Characteristics of Jurassic Continental Arc Magmatism in South Korea: Tectonic Implications

Study of whole-rock geochemical and isotope (Sr-Nd) characteristics of Jurassic plutons from South Korea shows differences in chemical compositions that might reflect the Jurassic tectonic environments of East Asia. These plutons are dominated by granites and granodiorites with subordinate intermediate monzodiorite to diorite phases. All analyzed samples show depletions in Ta-Nb, P, and Ti and enrichment in large ion lithophile elements, characteristic of high-K calc-alkaline I-type granitoids of the active continental margin. Early Jurassic plutons fall into two groups: Sr-87/Sr-86(I) = 0.708730-0.716792, epsilon(Nd) (T) = -19.6 to -9.4, from the middle of the Yeongnam Massif (i.e., Beonam, Geochang and Gimcheon), and Sr-87/Sr-86(I) = 0.704794-0.706967, epsilon(Nd)(T) = -6.3 to 0.9, in the southeastern end of the Yeongnam Massif (i. e., Chilgok and Hapcheon). Foliated leucogranite, foliated K-feldspar megacryst-bearing granodiorite, and massive biotite granite of the Early Jurassic Beonam pluton have SHRIMP zircon U-Pb ages of 196, 191, and 190 Ma, respectively. Middle Jurassic plutons of the Okcheon Belt (i. e., in the Iksan, Jeonju, and Sunchang areas) have high Sr-87/Sr-86(I) ratios (0.710453-0.717192) and strongly negative epsilon(Nd)(T) values (-20.8 to -13.9). These differences suggest that paleo-Pacific plate subduction along the continental outmost margin during the Early Jurassic gave rise to Andean-type magmatism, represented by minor crustal thickening and subsequent lithospheric delamination. This resulted in the upwelling of asthenospheric mantle and formation of juvenile crust by underplating of mantle-derived magma in the lower crust (e.g., Chilgok and Hapcheon plutons). A subsequent underplating and heating event from the asthenosphere partially melted the overlying preexisting underplated mafic rocks and ancient crust, leading to the formation of Early Jurassic plutons (e.g., Beonam, Geochang, and Gimcheon) and Middle Jurassic plutons that are preserved more inlandward within the Korean Peninsula.