Summary

The Eriksfjord Group comprises ∼3000 m of lavas and sediments rapidly deposited in a rift which developed within an Andean-type batholith in juxtaposition to the southern margin of the Laurentian Shield in South Greenland at ca. 1300 Ma. The lavas have been shown to preserve a detailed record of the geomagnetic field at the time of eruption, incorporating normal, reversed and transitional directions. This study has examined the magnetic properties of the intervening red sediments. They are found to possess a diagenetic remanence imparted by mediating fluids at later times. The impact of diagenesis is stratigraphically controlled: the base of the rift infill has magnetizations partially resident in magnetite which are either unstable to thermal cleaning or record a single polarity ‘B’ magnetization (D/I=284/67°, 31 samples, α95=5.5°, palaeopole at 244.1°E, 47.5°N, dp/dm=7.5/9.1°). This corresponds in polarity, and closely in direction, to remanence observed in mid-Gardar gabbro giant dykes and dyke swarms emplaced along the axis of the rift system at ca. 1160 Ma; the causative diagenetic magnetite appears to have grown from hydrothermal systems motivated by this magmatism in a sealed reservoir setting within the lower part of the rift infill.

The Ilímaussaq alkaline igneous complex was emplaced into the southern extension of the rift at ca. 1130 Ma and possesses a dual polarity magnetization (D/I=327/81°, α95=6.4°, 10 sites). Eriksfjord lavas within the thermal aureole are overprinted to varying degrees by comparable magnetizations with steep inclinations. The mean pole position (283°E, 71°N, dp/dm=12/12°) lies near the apex of an apparent polar wander loop incorporating the Gardar Track (ca. 1300–1140 Ma) and the Keweenawan Track (ca. 1115–1050 Ma). Magnetizations in the Eriksfjord sedimentary succession have not been significantly reset by emplacement of the Ilímaussaq complex, but higher levels of the rift infill are dominated by an ‘A’ magnetization (D/I=305/34°, α95=4.3°, 57 samples, palaeopole at 202.1°E, 32.4°N, dp/dm=2.8/4.9°) resident in haematite. The pole position does not correspond with any part of the Gardar Track, but does correlate with the return Keweenawan Track at ca. 1090 Ma, close to the time of Grenville orogenesis along the bordering southeastern margin of the Laurentian Shield. This remanence is attributed to diagenesis during extensional tectonism linked to the collapse of the Grenville Orogen formerly sited 100–200 km to the south.