Palaeomagnetic studies of Proterozoic rocks from the Lake Onega region, southeast Fennoscandian Shield

Abstract

SUMMARY In a recent compilation, Elming et al. (1993) presented a new apparent polar wander path for the Proterozoic of the Fennoscandian Shield. However, only a minor portion of the data used for this and for the earlier compilations has been obtained from the eastern part of the Fennoscandian Shield, thus prompting the present study. In this paper we present new palaeomagnetic data for the Lake Onega region. The sections studied comprise quartzites, as well as extrusive and intrusive rocks of Vepsian (1900-1650 Ma) and Jatulian (2300-2100 Ma) ages, which represent Svecofennian and Jatulian tectonomagmatic events respectively. Thermal and alternating-field demagnetization experiments reveal the presence of several components of the natural remanent magnetization of normal and reversed polarity. Component AN has been identified in a sill of Vepsian gabbro-dolerites (1770 Ma) exposed at Rybreka locality and yields a palaeopole at 37.8dN, 210.6dE (dp = 4.0d, dm = 7.7d). This magnetization is interpreted to be of thermoremanent origin. In the Vepsian Shoksha quartzite (ca. 1800 Ma) cut by the sill and in the underlying basalts, the palaeopoles indicated by the presumably primary components AN and AR are at 45.1 dN, 206.4dE (dp = 3.6d, dm = 6.5d) and 44.2dN, 241.5dE (dp = 6.1d, dm = 9.6d) respectively. Component AN was isolated in the Jatulian basaltic lavas (ca 2200-2300 Ma) at Girvas locality also; however in this case it appears as secondary and yields nearly the same palaeopole: 43.1dN, 211.5dE (dp = 4.9d dm = 8.8d). Component BR was isolated in Vepsian gabbro-dolerites and quartzites, sampled from zones altered by monzonite veins representing the final phase of sill emplacement. This component yields a palaeopole at 13.9dN. 171.3dE (dp = 2.4d, dm = 4.8d) for gabbro-dolerites and at 10.5dN, 201.8dE (dp = 8.4d, dm = 14.9d) for quartzites. The Vepsian and Jatulian rocks also show the presence of presumably younger (< 1500 Ma) magnetization components C, D and E. The presumably primary stable dual-polarity components determined in the Jatulian lavas and interlayers of the red siliceous rocks yield a palaeopole at 0.8dS, 245.4dE (dp = 7.2d, dm = 11.6d) for the upper part of the section (component F) and a palaeopole at 36.0dS, 307.4dE (dp = 9.9d, dm = 14.9d) for the lower part of the section (component G). These palaeomagnetic poles are compared with the apparent polar wander path of Fennoscandia for the interval 2400-1200 Ma (after Elming et al. 1993) and with some recent results from Vepsian and Jatulian rocks.