolution,and fractional crystallization occurred during the emplacement process.It may be caused by the Neo-Tethys Oceanic slab break-off after the initial collision between the Indian and Asiancontinents,which triggered partial melting of the mantle material.