Geochemical and isotopic studies of the Cretaceous igneous rocks in the Yeongdong Basin, Korea: Implications for the origin of magmatism in pull-apart basin

The Yeongdong Basin is one of the pull-apart basins in the southwestern part of the Korean Peninsula that has developed during Cretaceous sinistral fault movement. The bimodal igneous activities (basalts and rhyolites) in the basin appear to be closely associated with the basin development. Here, we discuss the origin of the igneous rocks using chemical and radiogenic isotope data. Basaltic (48.4-52.7 wt% SiO₂) and rhyolitic (70.3-70.8 wt% SiO₂) rocks have transitional alkalinity in a total alkali-silica diagram. The rhyolitic rocks have relatively high K₂O contents (5.2-6.0 wt%) for given SiO₂. The basaltic rocks have a distinct negative anomaly of Nb and Ta in a MORB-normalized spider diagram, indicating a significant amount of crustal component in the magma. The basaltic rocks plot generally within the calc-alkaline basalt field in the trace element discrimination diagrams. The εNd(T) values of the basaltic rocks (-13.6 to -14.3) are slightly higher than those of the rhyolitic rocks (-14.7 to -15.2), and the initial Sr isotopic ratios of the former (0.7085-0.7093) are much lower than those of the latter (0.7140-0.7149). Nevertheless, the initial Nd and Sr isotope ratios of the igneous rocks in the Yeongdong Basin are similar to those of the neighboring Cretaceous igneous rocks in the Okcheon belt. The Pb isotope ratios plot within the field of Mesozoic granitoids outside of the Gyeongsang Basin in Pb-Pb correlation diagrams. Since a basaltic magma requires the mantle source, the enriched isotopic signatures and negative Nb anomaly of the basaltic rocks suggest two possibilities for their origin: enriched mantle lithospheric source, or depleted mantle source with significant amount of crustal contamination. The slightly more enriched isotopic signatures of rhyolites suggest that they originated from differentiation of the basaltic magma with some crustal contamination, or from direct partial melting of the lower crust. Although larger exposed volume of the rhyolites than the basalts may support the latter possibility, distinct difference in isotopic ratios between the rhyolites and the Precambrian basement rocks favor the former one.