Extensive Removal of the South Korean Lithospheric Root Evidenced by Widespread Triassic Mafic Magmatism

The exact timing and process of the dramatic thinning of the East Asian lithosphere during the Phanerozoic are not precisely known. It seems that clues to this problem lie in a series of tectonic sequences involving magmatism and regional metamorphism that occurred on the Korean Peninsula from Permian to Triassic. During the Permian, extensive intrusion of granitic plutons occurred in the connected Korea-Japan region as a result of the subduction of the paleo-Pacific plate. The igneous rocks of ~250 Ma, distributed in the southern part of the Korean Peninsula, were converted into granite gneiss during the subsequent regional metamorphism. Then, Korea experienced widespread mafic-felsic magmatism during 240-220 Ma, producing scattered small plutons including gabbro, monzonite, syenite, granite, etc. Contrary to general expectations, most mafic rocks exhibit significant Nb negative anomalies, and their Nd or Hf isotope compositions show lower values than felsic rocks. The radiogenic isotope composition of the mafic rocks reflects the old continental crust rather than the depleted mantle, indicating an involvement of the crustal materials in their formation. The mafic rocks with a crustal isotopic composition may be due to the crustal material involved in the partial melting of the asthenosphere or the assimilation of the mafic magma during ascent through the crust. All of these mafic-felsic plutons are small, and the magmatism did not last long, indicating that the process that caused the melting did not result in very extensive melting and was soon stopped. These mafic plutons are usually small and the magmatism did not last long, indicating that the cause of the melting had soon ceased. Presumably, after subduction-related Permian magmatism and the subsequent Early Triassic metamorphism, extensive initial destruction of the gravitationally unstable lithosphere occurred, which induced Late Triassic magmatism by the upwelling asthenosphere. Some of the gaps in the destructing lithosphere were filled with the upwelled asthenosphere, stopping destruction. However, the lower portion of the lithosphere appears to be recycled into the convecting asthenosphere.