Permian Magmatic Rocks In The Eastern End Of The Gangdese Arc,and Their Cenozoic Metamorphism And Tectonic Significance

The Gangdese magmatic arc,located at the southern Tibet,consists mainly of Mesozoic-Cenozoic magmatic rocks,with minor Late Paleozoic magmatic rocks.Revealing genesis and metamorphism of the Late Paleozoic magmatic rocks can provide important constraints on the formation and evolution of the Lhasa terrane and Gangdese magmatic arc.In this paper,we conducted a petrological,geochemical,phase equilibria modeling and zircon U-Pb dating study for the migmatic garnet amphibolites with the Permian protolith and Cenozoic metamorphic ages from the eastern Gangdese magmatic arc.Petrographic observation shows that the garnet amphibole has a peak mineral assemblage of garnet+amphibole+plagioclase+epidote+quartz+rutile.Whole rock chemical compositions show that the garnet amphibolite is intermediate-basic magmatic rock,with SiO₂=47.90%⁵5.41%,Al₂O₃=17.7%¹9.5%,Na₂O+K₂O=4.09%⁴.97%,MgO=3.99%⁶.02%,belonging to calc-alkaline metaluminous rock?A/CNK=0.82⁰.91?.The garnet amphibolite is riched in light rare earth elements?LREE?and depleted in heavy rare earth elements?HREE?,with slight negative Eu anomalies and negative Nb,Ta and Ti anomalies,indicating chemical characteristics of arc-realeted magmatic rocks.Mineral chemical compositions show that the porphyroblastic garnet has obvious compositional zoning.Pyrope components increase and grossular and spessartine components decrease from the core to the mantle,indicating a growth zoning;the pyrope components decrease and grossular and spessartine components increase from the mantle to rim,indicating a diffusion zoning.Phase equilibria modeling shows that the peak metamorphic conditions of the garnet amphibolite are 770⁸10°C and13.5¹5.8kbar,with a melt produced of about 10 vol.%,corresponding to a formation depth of⁵5 km.This indicates that the garnet amphibolite underwent high pressure granulite facies metamorphism and partial melting.Zircon U-Pb dating results show that the garnet amphibolite has a protolithic age of Early Permian?294²75Ma?,and late metamorphic age of Eocene-Oligocene?55³0Ma?.The resultes show that the southeast Lhasa terrane underwent the Early Permian Andean-type orogen that resulted in the formation of arc magmatic rocks.These arc magmatic rocks were buried into the thickened lower crust,and underwent high pressure granulite facies metamorphism and partial melting during the Cenozoic collision between India and the Asia and associated mantle-derived magma underplating.The present study indicates that the Gangdese magmatic arc experienced distinct crustal thickening,and the juvenile crust underwent high grade metamorphic and anatectic reworking during the Cenozoic.