Late Cretaceous Metamorphism Of Sedimentary Rocks In The Eastern Gangdese Magmatic Arc And Its Tectonic Significance

The Gangdese magmatic arc is located in the southern part of the Lhasa terrane,southern Tibet.It is a typical composite continental magmatic arc,which was formed during the Mesozoic subduction of the Neo-Tethys oceanic lithosphere,and overprinted by the Cenozoic magmatism during the collision between Indian and Asian continents.In the eastern segment of the Gangdese magmatic arc,there are a large number of middle-to high-grade metamorphic rocks formed by metamorphism of Mesozoic and Cenozoic magmatic rocks and a small amount of sedimentary rocks,which represent the lower crust of the Gangdese magmatic arc.Revealing spatial changes in composition,metamorphic conditions and metamorphic time of these middle-to high-grade metamorphic rocks are the key to establish the crustal profile,and formation and evolution history of the Gangdese arc.In this paper,we conducted a petrological and geochronological study for the metasedimentary rocks from Milin area,the eastern Gangdese arc,in order to provide important constraints on the composition,metamorphism condition and time of the arc crust.The metamorphic sedimentary rocks occur as lenses within the metamorphosed gabbro of the Late Cretaceous Lilong batholith,and are composed of marble,calcic-silicate rock,gneiss and schist.Petrographic observation shows that the garnet-sillimanite-biotite schist consists of sillimanite,biotite,plagioclase,K-feldspar,garnet,quartz and ilmenite.Geobarometer and geothermometer calculations show that the schist was metamorphosed under P-T conditions of 7.5 kbar and 820?.Zircons U-Pb dating from marble,calcic-silicate rock and schist yielded metamorphic age of 83-88 Ma.This paper and the existing study results show that the metamorphic rocks exposed in the Zhaxi-Milin-Bujiu area,the eastern Gangdese magmatic arc,experienced high-pressure granulite facies metamorphism during the Late Cretaceous to Early Cenozoic,which represent the lower crustal component of the arc.From the northeastern Zhaxi area to the southwestern Tianxing(the present study area),and Caiba areas,the metamorphic degree gradually decreased to amphibolite facies,the metamorphic pressure decreased from 9-16.7 kbar to 7.5 kbar,and 5-6 kbar,the metamorphic time changed from 87-89 Ma to 86 Ma,and 74-79 Ma,and the crustal levels of the magmatic arc changes from the lower crust to the middle-lower crust.We suggest that the flat subduction of Neo-Tethyan oceanic lithosphere,and accretion and loading of voluminous mantle-derived magma during the Late Cretaceous resulted in the juvenile crustal growth and thickening of the Gangdese magmatic arc,and that the gabbro-dioritegranodiorite and sedimentary xenoliths in the Lilong batholith were transported into the middle and lower crusts,and underwent amphibolite-to granulite-facies metamorphism and partial melting.The downward transport of felsic magmatic rocks and sedimentary rocks into the deep crustal levels may be an important mechanism in transforming the mafic lower arc crust to andesitic one.The partial melting of the thickened lower crust resulted in the formation of granites in the upper crust,indicating that the juvenile crust of the magmatic arc underwent intracrustal differentiation and reworking during the Late Cretaceous subduction.