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The Association of Korean Geoscience Societies (한국지질과학협의회)
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Petrologic, chemical, and isotopic evaluation of pitchstones in the Samho area, southwestern Okcheon Belt, South Korea

  • Cho, Kyu-Seong (Department of Earth Science Education, Chonbuk National University) ;
  • Rajesh, V.J. (Department of Earth and Space Sciences, Indian Institute of Space Science and Technology) ;
  • Kang, Seong-Seung (Department of Energy and Resources Engineering, Chosun University) ;
  • Kim, Cheong-Bin (Department of Physics Education and Korea Institute of Landscape and Geology, Sunchon National University)
  • Published : 2011.06.01
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Abstract

The Samho area in the southwestern Okcheon Belt, Korean Peninsula, contains spatially related eruptions of lapilli tuff, rhyolite, and pitchstone, considered to be of Late Cretaceous age. The pitchstones, which are the youngest rocks in this volcanic suite, are chemically highly evolved, strongly metaluminous to weakly peraluminous, and calc-alkaline. They have moderately fractionated chondrite-normalized rare earth element (REE) patterns, with relative enrichment of light REEs and Eu/$Eu^{\ast}$ values of 0.53-0.57. A distinct negative Nb anomaly is observed on a primitive mantlenormalized trace element diagram. These chemical features correspond to an I-type granite derived from a continental/oceanic arc. The pitchstones have Zr contents of 45-92 ppm, and zircon thermometry yields temperatures of $650-743^{\circ}C$ (mean value, $722^{\circ}C$). The pitchstones have an initial $^{87}Sr/^{86}$Sr isotopic ratio of 0.712, an initial $^{143}Nd/^{144}Nd$ ratio of 0.5120, and ${\aa}Nd_{(t)}$ values of -10.6 to -12.0. The mean K-Ar whole-rock ages of the lapilli tuff, rhyolite, and pitchstone are 81.9 ${\pm}$ 2.4 Ma, 65.5 ${\pm}$ 1.9 Ma, and 12.7 ${\pm}$ 0.9 Ma, respectively. The depleted mantle model ages (TDM) range from 1.46 to 1.58 Ga. The $^{206}Pb/^{204}Pb$, $^{207}Pb/^{204}Pb$, and $^{208}Pb/^{204}Pb$ isotopic ratios are 18.477-18.569, 15.672-15.702, and 38.889-38.999, respectively. The Sr and Nd isotopic compositions of the pitchstones are relatively enriched compared with depleted MORB mantle (DMM), indicating the involvement of lower crustal components in their source. We conclude that the pitchstones in the Samho area formed in a continental arc setting by partial melting of a probable Mesoproterozoic parental source. The magma series, petrogenesis, and tectonic setting of Late Cretaceous magmatism in the Samho area can be correlated with those of the Haenam volcanic fields located east of the present study area.

Keywords

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