Spatio-temporal evolution of a dispersed magmatic system and its implications for volcano growth, Jeju Island Volcanic Field, Korea

Abstract

Jeju Island is the emergent portion of a basaltic volcanic field developed over the last c. 1.8 Ma on continental crust. Initial volcanism comprised dispersed, small-volume (< 0.01 km3) alkali basaltic eruptions that incrementally constructed a tuff pile. Lavas and scoria from continuing small-scaled monogenetic volcanism capped this foundation. From c. 0.4 Ma large-volume (> 1 km3) eruptions began, with lavas building a composite shield. Three magma suites can be recognized: Early Pleistocene high-Al alkali (HAA), and Late Pleistocene to Holocene low-Al alkali (LAA) and subalkali (SA). The chemical similarity between small-volume and primitive large-volume eruptions suggests analogous parent magmas and fractionation histories that are independent of erupted volumes. The large-volume magmas evolved to trachyte, which erupted in two distinct episodes: the HAA Sanbangsan suite at c. 750 ka and the LAA Hallasan suite at c. 25 ka. Sr and Nd isotopes indicate that the early trachytes were contaminated by upper crustal material, whereas the later magmas were not. Both suites bear a Nd isotope signature indicative of lower crustal interaction. Sub-suites transitional between HAA and LAA, and between LAA and SA, indicate that melting occurred in discrete, but adjacent, mantle domains. Throughout the evolution of this volcano, each magma batch erupted separately, and a centralized plumbing system was never created. The Island's central peak (Mt. Halla 1950 m a.s.l.) is therefore not a sensu stricto stratovolcano, but marks the point of peak magma output in a distributed magmatic system. Jeju's shape and topography thus represent the spatial variation of fertility of the mantle below it. An increase in melt production in the Late Pleistocene was related to a deepening of the melting zone due to regional tectonic rearrangements. Temporal coincidences between magmatic pulses on Jeju and large-scale caldera eruptive events along the nearest subduction system in Kyushu, Japan, suggest that tectonic extension and changing strain rates may drive volcanism on a regional basis, influencing the intraplate volcanism of Jeju Island.