Multi-stage magmatic plumbing system of the volcano: A case study from Ulleung Island, South Korea

Abstract

The multi-stage magmatic plumbing system beneath the Ulleung, Dokdo, and Oki-Dogo Island has attracted considerable attention but has seldom been investigated in detail. Herein, we present a comprehensive geochemical analysis of volcanic rocks sampled at the Ulleung Island, including whole–rock major and trace elements as well as Sr and Nd isotopic compositions. Late Cenozoic alkaline volcanic rocks from the Ulleung Island are commonly divided into basaltic rocks (tephrite basanite and basalt) and acidic rocks (trachyte and rhyolite) based on the large petrologic and geochemical variations. All rocks are significantly enriched in light rare-earth elements (REEs) and large-ion lithophile elements and relatively depleted in heavy REEs. The acidic rocks have slightly enriched 87Sr/86Sr and εNd ratios (0.704895–0.705173; −2.48 to −1.91) and distinctly negative Ba, Sr, P, Eu, and Ti anomalies relative to the basaltic rocks (0.704733–0.705006, -2.61– -1.17). A multi-stage magma evolution model was finally developed beneath the Ulleung, Dokdo, and Oki-Dogo Island, characterized by at least two magma reservoirs at different depths approximately ranging between 20 and 100 km. These ascending magmas were retained in a shallower reservoir for ~1.0, ~0.5, and ~4.5 Ma, respectively, prior to eruptions with significant crystal fractionation. Mantle-derived basaltic melts either directly travelled to the surface to erupt as tephrite-basanite and basalt, or were stagnated in shallower reservoirs where they differentiated into acidic magma to erupt as trachyte and rhyolite. The Late Cenozoic Korean Peninsula basalts are mainly divided into two groups, i.e., the depleted mantle (DM) and enriched mantle 1 (EM1) mixing trend beneath the Ulleung and Dokdo Island, and the DM-EM2 mixing connection beneath the Jeju Island, which likely indicates the traversed boundary beneath Korean Peninsula that represents a simple lateral continuation of the boundary between the southeast and northeast China Blocks.