SR, ND, AND PB ISOTOPE SYSTEMATICS OF GRANITIC-ROCKS IN THE CENTRAL OGCHEON BELT, KOREA

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 thr ee types; the Permian to Triassic granites distributed near the Boeun area (PTG), the Daebo granitic batholith (DBG), and Cretaceous Sogrisa n Granite (SOGR). Well fitted Rb-Sr whole rock isochrons of the PTG yi eld geologically significant ages (256 Ma for the Baegrock Granodiorit e (BRGD), 230 Ma for the Boeun Granite (BOGR), and 216 Ma for the Chun gsan Granite (CHGR)). Rb-Sr isotope data of the DBG do not form isochr ons of geologically reasonable ages, suggesting that the batholith was formed by discrete magmatic events. Variation of initial Sr and Nd is otope ratios in the PTG can be explained by a simultaneous assimilatio n and fractional crystallization (AFC) model. The BRGD has the most pr imitive Sr-Nd-Pb isotopic signature among the PTG. Based on mass balan ce calculation, the relatively high but strongly negative epsilon(Nd)( t) of the BRGD is difficult to explain by an interaction of mantle-der ived melt with crustal materials. Field observation and low initial Sr -87/Sr-86 ratios of the BRGD contradict the possibility of its derivat ion exclusively from the upper crustal source region. The isotopic sig nature of the BRGD is taken to be largely inherited from the continent al lower crust. Primitive BRGD samples have relatively higher epsilon( Nd)(t) values than the other granites, indicating that the crustal for mation 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. epsil on(Nd)(t) values of the DBG are strongly related to their locality, in dicating local heterogeneity in the source region. Inconsistent variat ion of the DBG in epsilon(Nd)(t) vs. epsilon(Sr)(t) plot precludes a m ixing model between primitive melt and upper crustal materials. Relati vely constant epsilon(Nd)(t) and variable epsilon(Sr)(t) of the SOGR r eflects the effect of simultaneous assimilation and fractional crystal lization of feldspars. Ranges of initial Sr, Nd and Pb isotopic compos itions of the SOGR are quite similar to those of the other, older gran ites, which implies a similarity of source materials between them.