New 1.72-1.76 GA paleointensity data obtained on Proterozoic volcanic rocks from the Ukrainian Shield

The Precambrian period occupies ≈ 85% of the Earth’s geological history and accommodates all the main formation stages of the Earth as a planet, including the emergence of its magnetic field. Variations in the time-averaged geomagnetic dipole moment have the potential to learn about the long-term development of the geodynamo and its response to mantle forcing and thermal evolution of the core. But determinations of paleointensity (Banc) of the geomagnetic field during this period are sparse and of limited reliability. Here we report detailed palaeomagnetic and paleointensity studies combined with comprehensive investigations of magnetic properties of Proterozoic volcanic rocks from the Ukrainian Shield.

The Ukrainian Shield comprises the crust of the Palaeoproterozoic protocraton Volgo-Sarmatia, which together with the Fennoscandian crustal segment constitutes the East European Craton (Baltica). The different megablocks of Ukrainian Shield can be treated as a coherent unit since 1.77 Ga.  Our studies has been performed on gabbro-anorthosite complexes from Ingul megablock within the Korsun-Novomigorodsky Pluton (ages 1.75-1.72 Ga) and North-Western megablock within the Korosten Pluton (age ca 1.76 Ga). The high-temperature stable ChRM component was isolated in the interval of blocking temperatures of 500-580°C by more than 300 samples from 7 sites. The presence of dual-polarity high-temperature component, lack of signs of metamorphism and good agreement of the mean palaeomagnetic pole position obtained from the Ingul block with age ca.1.75 Ga (Φ=22.5º, Λ=167.3º, dp/dm=4.0/7.7) with previous studies of anorthosites (Elming et al., 2001) of similar age suggests a primary origin of ChRM.

Comprehensive investigations of magnetic properties of rocks, the electron microscopic images of thin sections and X-ray diffractograms were performed. Rocks demonstrate thermally stable successive Msi(T) curves with clearly pronounced near-magnetite Tc. The carriers of remanent magnetization are fine magnetite isolated needle-like and/or lamellar ferromagnetic particles dispersed in plagioclas. According to the thermomagnetic criterion, high-temperature pTRMs show typical SD-PSD behavior. Palaeointensity determinations were successful on samples from 5 sites carrying well-identified ChRM components using the Thellier-Coe method with pTRM checks and the Wilson protocols. Reliable Banc values give generally low palaeofield (3.7-6.6 µT) with corresponding VDM values in the range (0.93-1.6)×10²² Am². These findings agree with our previous results for Proterozoic rocks of Kola Peninsula (age 1.86 GA) and with the data reported in the World paleointensity databases (http://wwwbrk.adm.yar.ru/palmag/index_e.html and others data), which also provide a noticeably low paleofield intensity with mean VDM = 3.2×10²² Am² for the Paleo-Proterozoic period. Thus, our new data support the Proterozoic dipole low hypothesize by Biggin et al., 2009. The work was supported by the state assignment 17-05-00259 and the RFBR grant 19-05-00433.