The palaeomagnetism of major (middle proterozoic) igneous complexes, South Greenland and the Gardar apparent polar wander track

Abstract

The Gardar Province comprises a major rift-related igneous suite emplaced in three pulses at ca. 1300–1280 Ma, ca. 1220 Ma and 1160–1125 Ma. This study reports a palaeomagnetic study of the northeastern sector of the present outcrop and incorporates igneous units belonging to the first and last episodes. Early lavas from the basal part of the rift infill record a reversal of magnetisation and define a palaeomagnetic pole at 207°E, 10°N ( 5 sites, dp/dm= 7 14° ). This primary magnetisation record has been partially overprinted in the aureoles of large alkaline complexes. The latter plutons include the North Qoroq intrusion (1291 ± 61 Ma) yielding a pole position at 203°E, 13°N ( 65 samples from 12 sites, dp/dm= 6 12° ), the Motzfeldt intrusion (1291 ± 31 Ma) yielding a pole position at 212°E, 10°N ( 25 samples, dp/dm= 6 12° ), and the Narssarssuaq Stock yielding a pole position at 207°E, 9°N ( 23 samples, dp/dm= 4 7° ). The younger South Qoroq complex (1160 ± 8 Ma) yields a pole position at 216°E, 42°N ( 67 samples from 9 sites, dp/dm= 11 16° ). Positive contact tests indicate primary cooling-related magnetisations. These bodies are predominantly of single polarity equivalent to W + directions of magnetisations although reversals are recorded. The majority of a suite of NE-SW trending dykes emplaced at ca. 1154–1143 Ma are magnetised in a similar direction [ D/I= sol3 51° , pole position at 231°E, 33°N ( 18 dykes, dp/dm= 5 7° )] to the slightly-older South Qoroq complex. Together with earlier results from the Gardar Province, these data define a motion of the region from shallow to equatorial and then back to intermediate palaeolatitudes between 1290 and 1140 Ma. Pole positions trace out an apparent polar wander path comprising a small loop followed by a long track. This track is the outward path of the “Great Logan” palaeomagnetic loop for which data from the Keweenawan Province of North America (ca. 1100–1050 Ma) define the return path. The loop was executed between ca. 1200 and 1050 Ma following a near-static interval between ca. 1290 and 1200 Ma. This long static interval was also characterised by constant field polarity. Asymmetry of geomagnetic field reversals is recognised in early Gardar events; it is not apparent in younger Gardar data although it recurs in Keweenawan times.