PETROCHEMICAL CHARACTERISTICS OF THE QUATERNARY VOLCANIC ROCKS IN JEJU VOLCANIC ISLAND, KOREA

Jeju(=Cheju) Volcanic Island is a major Quaternary volcanic field in Korea, and is a shield volcano that lies on the ca. 100 m-deep continental shelf off the Korean Peninsula. The island, 74 km long and 32 km wide, is mainly composed of plateau- and shield-forming basaltic lavas and is covered with numerous (~365) monogenetic volcanic cones, most of which are scoria cones formed by Strombolian or Hawaiian eruptions, whereas about 20 are tuff rings and cones formed by explosive hydrovolcanic eruptions. Major, trace elements and REE analyses for lavas in Jeju Island were carried out to interpret the origin and evolution of magma. The lavas mainly belong to the alkaline rock series in chemistry, which show alkali basalt-hawaiite-megearite-benmoreite-trachyte composition, and partly belong to the tholeiitic rock series with basalt-basaltic andesite composition. For given MgO contents, tholeiitic lavas have characteristically lower TiO2, P2O5, Fe2O3, K2O and other incompatible elements content, but higher SiO2 than alkali lavas. Total REEs contents of tholeiitic lavas are lower than those of alkaline rocks, but chondrite-normalized REE patterns are subparallel to those of the alkaline rocks. On the tectonomagmatic discrimination diagram for basaltic rocks, they fall into the fields for within-plate and oceanic island alkalic basalt and tholeiitic basalt. Tholeiitic lavas have higher ratios of Zr, Y, Yb and P2O5 to LIL elements (i.e. P2O5/K2O, Yb/La, Y/Nb) than alkaline rocks. Both tholeiitic and alkaline lavas are similar in their K/Ba ratios. The alkaline and tholeiitic lavas originated from different degrees of partial melting of a homogeneous source with primitive garnet-peridotite mantle materials. Degrees of partial melting of tholeiitic lavas were higher than alkaline rocks.