International Geologiical Congress - Oslo 2008


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EIL-01 General contributions to the lithosphere


Geochemical and textural implications of gabbroic xenoliths in alkaline basalts from Jeju Island, South Korea


Jae-eun Yu, Pusan National University (Republic of Korea)
K Yang, Pusan National University (Republic of Korea)
Y Eom, Pusan National University (Republic of Korea)
B Hwang, Pusan National University (Republic of Korea)
J Kim, Pusan National University (Republic of Korea)
C Szabo, Eotvos University (Hungary)


Gabbroic xenoliths and plagioclase megacrysts trapped in alkaline basalts of Jeju Island, South Korea, a ∼ 1.7 to historical times shield volcano. Jeju Island (75 35 km) is also known as a intraplate volcano showing signatures of oceanic island basalt although the island is located at the eastern margin of the Eurasian plate which is known as convergent plate margins where Pacific and Philippine Sea plates are being subducted beneath the Eurasian Plate. The gabbroic xenoliths trapped in basaltic rocks are 1-10 cm in size and gabbro norite in composition. Plagioclase is the most dominantly present (42-94 vol%), while olivine crystals are absent. Opx is subordinate to cpx. Gabbroic xenoliths have distinctively different textural types; poikilitic and intergranular characteristics. Pokilitic xenoliths, interpreted as cumulates, include euhedral opx+cpx in oikocryst plagioclase.

Equigranular xenoliths include subhedral to anhedral opx+cpx+pl. Based on the textural and geochemical natures and occurrence mode at the outcrops, all mineral compositions of the gabbroic xenoliths in this study are compatible with those from the basaltic rocks in Jeju Island, implying that the studied xenoliths belong to the Jeju magma system. Furthermore, poikiltic and equigranular xenoliths and plagioclase megacrysts were from rigid zone, mushy zone and crystal-suspended zone of the magma chamber, respectively. The gabbroic xenoliths in alkaline basalts of the Jeju Island represent crystallization products at the upper part of a magma chamber. The parental magma from which gabbro xenoliths and plagioclase megacrysts crystallized is similar to that represented by the host lava, but more evolved one and pre-dating. The studied xenoliths represent gabbro fragments that went through crystallization-isolation-capture processes associated with magma batches temporarily occupying magma chamber during the formation of the Jeju Island.
Key words: Jeju Island, gabbroic xenoliths, poikilitic, intergranular, magma chamber


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