Geochronology, Geochemistry And Petrogenesis Of Mesozoic Magmatic Rocks In Shiquanhe Area, Tibetan Plateau

Mesozoic magmatism provides a prime opportunity to understand geological evolution and deep processes of Lhasa terrane,southern Tibet.Shiquanhe area is at the western end of the Lhasa terrane,where Indus-Yarlung suture zone and Bangong-Nujiang suture zone shrink in north-south direction due to the convergence and collision between India and Eurasia.Due to lack of systematic and high-quality geochemistry data,the petrogenesis and the dynamic mechanisms of Late Jurassic to Early Cretaceous magmatic rocks in this area is in urgent need of new research data.On the basis of field investigation and sampling,we present zircon U-Pb dating,in situ Hf isotopic analysis,and high precision whole-rock Sr-Nb-Pb-Mg isotopic study on the Late Jurassic to Early Cretaceous granitoids in Jiangba and Bangba plutons,Shiquanhe area attempt to reveal the characteristics and petrogenesis and provide a new constraint for exploring the Mesozoic evolution of the Lhasa terrane.Zircon U-Pb dating results show that the magmatic rocks from Jiangba and Bangba plutons resulted from two stages of magmatism,155-166 Ma in Jiangba pluton and 137-145 Ma in Bangba pluton.The MMEs and host granitoids have similar crystallization ages.Magmatic rocks from Jiangba pluton are metaluminous to slightly peraluminous and calc-alkaline to high K calc-alkaline ?-type granitoids,and they may be experienced low degree fractional crystallization of Pl,Bi,Amp and magma mixing.The host granites from Bangba pluton show peraluminous and calc-alkaline to high K calc-alkaline ?-type,while MMEs basically belongs to metaluminous series.Magmatic rocks from Bangba pluton also identified by mixing of granitic and mantle-derived magma.Evidence from major and trace elements,Sr-Nd-Pb-Mg and zircon Hf isotopic compositions exhibit diffenent petrogenesis of Jiangba and Bangba plutons.Magmatic rocks from Jiangba pluton exhibit overall negative zircon?_Hf?t??-13.8 to+1.8?,while Bangba pluton shows positive zircon?_Hf?t??+0.2 to+7.7?.The magmatic rocks from Jiangba pluton may originate from magma mixing between mantle-derived and ancient crust-derived melts,and magmatic rocks from Bangba pluton derived from juvenile lower crust with a higher proportion of mantle-derived magma.Mg isotope study of granitoids in Shiquanhe area was carried out for the first time in this study.The results show that the Mg isotope compositions of Bangba pluton(?²⁶Mg=-0.27 to-0.23‰)is closer to the average mantle value(?²⁶Mg=0.25±0.07‰),which also proved the proportion of mantle-derived magma was significantly increased in this periodBased on the data in this study and other literatures,we propose that late-Jurassic to early Cretaceous granitoids in Shiquanhe area were controlled by the southward subduction of Bangong-Nujiang Neo-Tethyan oceanic seafloor and significant change of tectonic settings like slab roll-back may have been recorded by the higher proportion input of mantle-derived magma.