Petrology of peridotite xenoliths in alkali basalt (11 Ma) from Boun, Korea: An insight into the upper mantle beneath the East Asian continental margin

Peridotite xenoliths in alkali basalt of about 11.0 Ma in age from Boun, which erupted in the Ogcheon belt, a tectonic belt between the Sino-Korean and Yangtze cratons, are examined to understand the petrological nature of the upper mantle beneath the Korean Peninsula. The xenolithic suite is almost composed of lherzolite containing spinel of low Cr# [=Cr/(Cr+Al) atomic ratio] around 0.1, and clinopyroxene with relatively high Na2O content (about 1 to 2 wt%). Predominance of the fertile lherzolite within the mantle may indicate that appreciable amount of melts did not pass through it and not leave dunite and/or pyroxenites with aureole of restite with higher degree of melting. The lherzolite from Boun has mineralogical characteristics of continental rift-zone mantle peridotite and is distinctly different from sub-arc and abyssal mantle peridotites. It is noteworthy that the peridotite xenoliths with arc or abyssal mantle signatures apparently have not been found from the eastern margin of the Asian continent, including the Korean Peninsula, despite possible arc settings experienced through geologic time. It is highly speculative but is possible that the arc-type mantle material had not been accreted upon arc crust accretion. Alternatively the arc type mantle which had once been present beneath the continental margin was replaced later by the continental rift-zone type mantle.