Geochemistry And Petrogenesis Of Late Cretaceous Granites In Gonggar,Southern Tibet

The Late Cretaceous period is an very important time for the tectonic evolution of the continental arc in the southern Lhasa terrane,and the Neo-Tethys ocean slab subdution.The thesis studied the Late Cretaceous granitic intrusive rocks from Gonggar area,Southern Lhasa terrane,including zircon U-Pb chronology trace elements and Hf isotopes,and bulk-rock element geochemistry,in order to reveal the genesis and tectonic environment of the rocks,as well as provide new evidence for the magmatic evolution of the Southern Lhasa block.The Gonggar intrusive host many MMEs?Mafic Microgranular Enclaves?.Being illustrated by TAS diagram,the host rock are adamellites and granodiorites,and the MMEs are monzonites.Zircon U-Pb chronology shows that both the host rocks and MME intruded in the Late Cretaceous time?⁹6 Ma?.In terms of zircon trace elements,the samples were depleted in light rare earth elements?LREEs?and enriched in heavy rare earth elements?HREEs?,with significant Ce positive anomaly and weak Eu negative anomaly?Eu=0.26⁰.70?,as well as low Pb and high Th/Pb and?Nb/Pb?_N ratio,indicating I-type granite feature.Zircon Hf isotopes show that the?_Hf?t?is positive,indicating that the samples have come from the depleted source area.Meanwhile,the mantle model age and crustal model age are relatively young,suggesting that these rocks were from the juvenile crust.In terms of major elements,host rocks are alkali-rich and potassium-rich,belonging to high potassium cal-alkaline series.The A/CNK=0.92⁰.98,belonging to metaluminous-peraluminous rocks.The MMEs are alkali-rich,distributed in the series of calc-alkaline,high potassium cal-alkaline to potassium series.The A/CNK=0.79⁰.85,belonging to metaluminous rocks.In terms of trace elements,both the host rock and MMEs are enriched in LREEs and depleted in HREEs,and also enriched in large ionic lithophile elements?LILEs?and depleted with high field strength elements?HFSEs?.In addition,they have high content of Sr,and have low content of Y and Yb,showing the adakitic affinity.The Late Cretaceous granitic intrusive rocks in Gonggar came from the partial melting of the mafic juvenile crust.During the Neo-Tethys oceanic crust subduction stage,basaltic magma underplated into the bottom of continental crust and brought a lot of heat,which caused the lower crust melting and generated granitic magma.Then the basaltic magma mixed with the granitic magma,which formed the intermediate-acid intrusive rocks in Gonggar.Besides,magma underplating and magmatic mixing are important ways to thicken the continental crust during the Neo-tethys oceanic crust subduction period.