SR, ND AND PB ISOTOPIC COMPOSITIONS OF THE CENOZIC ALKALI BASALTS OF THE ULLEUNG-DO AND DOK-DO IN THE SEA OF KOREA: COMPARISON WITH THE DELAMINATED CONTINENTAL LITHOSPHERIC MANTLE OF EAST ASIA

We report the newly determined Sr, Nd and Pb isotopic compositions of the Cenozoic basaltic rocks from the Ulleung-do and Dok-do located in the middle of the Sea of Korea. The basaltic rocks have OIB-like trace element abundance patterns without any sign of significant contributions from the westward subduction at the Pacific margin along the Japanes Islands. Their radiogenic isotope compositions of Sr, Nd and Pb are quite enriched compared to the normal depleted mantle. It is well known that Cenozoic basalts throughout East Asia and Southeast Asia tend to form two distinct isotopic compositional domains, i.e. a DMM-EM1 array for northeast China and a DMM-EM2 array for south China and Southeast Asia. The Cenozoic basalts from Korean Peninsula also follow such trends and most of them follow the DMM-EM1 trend of north China but basalts from the southernmost Jeju-do island show similarities to the DMM-EM2 trend of the southern group. Peculiarly enough, Ulleund-do and Dok-do basalts show isotopic compositions right in between the two mantle compositional domains. It is not clear whether such a difference in isotopic compositions of the Ulleung-do and Dok-do basalts indicates the existence of the third mantle domain or the mixing of materials derived from the two mantle domains. Isotopic compositions reported for the mantle xenoliths carried by the Cenozoic basalts of East and Southeast Asia mimics the variations of their host basalts. Such similarities suggest that enriched components within the source mantle of the Cenozoic basalts of East and Southeast Asia may have originated from the subcontinental lithospheric mantle by delamination process.