Cenozoic magmatism of extension zones in continental margins: An example of the Korea-Japan area

Cenozoic volcanic rocks of extension zones in the eastern margin of the Eur asian continent display lateral and temporal variability depending on the i ntensity of extension. The sequence of postsubduction volcanics in the cont inental margin begins with Oligocene shoshonites, which gave way to Miocene -Holocene rocks, predominantly alkaline basalts of the within-plate type. T he Paleogene shoshonites are characterized by high K concentrations and sho w some features of calc-alkaline rocks. The Lower Miocene basalts of Korea are highly alkaline and contain a subduction-related component (lower conce ntrations of Ti, Ta, and Nb). The HFSE richest Middle Miocene and Pliocene- Holocene alkaline basalts of Korea additionally include single tholeiite fl ows similar to E-MORB, and, more rarely, N-MORB. In the southeast of the Ko rea Peninsula and in the Sea of Japan, the Early Miocene interval correspon ds to tholeiites, whose depletion degree increases with decreasing age. The evolution of the postsubductional Cenozoic magmatism was caused by the eff ects of several sources. The Oligocene lavas, which were produced in an env ironment of the initial extension of a continental margin, were derived fro m shallow sources of the EMU type of the subcontinental lithosphere, which had been extensively metasomatized in connection with subduction during the earlier, Mesozoic-Paleocene stage. The genesis of the Miocene-Holocene alk aline basalts, whose Nd and Pb isotopic signatures suggest relations with E MI compositions, was most probably controlled by a lower-mantle plume. The Lower Miocene tholeiites in the southeastern Korea Peninsula, adjacent to t he opening Sea of Japan were related to an asthenospheric source. The conti nental extension-zone tholeiites seem to be heterogeneous, and the causes o f their calc-alkaline features are different.