Final Magma Storage Depth Modulation of Explosivity and Trachyte–Phonolite Genesis at an Intraplate Volcano: a Case Study from Ulleung Island, South Korea

Abstract

Ulleung Island is the top of a 3000 m (from sea floor) intraplate al-kalic volcanic edifice in the East Sea/Sea of Japan. The emergent 950 m consist of a basaltic lava and agglomerate succession (Stage 1, 1·37^0·97Ma), intruded and overlain by a sequence of trachytic lavas and domes, which erupted in two episodes (Stage 2, 0·83^ 0·77Ma; Stage 3, 0·73^0·24Ma). The youngest eruptions, post 20 ka BP, were explosive, generating thick tephra sequences of phonoli-tic composition (Stage 4), which also entrained phaneritic, porphy-ritic and cumulate accidental lithics. Major element chemistry of the evolved products shows a continuous spectrum of trachyte to phonolite compositions, but these have discordant trace element trends and distinct isotopic characteristics, excluding a direct genetic relationship between the two end-members. Despite this, the Stage 3 trachytes and some porphyritic accidental lithics have chemical char-acteristics transitional between Stage 2 trachytes and Stage 4 phono-