Palaeomagnetism of the Vendian traps of Volyn, southwestern margin of the East European Platform, P. 2: magnetostratigraphy

Abstract

We present new results of palaeomagnetic studies of the Vendian (Ediacaran) rocks of the Volyn Basalt Province revealed by six boreholes on the NW Ukraine. This is a continuation of previous studies of upper part of the Volyn series described by Bakhmutov et al. [2021]. In the recent works [Shcherbakova et al., 2020; Thallner et al., 2022], the results of palaeointensity determinations of the Volyn series basalts showed the ultra-low dipole moment of Earth’s magnetic field which coincident with other palaeomagnetic data for Ediacaran indicating an extremely weak geomagnetic field. Clear stratification and correlation of the basalt and tuff layers by magnetic parameters allow us to determine the magnetic field reversals throughout the stratigraphic succession of the Volyn series of about 400 m thick. In basalts, tuffs and baked contact rocks, a high-temperature characteristic component of remanent magnetization (ChRM) with all signs of primary magnetization have been isolated. For the directions of inclination of ChRM-components at least six magnetic polarity reversals were revealed. The geochronological ages of rocks is in the range of 580—545 Ma, but the errors in the age estimation are too large for separation of individual formations within the pulses of activity or differentiation of individual eruptions. Therefore the formation time of the entire stratum remains uncertain, and the key issue for interpretation of the magnetic polarity reversal frequency is the duration of accumulation of traps. Two possible interpretations of palaeomagnetic results are considered. In the first, which takes into account different stages of magmatic activity, the time interval of accumulation can be about 10 million years. Then the average frequency of geomagnetic inversions is close to mean for the Phanerozoic. We prefer the other interpretation when the formations of the Large Igneous Provinces have been occurred over a short time interval (e.g. 0.5 Myr). Taking into account the results of palaeointensity estimations for the same samples, which shown the extremely weak geomagnetic field, the hypothesis of the «hyperactivity» of the field with a frequency of at least 12 reversals per one million years on the end of the Ediacaran gain the additional confirmation.