Polyphase tectono-magmatic episodes as revealed by SHRIMP U-Pb geochronology and microanalysis of zircon and titanite from the central Okcheon belt, Korea

The south central Okcheon belt of the southern Korean peninsula experienced polyphase granitic magmatism and deformation during the Mesozoic. This study presents integrated results of ion microprobe U-Pb geochronology and microtextural and microchemical analyses of zircon and titanite from pre-tectonic (Baegnok and Cheongsan) and post-tectonic (Boeun and Sokrisan) plutons with respect to the Cheongsan shear zone. The Baegnok granodiorite sample yielded coherent zircon and titanite ²⁰⁶Pb/²³⁸U ages of 225.6±1.8 and 223.9±4.4Ma, indicating a rapid early cooling of the pluton. The Cheongsan K-feldspar megacryst-bearing biotite granite sample yielded a coeval zircon ²⁰⁶Pb/²³⁸U age of 224.7±1.8Ma. On the other hand, the titanite grains from this rock are categorized into two age populations corresponding to the late Triassic (214.3±7.9Ma) and the late Cretaceous (96.2±8.2Ma) growth. The older population is clearly of igneous origin, considering partly preserved oscillatory zoning in back scattered electron images. Therefore the age indicates contrasting early cooling paths of the two pre-tectonic plutons. The younger population is interpreted to be the product of superimposed metasomatic reactions involving plagioclase, ilmenite, rutile, epidote, and calcite. The two populations show different patterns in their atomic Al/Fe ratios. Zircons from the Boeun and Sokrisan plutons yielded ²⁰⁶Pb/²³⁸U ages of 172.2±2.1 and 94.2±2.1Ma, respectively, with the latter being indistinguishable from the younger titanite age of the Cheongsan pluton. The SHRIMP U-Pb age of syn-deformational fine-grained titanite grains lying on sigmoidal foliations in a mylonite from the Cheongsan shear zone has a large error (165±47Ma) due to their low U/Pb ratios possibly affected by post-tectonic magmatic activities. This study highlights the importance of detailed microanalysis prior to the isotopic dating of samples that have experienced complex tectono-magmatic episodes.