Geochemistry and petrogenesis of Jeungok basalts in mid-Korean peninsula

Abstract

Major, trace element concentrations and Sr-Nd-Pb isotope systematics are reported for Jeungok basalts from the Choogaryong rift valley in mid-Korean peninsula. The lavas formed narrow and long basalt-plateau showing the layers of 10 to 20 m thick and about 95 km in length along the old river path. These lavas are olivine alkali basalts which formed in response to the subduction of the western Pacific plate beneath the northeastern part of the Eurasian plate margin. The Jeungok basalts are enriched in highly incompatible elements, and are enriched relative to bulk silicate earth (BSE) based on Sr, Nd and Pb isotope covariation diagrams. The basaltic rocks have very small variations in ⁸⁷Sr/⁸⁶Sr (0.7047-0.7056) and ¹⁴³Nd/¹⁴⁴Nd (0.51259-0.51265) ratios. The ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb vs ²⁰⁶Pb/²⁰⁴Pb plots show linear arrays parallel to the NHRL, and ²⁰⁸Pb/²⁰⁴Pb data points are displace considerably above NHRL.
     Trace elements and isotopic signatures indicate that the chemical composition of the rocks have not been modified by crustal contamination, and chemical variation of the rocks are mainly caused by fractional crystallization of olivine, plagioclase, clinopyroxene and magnetite. Trace element signatures such as Rb/Sr, Sm/Nd, Ba/Nb and La/Nb ratios are similar to OIB. These characteristics in conjunction with isotopic data suggest that these rocks were derived from enriched magma source result from mixing of the asthenospheric magma with lithospheric materials, and do not show any subduction related characteristics.