Geochemistry, K–Ar geochronology and Sr–Nd–Pb isotope compositions of pitchstone in Gohado, southwestern Okcheon Belt, South Korea

Abstract

Abstract Geochemical and Sr–Nd–Pb isotope characteristics, as well as K–Ar geochronology of a massive pitchstone (volcanic glass) stock erupted into Late Cretaceous lapilli tuff and rhyolite in the Gohado area, southwestern Okcheon Belt, South Korea, are reported. The pitchstones are highly evolved with SiO2 contents ranging from ∼72 to 73 wt%, K2O/Na2O ratios of 1.04–1.23 and low MgO/FeOt values (0.17–0.20). The pitchstones are weakly peraluminous and the ASI (molar Al2O3/Na2O + K2O + CaO) values are significantly lower than 1.1. The pitchstones also display a general calc‐alkaline nature with significant alkali contents. The rare earth elements (REE) compositions show moderately fractionated nature with (La/Yb)N ranging from 11 to 16. Chondrite normalized REE patterns show relative enrichment of light REE over heavy REE and moderate Eu anomaly (Eu/Eu* ratio varies from 0.53 to 0.57). A distinct negative Nb anomaly is observed for all pitchstones on a primitive mantle normalized trace element diagram, typical of subduction‐related magmatism and crustal‐derived granites. All these features are characteristic of I‐type granites derived from a continental arc. The pitchstones have Zr contents of 98.5–103.5 ppm with zircon thermometry yielding temperatures of 749–755°C (mean 752°C). The K–Ar analyses of representative pitchstone samples yielded ages of 58.7 ± 2.3 and 62.4 ± 2.1 Ma with a mean age of 61 Ma. The rocks show nearly uniform initial 87Sr/86Sr isotopic ratios of 0.7104–0.7106 and identical 143Nd/144Nd initial ratio of 0.5120. The rocks display negative εNd (61 Ma) values of −12. The depleted mantle model ages (TDM) range from 1.54 Ga to 1.57 Ga. The Pb isotope ratios are 206Pb/204Pb = 18.522–18.552, 207Pb/204Pb = 15.642–15.680 and 208Pb/204Pb = 38.794–38.923. These ratios suggest that the Gohado pitchstones were formed in a continental arc environment by partial melting of a 1.54 Ga to 1.57 Ga parental sources of lower crustal rocks probably of mafic or intermediate compositions.