Palaeomagnetism of the Jotnian lavas and sediments and post-Jotnian dolerites of central Scandinavia

Publisher Website
Google Scholar
Abstract
Alternating field and thermal demagnetisation studies of lavas and sediments assigned to the Proterozoic Jotnian of western Sweden (Dalarna) and eastern Norway are reported. Hematite is the predominant remanence carrier in the sediments. These locally record a dipolar axis of magnetisation D = 14° and I = 10°, but the bulk of the sediments, including the western part of the outcrop and all of the upper sequence, have a characteristic remanence ca. D = 160°, I = 35°, interpreted as a magnetic overprint during the Sveconorwegian mobile episode (1100-800 Ma). The lavas have low deuteric oxidation states but were subjected to widespread hematisation during later hydrothermal activity. The majority of the lavas, including the least altered, record a remanence with a mean D = 4°, I = −2° interpreted as a primary field direction, and an antiparallel remanence locally present in hydrothermal hematite; a Sveconorwegian overprint has displaced this at some sites. Stratigraphic sections identify a polarity history N→R in the lower sediments followed by N→R in the lavas. The post-Jotnian dolerites (D = 37° I = −33°) intruded at ca. 1250-1220 Ma did not achieve significant remagnetisation of the Jotnian rocks. Palaeomagnetic poles are at present insufficient to assign an age to the Jotnian rocks within the interval 1600-1250 Ma.