Palaeomagnetism of the Late Cretaceous–Tertiary volcanics from Disko

Various magnetic properties of three lava profiles (45 flows), basalt breccia and intrusives from the southernmost part of the west Greenland basalt area have been measured. All samples contain a stable remanence com-ponent of reverse (R) polarity, in accord with most other rocks of the Brito-Arctic Lower Tertiary province. Three magnetic groupings were found: Class r. In the majority of lava samples, the NRM is dominated by the stable R component, which remains stable in direction under AF cleaning and has a characteristic single, high Curie point (T, = 550-570 "C). Class n. One-quarter of the lava samples had a soft, normal NRM whose polarity switched to R under AF treatment at 50-100 Oe and became stable in higher fields. Class n samples often show both a low T, (G300 "C) and a high T,, and their mean Koenigsberger ratio (0.43) is low compared to class r (2-9). Class r ' behaviour, combining r and n features, occurs especially in the breccia, intrusives and remaining lavas. Laboratory viscous build-up tests indicate that the soft component prominent in n and r ' rock is a VRM acquired during the present (Brunhes) geomagnetic epoch. Rock of different classes frequently co-exists even in single flows; this is attributed simultaneously to (1) spatial dif-ferences in oxygen fugacity during initial cooling, and (2) variations in titanomagnetite domain-size distribution near the singledomain-super-paramagnetic threshold. Spatial variations in the rate of lava extrusion and of cooling can explain both effects. One flow has a K-Ar age of 70+4 My. From the magnetic results on 37 flows, it is concluded that the stable remanence is of primary origin and that its between-flow dispersion (19") is due largely to secular variation. The average palaeo-field direction calculated from these flows corresponds to a reverse pole at 62 " N, 169 " W, with dp = 8", dm = 9". This pole agrees with a published Lower Tertiary pole for east Greenland but is displaced from poles of similar age for north-western Europe. This is compatible with an opening of the Norwegian Sea that occurred mainly after these rocks were laid down. 1