Dykes And Extension Events In The Southern Lhasa Subterrane During The Cretaceous And Cenozoic

The tectonic evolution of the southern Lhasa subterrane is closely related to the subduction and closure of the Neo-Tethys and the collision between India continent and Eurasia continent.However,there is currently no systematic study of the extension events it has experienced since the Cretaceous.In this paper,we carried out systematic field investigations,petrology,chronology,geochemistry,and zircon Hf isotope studies of dykes from Xigaze,Dazi,and Linzhou as carriers,and combined with literature data to systematically determine Multi-stage stretching events in southern Lhasa subterrane since the Cretaceous.The basic and acidic dykes in the Dazi and Linzhou emplaced at 67-64 Ma,both of which are calc-alkaline to high-K calc-alkaline series,with enrichment of light rare earth and large ion philophile elements,depletion of heavy rare earth and high field strength elements,with displaying positive zircon?_Hf?t?values and young mantle and crust model age.The source area of basic dykes is the lithospheric mantle replaced by subduction fluid,and source area of acid dykes is crustal material newly generated during subduction.The EW trending intermediate and acid dykes in Xigaze emplaced at 15-14 Ma,both of which are calc-alkaline series,and belong to the adakitic rock with high Sr and low Y,enriched with light rare earth and large ion lithophile elements,depleted in and heavy rare earth and high field strength elements,with depleted Hf isotope and young crust model age.These features indicate that their source area is a thickened lower crust newly formed,melted in the background of post-collision east-west extension.Combining dykes and regional magmatism,graben basins and normal fault systems since the Cretaceous in the South Lhasa subterrain,four stages of extension events were preliminarily determined,namely:?1?N-S forearc extension during the early Cretaceous?130-107 Ma?.It may be caused by the southward retreat of the subducted Neo-Tethys ocean trench,forming the mafic dykes in the Xigaze forearc basin and the Yarlung Zangbo ophiolite;?2?Backarc extension and intra-arc extension during the late Cretaceous?95-82 Ma?.It may be triggered by slab rollback of the Neo-Tethys Ocean,forming a magma‘flare up'widely distributed in Gangdese,basalts in the Linzhou,complex and dykes in Namling and Quxu;?3?N-S syn-collision extension during the Paleogene?57-53 Ma?,which may be caused by the slab break-off of the Neo-Tethys ocean connected to the front of the lithosphere of the Indian continent shortly after the collision,forming the widely distributed Linzizong volcanic strata,contemporaneous intrusions,and mantle-derived mafic dykes in Dazi and Lizhou.?4?E-W extension during the Miocene?18 Ma-present?,which was probably triggered by the delamination of the thickened lithosphere of the Lhasa terrane,causing upwelling and decompression melting of the asthenosphere to form potash-superpotassic magma,melting of thickened new lower crust to form adakitic magma,forming a great number of N-S normal fault systems with many dykes intruding in.