Magnetic stratigraphy and magnetic-petrologic properties of Precambrian Gardar lavas, South Greenland

Abstract

The Gardar lavas ( 1310 m.y.) within the Eriksfjord Formation of South Greenland were a prelude to major hypabyssal magmatism within the Gardar alkaline igneous province. This paper reports a palaeomagnetic study of the lower two-thirds of the lava stratigraphy comprising three groups of flows separated by sediments. The lower group (palaeomagnetic pole at 239°E, 7°N, 7 sites, dΨ = 7.6°, dχ = 14.1°) includes one reversal of magnetisation and two intermediate directions, and the upper group (pole at 222°E, 16°N, 24 sites, dΨ = 4.8°, dχ = 9.0°) includes four reversals and two intermediate directions. The middle group of 13 lavas yields anomalous directions with respect to the other groups and appears to have been erupted during a polarity excursion of uncertain significance. Opaque petrographic analysis reveals all stages of deuteric oxidation (800–500°C) plus lower-temperature hydrothermal alteration to epidote grade. In addition to high reversal frequency, the succession illustrates asymmetry of the dipole source, and there is a suggestion that lavas with easterly directed magnetisations have higher deuteric oxidation states than those with westerly directed magnetisations. Collective palaeomagnetic results from the Gardar Province are linked to a number of Rb Sr isochrons, and comparison with contemporaneous North American data confirms the 500-fathom continental reconstruction for Proterozoic times. The poles define a movement of the Laurentian craton from low palaeolatitudes (1400–1220 m.y.) to high palaeolatitudes (1168–1070 m.y.) with a quasi-static interval at 1280–1220 m.y. coinciding with the Mackenzie igneous episode. The youngest Gardar data correlate with the oldest Keweenawan palaeomagnetic poles and restrict deposition of this group to the interval ca. 1160–1040 m.y.