Mesozoic Magmatic Rocks In Mamba Area,Central Lhasa Subterrane:Geochronology,Geochemistry,and Geodynamic Implications

Lhasa terrane as a natural laboratory of the continental-continental collision research,has received considerable attention by home and abroad scholars.Recently,Mesozoic-Cenozoic magmatism in the southern Lhasa subterrane has been intensively studied and proposed the dynamic model of the magma evolution.While,the Triassic-Cretaceous magmatism records also have been found in central Lhasa subterrane,which still lack systematic study of magma nature and source,as well as the dynamic mechanisms.In this paper,we focus on Mamba area,central Lhasa subterrane,selecte granitic rocks and mafic rocks(including dioritic enclaves,gabbroic enclaves and diabase)and carried out a systematic petrography,zircon U-Pb geochronology and Hf isotopes,whole rock elements and Sr-Nd isotopic study of Mesozoic magmatism attempt to reveal the mechanism and kinetics of the process deep in the area,providing new evidence for the pre-Cenozoic evolution.According to U-Pb zircon geochronology established Mamba Late Triassic to Late Cretaceous magmatism grid reveals the magmatic properties and process.Magmatism in the study area can be divided into four stages,Late Triassic-Early,Jurassic,Late.Jurassic,Early Cretaceous and Late Cretaceous.(1)Late Triassic-Early Jurassic magmatic rocks include mafic rocks,host granite and enclaves:host granite are high-K calc-alkaline series,with enrichcd Nd-Hf isotope compositions,may be derived from partial melting of the ancient Al-rich basement,and varying degrees of fractional crystallization occurred in the generation process;mafic rocks probably derived from partial melting of enriched mantle metasomatised by subduction fluids that have mixed with granitic magma.(2)Late Jurassic granite probably originated in ancient partial meiting of thickened lower crust,experience significant fractional crystallization;coeval diabase propably derived from enriched mantle material which have been metasomatised by the sediments and fluids.(3)Karly Cretaceous granitic rocks and mafic lower crust probably derived form ancient lower crust and mantle material enriched by metasomatism of oceanic sediments and fluid,respectively.Gabbroic enclaves underwent significant fractional crystallization.(4)Late Cretaceous host granodiorite has typical adakitic-gcochemical affinities.Mafic enclaves have high K2O and Zr content and high Zr/Y ratio.They likely generated from ancient thickened lower crust and mantle material enriched by fluids,respectively,and experienced magma mixing.This paper attempts to establish deep dynamics model of Mesozoic magmatism in Mamba area,with geochronology and geochemistry herein newly acquired data,combined with data on the area has been published.Late Triassic-Early Jurassic magmatism may relate to southward subduction and roll-back of Bangong-Nujiang lithospheric plage.Whereas the late Jurassic-early Cretaceous magmatism probably generated by normal angle north subduction of Neo-Tethys(Yarlung-Zangbo Ocean),Late Cretaceous magmatism may be formed in back-arc extension settings related to northward subducted Yarlung-Zangbo plate beneath to Lhasa block.This paper reveals the magmatism in the central Lhasa block corresponding to the northward subduction of Noe-Tethys,and southern-central Lhasa block underwent more complicated kinetic process.