Rare earth element geochemistry and Nd isotope composition of stream sediments, south Han River drainage basin, Korea

Chemical analyses of parent rock, soil and stream sediments were done to determine to what extent the river sediments retain the geochemical signature of the parent rocks. The study area consists mainly of Mesozoic biotite granite and Precambrian biotite gneiss, located in the central Korean peninsula. The drainage system consists of tributaries of the South Han River, such as Bokhacheon, Shinduncheon and Yanghwacheon. Stream sediments were collected every March during 2004–2006. The sediments collected were divided into coarse (gravel to sand size) and fine (silt to clay size). More than 90% is coarse sediment. The major element composition of the coarse and fine sediments are not identical to the weathering trend of granite. However, molar proportions of Al2O3, CaO+Na2O, and K2O in the sediments show that they were derived from granitoids rather than from biotite gneiss. The total Fe2O3, CaO, MgO, Na2O, and MnO contents and REE concentrations of the fine sediments are higher than those of the coarse sediments. In a weathering profile from the granite and gneiss area, there was no change of REE distribution pattern from the fresh rock to soil. Compared to biotite granite-normalized REE pattern for the sediment, both coarse and fine sediments have relatively flattened REE patterns, and all the coarse sediment have positive Eu anomalies, whereas in the fine sediments negative Eu anomalies prevail. Nd isotopic composition of the sediments is similar to that of the biotite granite rather than the biotite gneiss. Stream sediments in the Icheon area were derived from the granite. Nd isotope geochemistry as well as REE geochemistry is useful tool for tracing the provenance of the river sediments.