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, and formed during the Mesozoic subduction of the Neo-Tethyan oceanic lithosphere, and overprinted by the Cenozoic magmatism and metamorphism during the collision between Indian and Asian continents. Middle -to high-grade metamorphic rocks exposed in the eastern Gangdese magmatic arc represent the middle and lower crusts of the arc, which are ideal area for studying the deep component and evolution of the continental magma arc. In this paper, we conducted a petrological and geochronological study for the metasedimentary rocks from the Tianxing area, Milin County, the eastern Gangdese arc. The studied rocks include garnet-sillimanite-biotite schist, garnet -bearing calc-silicate rock, biotite-plagioclase gneiss and marble. The garnet-sillimanite-biotite schist consists of garnet, sillimanite, biotite, plagioclase, K -feldspar, quartz and ilmenite, and experienced medium -pressure granulite-facies metamorphism under P-T conditions of 6. 4 7. 8kbar and 810 820 C. Zircons U-Pb dating from garnet-sillimanite-biotite schist, calc-silicate rock and marble yielded metamorphic age of 87 83Ma. This study and the existing results show that the high-pressure granulite-facies metamorphic belt in the Linzhi-Milin area, the eastern Gangdese magmatic arc, represents the lower crustal component of the arc, while the medium pressure granulite-facies and amphibolite-facies metamorphic belt represents the middle -lower crust of the arc. We suggest that the accretion and loading of voluminous mantle -derived magma and subsequent flat subduction of Neo-Tethyan oceanic lithosphere during the Late Cretaceous resulted in the crustal growth, thickening and underthrusting of the Gangdese magmatic arc, which transported the intrusions and hosting sedimentary xenoliths in the upper crust into the middle and lower crust. The thickened middle and lower rocks underwent high temperature metamorphism and partial melting to generate the granites in the upper crust, indicating that the juvenile arc crust underwent reworking during the Late Cretaceous subduction. The downward transportation of the upper crustal felsic magmatic rocks and sedimentary rocks is probably an important mechanism in transforming the mafic lower arc crust to andesitic one.

Automated summary

The Gangdese magmatic arc in southern Tibet formed during Mesozoic subduction of the Neo-Tethyan oceanic lithosphere and was later affected by Cenozoic magmatism and metamorphism. Petrological and geochronological studies in the Tianxing area of the arc revealed high-pressure and medium-pressure granulite-facies metamorphism, indicating different crustal components. The Late Cretaceous accretion and loading of mantle-derived magma resulted in crustal growth, thickening, and underthrusting of the arc, transforming the lower arc crust to andesitic one through downward transportation of upper crustal felsic magmatic rocks and sedimentary rocks.