The Petrology and Geochemistry of Volcanic Rocks on Jeju Island: Plume Magmatism along the Asian Continental Margin.

The incompatible element signatures of volcanic rocks forming Jeju Island, located at the eastern margin of the Asian continent, are identical to those of typical intraplate magmas. The source of these volcanic rocks may be a mantle plume, located immediately behind the SW Japan arc. Jeju plume magmas can be divided into three series, based on major and trace element abundances: high-alumina alkalic, low-alumina alkalic, and sub-alkalic. Mass-balance calculations indicate that the compositional variations within each magma series are largely governed by fractional crystallization of three chemically distinct parental magmas. The compositions of primary magmas for these series, using inferred residual mantle olivine compositions, suggest that the low-alumina alkalic and sub-alkalic magmas are generated at the deepest and shallowest depths by lowest and highest degrees of melting, respectively. These estimates, together with systematic differences in trace element and isotopic compositions, indicate that the upper mantle beneath Jeju Island is characterized by an increased degree of metasomatism and a change in major metasomatic hydrous minerals from amphibole to phlogopite with decreasing depth. The original plume material, having rather depleted geochemical characteristics, entrained shallower metasomatized uppermost mantle material, and segregated least-enriched low-alumina alkalic, moderately enriched high-alumina alkalic, and highly enriched sub-alkalic magmas, with decreasing depth. [ABSTRACT FROM PUBLISHER]