Palaeointensity and palaeodirectional studies of early Riphaean dyke complexes in the Lake Ladoga region (Northwestern Russia)

Abstract

SUMMARY Results of palaeointensity and palaeomagnetic studies for the volcanic rocks of 1450 Ma, from Early Riphaean Baltic shield dyke complex sampled in Lake Ladoga region (Karelia, Northwestern Russia) are reported. Electron microscope observations, thermomagnetic and hysteresis measurements indicate the presence of single domain (SD) to pseudo-single domain (PSD) titanomagnetite (TM) with low Ti content as the main magnetic mineral. Stepwise alternating field (AF) and/or thermal demagnetization revealed a two-component natural remanent magnetization (NRM) for most of the samples. The characteristic remanent magnetization (ChRM) component was isolated between 440 and 590 ◦ C. Note that the ChRM amounts to 95 per cent of the NRM intensity. The geographic position of the ChRM palaeopoles does not contradict the ‘key poles’ of the [1270; 1580] Myr time interval, testifying anticlockwise rotation of whole East Europe Craton between 1450 and 1500 Ma. Palaeointensity determinations were performed by Coe-modified Thellier procedure. 35 samples passed our palaeointensity selection criteria and show large linear segments on Arai-Nagata plots. The site mean virtual dipole moment (VDM) varies from 2.00 to 3.91 (× 10 22 Am 2 ). Based on these and other observations, we suggest that the Palaeo- and MezoProterozoic was dominated by low VDMs.