Geochemistry of anorthositic xenolith and host tholeiite basalt from Jeju Island, South Korea

Abstract

Volcanic rocks on Jeju Island consist mainly of intraplate alkaline lavas with subordinate intercalated tholeiite basaltic rocks. The alkaline suite can be divided into high-Al alkali (HAA) and low-Al alkali (LAA) sub-suites. We determined the 87Sr/86Sr ratios of anorthositic xenolith plagioclases and the Sr-Nd-Pb isotope compositions of the host tholeiite basalt (Taeheungri basalt). The anorthositic xenoliths are considered to represent buoyancy-induced floating cumulates from previous tholeiite magmatism on Jeju Island. The 87Sr/86Sr ratios of the xenolith plagioclases are highly elevated compared with those of the Jeju tholeiite lavas, including the host basalt. The 87Sr/86Sr ratios of the Jeju volcanic rocks increase from the LAA through the HAA to the tholeiite suite. The tholeiite suite generally exhibits less radiogenic 143Nd/144Nd ratios than the alkaline suite. In Pb-Pb space, the tholeiite suite has, in general, elevated 207Pb/204Pb ratios compared with those of the alkaline suite at a given 206Pb/204Pb ratio. In contrast to the 87Sr/86Sr ratios, the two alkaline sub-suites (HAA and LAA) display indistinguishable Nd and Pb isotope compositions. Notably, some of the tholeiite and HAA suites exhibit distinctive positive anomalies in Eu and Sr in the trace element distribution pattern and the 87Sr/86Sr ratios increase as a function of the degree of Sr or Eu enrichment. These observations, along with the relatively fractionated nature of the tholeiite and especially the HAA suites, indicate that the suites might be products of assimilation of plagioclase-rich cumulates, possibly at lower crustal depth, during parent magma evolution. Jeju volcanic rocks exhibit a mixing line between Indian mid-ocean ridge basalt-like depleted mantle and enriched mantle type 2 (EM2) components in Sr-Nd and Pb-Pb isotope correlation diagrams. The enriched component, the source of which may have been the asthenosphere, is supposed to have been higher in the Jeju tholeiite magmatism than in the alkaline magmatism.