Recurrent Quaternary magma generation at Baekdusan (Changbaishan) volcano: New zircon U-Th ages and Hf isotopic constraints from the Millennium Eruption

초록

It is well known that Baekdusan (Changbaishan), an active 2744-m-high stratovolcano located on the border between North Korea and China, initiated trachytic/commenditic eruption in the Pleistocene but the petrogenetic and temporal evolutions of this alkaline magma system remain insufficiently constrained. The so-called "Millennium Eruption" (ME) in the mid-10th century, which resulted in the formation of the world's highest caldera lake (Cheonji, or Tianchi) atop the mountain, was one of the largest and most violently explosive volcanic eruptions to have occurred on Earth in the past 2000 years. Here, we present U-Th disequilibrium ages and Hf isotopic compositions of zircons from trachydacitic pumices ejected during the ME. Taken together with the previous U-Th isotope data, our laser ablation-multicollector ICPMS analyses of the zircons yielded multiple age components consisting of an earliest Holocene cluster (11.3 +/- 1.3 ka) and Middle-Late Pleistocene populations (ca. 230 and 110 ka), which indicates that Baekdusan magma has been generated episodically at intervals of ca. 120-100 ky. If the eruption ages assigned for the samples analyzed in the previous isotopic studies are correct, the consistent age pattern of zircon may suggest the persistent existence of the same magmatic plumbing system beneath Baekdusan since the ME. The wide range of zircon epsilon(Hr) values (+5.8 to -3.5) reflects an interaction between the EM1-type primitive magma and pre-existing crustal materials, evidenced by the presence of a xenocrystic zircon core showing a significantly negative epsilon(Hf) value (= -21). A broad negative correlation observed between zircon epsilon(Hf) and the published delta O-18 values can be ascribed to the progressive assimilation of weathered supracrustal rocks, likely in the course of magma chamber expansion. (C) 2018 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

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