Constrains on the Latest Volcanic Eruptions in Eastern-Central Europe: Zircon Geochronology and Magma Residence Time at the Ciomadul Volcano.

Abstract:

Evaluation of volcanic hazard is often difficult, particularly at those areas, where no volcanic events have been documented in the historical time or even longer. Here, we present new geochronological data to demonstrate that relatively young volcanism occurred in eastern-central Europe, at the Ciomadul volcano (southeast Carpathians, Romania) and there is a potential for future rejuvenation. The eruption ages of the Ciomadul are constrained by (U-Th)/He zircon dating corrected for U-series disequilibrium and this suggests that the volcanic activity occurred from 150 ka to 32 ka. Following a lava dome building stage, a more explosive eruption phase took place at 50-32 ka. Subplinian to plinian eruptions produced volcanic ash clouds that blanketed extended areas. The U-Th and U-Pb age spectra of the zircons from the 50-32 ka volcanic rocks ranges over 300 ka and is thus indicative of a long-lasting intrusive body beneath the volcano. Interior-rim age difference within individual zircons is as much as 340-88 ka. Thus, a subterraneous magma system was already under construction ca. 150-200 ka before the onset of volcanic eruptions. The peak zircon crystallization age falls between 220-100 ka, and there are only sparse age data <100 ka indicating that conditions became unfavourable for zircon crystallization, possibly due to sluggish precipitation at low temperatures or complete solidification within some parts of the magma system. Nevertheless, rim-analysis (outer 4 μm) of the youngest, 32 ka zircons shows that crystallization occurred also at 60-80 ka, but this still precedes the eruption age by about 30-40 ka. All samples contain zircons with widely differing ages which cover the entire 300 ka period of magma evolution at Ciomadul. These data reflect that a melt-bearing crystal mush body could have continuously existed beneath the volcano. Intermittent injections of hotter magma into the cold crystal mush could help to maintain the magma system over 100’s ka. Magnetotelluric data indicates that melt-bearing crystal mush body could still be present beneath Ciomadul. Although 32 ka has elapsed since the last eruption, the prolonged lifetime of the magma system and the presence of partial melt still beneath the volcano suggests that rejuvenation of volcanic activity cannot be dismissed.