抄録
The wide utilization of radiometric ages for orogen analyses would not been possible without monazite and zircon that are sufficiently robust to preserve their original U and Th and accumulated radiogenic Pb even through overgrowth during high temperature geologic events. Monazite and zircon discriminate strongly against the daughter Pb during crystallization. The CHIME (chemical Th-U-total Pb isochron method) is a new approach to the dating of micrometer-sized domains in monazite and zircon grains on the basis of the precise EPMA analyses of Th, U and Pb. Although the EPMA dating inherits the classic Th-U-Pb chemical dating, the CHIME method provides a significant advantage by adopting the regression-based technique that has the ability to work with minerals containing substantial initial Pb. The age- and composition-mapping can identify two or more chronologically homogeneous domains that are separated by age gaps within a single grain. Critical comments include the chemical criteria that discriminate possibly concordant EPMA zircon and monazite analyses from discordant ones. The in-situ CHIME dating and mapping of zircon and monazite in polished thin sections coupled with compositional mapping of associate garnet has a great chronological potential for the analysis of the detailed sequence of geologic events. The improved CHIME method revealed that (1) the garnet granulite and the eclogite in the Hongseong area of the Gyeonggi Massif within the Korean Peninsula have different evolutional histories as opposed to the current view of the formation by a single Permo-Triassic collision, (2) the widespread Permo-Triassic paragneiss in the massif are in fact an exotic metamorphic terrain thrusted over the Proterozoic basement complex at ca. 240Ma, and (3) Permo-Triassic overprints on the latter are attributable to contact metamorphism from the hot allochthon.
