Palaeomagnetism of the Almunge alkaline complex and Tuna dykes, Sweden: Mid-Proterozoic palaeopoles from the Fennoscandian Shield

Abstract

The Almunge alkaline complex dated 1700 and 1581 Ma (K-Ar biotite ages) preserves only a sporadic northwest positive-directed magnetisation record of uplift following the Svecofennian mobile episode (ca. 1925–1700 Ma). The characteristic remanence is dominated by easterly positive-directed components yielding a palaeopole at 90°E, 44°N (32 samples, dp/dm = 9/12°) not previously represented in the record from the Fennoscandian Shield. It probably postdates the uplift-related cooling of the Svecofennian terrane and is provisionally dated by reference to the APW path and regional age data at ca. 1580–1560 Ma. NNE-trending dykes belonging to the Tuna swarm (1371±50 Ma) in south-central Sweden possess a characteristic remanence D/I = 23/+13°. The palaeopole at 17°E, 21°N (6 sites, dp/dm = 7/13°) defines a link between the Svecofennian and post-Jotnian (1300–1200 Ma) segments of the APW path for this shield.

Key Words:

• Palaeomagnetism
• Almunge complex
• Tuna dykes
• mid-Proterozoic
• Fennoscandian Shield