Palaeomagnetism of some late Mesozoic dolerite sills of East Central Spitsbergen, Svalbard Archipelago

Two late Mesozoic dolerite sills, situated near Agardhbukta on the east coast of Vestspitsbergen and dated radiométrically at 100 ± 4 Myr bp, have been sampled in five localities and subjected to detailed mineralogical and rock magnetic studies to determine the direction and origin of their magnetization. Although the sills lie outside the Tertiary orogenic belt, one locality (no. 4) has undergone strong hydrothermal alteration and a small part of another locality (no. 3) has also been affected. A conventional procedure based on examination of Zijderveld diagrams, applied to specimens demagnetized by alternating fields and thermally, yielded similar remanence directions at all five localities, except at the altered part of locality 3. Using a least squares computer méthod of analysis of step demagnetization data, comparable directions were isolated from all localities, including the altered part of locality 3. Except in this last case, all directions were reversed. The adjusted mean direction obtained from this analysis is D=159.0°, I=62.2°, α95=9.0° yielding a palaeomagnetic pole situated at 225.0°, 54.3°N comparable with pole positions obtained from other late Mesozoic igneous rocks on Spitsbergen and distinct from palaeopoles derived from Mesozoic rocks in North America and Eurasia. This suggests that during the late Mesozoic Svalbard existed as a semi-independent microplate.