Petrogenesis and mantle source characteristics of volcanic rocks on Jeju Island, South Korea

Abstract

Abstract Volcanic rocks on Jeju Island, South Korea, are dominantly alkaline suites (basalt to trachyte) with small amounts of intercalated tholeiitic (TH) basaltic rocks. The alkaline suites can be subdivided into high-Al alkaline (HAA) and low-Al alkaline (LAA) sub-suites. We determined the major- and trace-element abundances, and Sr–Nd–Pb–Mg isotopic compositions for 60 samples from four boreholes and nine outcrops on Jeju Island to better understand their petrogenesis and mantle source characteristics. The samples yield whole-rock40Ar/39Ar ages of ~968–24\u202fka. The HAA suite formed slightly earlier than the other suites. On a primitive-mantle-normalized trace-element diagram, Jeju basaltic rocks exhibit typical oceanic island basalt (OIB)-like large-ion lithophile element (LILE) enrichments without significant Nb or Ta depletions. No meaningful correlation was observed between SiO2 and Sr–Nd–Pb–Mg isotope compositions. However, some samples, especially the TH and HAA suites, exhibit positive Eu and Sr anomalies, coupled with elevated 87Sr/86Sr, Δ7/4Pb, and Pb/Ce ratios, indicating an important role of ancient anorthosite assimilation during its evolution. Furthermore, they show negative K and Rb anomalies. The basaltic rocks have fractionated Nb/Ta and Zr/Hf ratios with negative Ti anomalies. They have elevated Dy/Yb ratios within the range of melts derived from garnet lherzolite, but have Yb contents much higher than those of garnet lherzolite melts, instead plotting close to those of eclogite-derived melts. They also have high Fe/Mn ratios (>~60) and FeO/CaO–3MgO/SiO2 values (>0.45), similar to melts from pyroxenite/eclogite sources. The Sr–Nd–Pb isotopic compositions display a good linear array between depleted mid-ocean-ridge basalt (MORB) mantle and enriched mantle type 2 (EM2). There are no discernible 143Nd/144Nd, Δ8/4Pb, and δ26Mg isotopic differences among the three suites, implying their derivation from a compositionally homogeneous source by variable degrees of melting. The Mg isotopic compositions yield δ26Mg values of −0.53‰ to −0.20‰, extending to a much lower value than normal mantle (δ26Mg\u202f=\u202f−0.25‰\u202f±\u202f0.04‰). These observations suggest the presence of recycled rutile-bearing eclogite, carbonate, and sedimentary components in the mantle source of the Jeju volcanic rocks together with a peridotite component. No significant spatiotemporal changes in the mantle sources and melting conditions were observed.