URANIUM-SERIES DATING OF ZIRCONS FROM MULTIPLE ERUPTIONS OF THE CHANGBAI VOLCANO, CHINA/NORTH KOREA BORDER

Recently, the Changbai (or Baitoushan, Paektu) volcano located on the border of China and North Korea has drawn much attention as a result of its large climactic caldera-forming eruption 1000 years (1 ka) ago and the recent detection of swarms of small tremors at the volcano between 2002 and 2006. In addition to the 1-ka eruption of 100 km³ of comendite ignimbrite and pumice, the Changbai volcano also erupted moderate–volume pre-caldera comendite lava at ~4 ka and small-volume post-caldera trachyte ignimbrite at ~0.3 ka. We report uranium-series ages of zircons from these 3 eruptions, in order to investigate potential genetic relationships of these eruptions and their magma residence time scales. Zircon isochron ages are 12.2±1.1 ka (2σ) for the 4-ka pre-caldera comendite lava and 12.2±1.7 ka for the 1 -ka syn-caldera comendite materials. In contrast, zircons from the 0.3-ka post-caldera trachyte ignimbrite exhibit 3 respective peaks at 2.6±1.8 ka, 130±10 ka and >230 ka. The indistinguishable zircon ages for the 4-ka pre-caldera eruption of comendite lava and the 1-ka syn-caldera explosive eruption suggest that the 4-ka lava provides an early sampling of a much larger magma body at depth and thus serves as a petrologic early-warning signal. Calculated magma temperatures indicate that the 4-ka lava may represent the lowest-temperature magma in the roof zone of a thermally zoned magma chamber that usually escapes first. The distinctive multi-modal zircon age distributions of the 0.3-ka trachytic eruption, however, reveal that this post-caldera eruption tapped a different magma body. Our results also indicate very short zircon and magma residence times for the Changbai volcano (8 kyr for the 4-ka eruption, 11-12 kyr for the 1-ka eruption and 2.3 kyr for the 0.3-ka eruption).