Sr, Nd, and Pb isotope systematics of granitic rocks in the central Ogcheon Belt, Korea.
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The source characteristics and magmatic evolution history of granitic rocks in the central Ogcheon Belt, Korea, were investigated based upon Sr-Nd-Pb isotope systematics. The granitic rocks are divided into three types; the Permian to Triassic granites distributed near the Boeun area (PTG), the Daebo granitic batholith (DBG), and Cretaceous Sogrisan Granite (SOGR). Well fitted Rb-Sr whole rock isochrons of the PTG yield geologically significant ages (256 Ma for the Baegrock Granodiorite (BRGD), 230 Ma for the Boeun Granite (BOGR), and 216 Ma for the Chungsan Granite (CHGR)). Rb-Sr isotope data of the DBG do not form isochrons of geologically reasonable ages, suggesting that the batholith was formed by discrete magmatic events. Variation of initial Sr and Nd isotope ratios in the PTG can be explained by a simultaneous assimilation and fractional crystallization (AFC) model. The BRGD has the most primitive Sr-Nd-Pb isotopic signature among the PTG. Based on mass balance calculation, the relatively high but strongly negative εNd(t) of the BRGD is difficult to explain by an interaction of mantle-derived melt with crustal materials. Field observation and low initial 87Sr/86Sr ratios of the BRGD contradict the possibility of its derivation exclusively from the upper crustal source region. The isotopic signature of the BRGD is taken to be largely inherited from the continental lower crust. Primitive BRGD samples have relatively higher εNd(t) values than the other granites, indicating that the crustal formation age of the lower crust is younger than that of the upper crust. Less radiogenic Sr and Pb isotope ratios of the BRGD indicate that Rb, U, and Th (and other large ion lithophile elements ?) depletion has been maintained in the lower crust for a geologically long time. εNd(t) values of the DBG are strongly related to their locality, indicating local heterogeneity in the source region. Inconsistent variation of the DBG in εNd(t) vs. εSr(t) plot precludes a mixing model between primitive melt and upper crustal materials. Relatively constant εNd(t) and variable εsr(t) of the SOGR reflects the effect of simultaneous assimilation and fractional crystallization of feldspars. Ranges of initial Sr, Nd and Pb isotopic compositions of the SOGR are quite similar to those of the other, older granites, which implies a similarity of source materials between them.
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日本地球化学会 | 論文
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