부산 기장군 신평리 해안 일대에 분포하는 백악기 이천리층의 미량원소 조성: 초고철질 기원암과 경상분지 발달사에의 의의 = Trace-element composition of the Cretaceous Icheonri Formation at Sinpyeong-ri Coast, Gijang-gun, Busan, Korea: Implications for ultramafic source and developments of the Gyeongsang Basin

다국어 초록 (Multilingual Abstract)

The Icheonri Formation is a Cretaceous non-marine sedimentary formation distributed in the southeastern part of the Gyeongsang Basin. Detrital chromian spinel, previously reported only from the Sindong Group of the Gyeongsang Supergroup, has recently been reported in the Icheonri Formation. The occurrence of chromian spinel, derived from ultramafic rocks, provides key evidence for reconstructing the tectonic setting during deposition, and sediments containing this mineral typically exhibit elevated concentrations of chromium (Cr) and nickel (Ni). In this study, we assessed the influence of ultramafic source rocks (e.g., mantle peridotite) using the trace element compositions of sandstones and mudrocks from the Icheonri Formation, in comparison with those from the Sindong and Hayang Groups of the Gyeongsang Supergroup. Trace element enrichment in the Icheonri sediments, relative to those in both the upper continental crust and active continental margin sediments, implies an input from ultramafic sources, albeit with a limited degree. While the extent of sediment supply from mafic rocks is comparable to that observed in the Sindong Group, such influence is not evident in the Hayang Group. Therefore, ultramafic rocks were likely exposed on a small scale along the eastern margin of the Gyeongsang Basin, in addition to the ultramafic bodies located to the further west that supplied mafic sediments to the Sindong Group. This suggests that ultramafic rocks were exposed in at least two distinct localities around the basin during both the early and late stages of its evolution.