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

摘要: Ulleung Island is the top of a 3000 m (from sea floor) intraplate alkalic volcanic edifice in East Sea/Sea Japan. The emergent 950 consist basaltic lava and agglomerate succession (Stage 1, 1·37^0·97Ma), intruded overlain by sequence trachytic lavas domes, which erupted two episodes 2, 0·83^ 0·77Ma; Stage 3, 0·73^0·24Ma). youngest eruptions, post 20 ka BP, were explosive, generating thick tephra sequences phonolitic composition 4), also entrained phaneritic, porphyritic cumulate accidental lithics. Major element chemistry evolved products shows continuous spectrum trachyte to phonolite compositions, but these have discordant trace trends distinct isotopic characteristics, excluding direct genetic relationship between end-members. Despite this, 3 trachytes some lithics chemical characteristics transitional 2 4 phonolites. Within tephras three subgroups can be distinguished. oldest, Tephra 5, considerably enriched incompatible elements chondrite-normalized rare earth (REE) patterns display negative Eu anomalies.The later tephras, Tephras 4^2, compositions intermediate early units samples, their REE do not significant last erupted,Tephra from small intra-caldera structure, has tephriphonolite composition. Trace as well textural suggest phaneritic host pumices.The are related earlier phases magmatism (Stages 1^3). They cannot reliably modelled fractional crystallization processes.The differences explosive 4) effusive 2^ 3) eruptions mainly due different pre-eruptive pressures temperatures, causing closedversus open-system degassing. Based on thermodynamic thermobarometric modelling, phonolites derived deeper (subcrustal) magma storage rose quickly, with volatiles trapped until eruption. By contrast, stored at shallower crustal levels for longer periods, allowing volatile exsolution degassing before