Sequence of construction of Ciomadul (Csomád) dacitic lava domes, East Carpathians, by unspiked K-Ar Cassignol-Gillot technique in comparison with other radiometric methods

Stéphane Dibacto (1, 2) , Pierre Lahitte (3) , Dávid Karátson (4) , Daniel Veres (5) , Ralf Gertisser (6)

1 GEOPS - Géosciences Paris Saclay
2 CEREGE - Centre européen de recherche et d'enseignement des géosciences de l'environnement
3 IDES - Interactions et dynamique des environnements de surface
4 ELTE - Eötvös Loránd University
5 Institute of Speleology
6 School of Physical & Geographical Sciences

Located at the southern tip of the Intra-Carpathian Volcanic Range in Romania, the Ciomadul (Csomád) volcanic complex (CVC) is the youngest eruptive center of the Carpatho-Pannonian Region (CPR). It consists of a dozen of dacitic lava domes that encompass two youngest explosive craters. Whereas, in the last decade, the Ciomadul explosive history for the last 50 ka was well constrained by numerous studies, the chronology of the dome sequence still lacks robust chronological constraints, since it is based on biased K-Ar ages. Here, we provide a detailed K-Ar dating approach to re ne the chronology of the domes, using the unspiked K-Ar Cassignol-Gillot techniques for the rst time in Carpathian Range. New eruption ages were performed on groundmass, the only material the cooling of which is doubtlessly contemporaneous with the eruption. Each age obtained is found to be younger than those proposed on previous whole rock K-Ar datings. Our analysis shows that the K-Ar ages in the previous studies su er from the unsuitability of the material used to obtain eruptive ages, since dating whole rock gives a mixed age due to various mineral fractions with di erent individual ages. A correlation between excess argon from these whole-rock measurements and the abundance of glomerocrysts is established. we also investigated the sequence of crystallisation of phenocrysts present in the lavas, with respect to the modi cation of the eruptive ages. We dated di erent phases containing potassium, i.e. feldspar, amphibole, biotite, plagioclase microlites, and groundmass. Ages obtained on feldspar, amphibole, biotite are systematically older than those obtained on the groundmass illustrating that these phenocrysts were formed before (or well before) the eruption, and brought up by the magma during the eruption. On the other hand, plagioclase microlites display a complex range of ages, from the age of groundmass to the age of plagioclase phenocrysts.The newly applied methodology, mainly consisting in a double full preparation rst at larger grain size and next at < 100 micrometres, allows us to propose an appropriate procedure to separate the adequate material in order to obtain the K-Ar age of the eruption, i.e. the groundmass, in which there is no risk of presence of older, inherited phenocrysts. Main issue is to separate by using heavy liquid in association with magnetic separator the groundmass from the important presence of inherited phenocrysts such as: biotite, amphibole, feldspars (plagioclase).

Domaines: Sciences de la Terre

Stéphane Dibacto, Pierre Lahitte, Dávid Karátson, Daniel Veres, Ralf Gertisser. Sequence of construction of Ciomadul (Csomád) dacitic lava domes, East Carpathians, by unspiked K-Ar Cassignol-Gillot technique in comparison with other radiometric methods. EGU General Assembly 2017, 2017, Vienna, Austria. ⟨hal-03662433⟩