Paleomagnetism, geochemistry, and uranium-lead geochronology of Proterozoic Mafic intrustions in the high Arctic: Relevance to the Nares Strait problem.

摘要

The paleomagnetism, geochemistry, and geochronology of mafic intrusions in Arctic Canada and Greenland were investigated with the primary purpose of resolving the Nares Strait Problem, a controversy regarding the location of a plate boundary between Greenland and North America and the relative displacement between the two plates. E/W-trending dykes in Arctic Canada and northwest Greenland have an age of 721+/-2 Ma and are associated with the Franklin magmatic event. Their geochemistry is comparable and the mean paleopole for the Canadian dykes (5.8°N, 188°E, N=12, A95=9.9°) is broadly similar to that of the Greenlandic dykes (8.8°N, 178.7°E, N=10, A95=7.2°) indicating that they are of the same swarm, but that of the Canadian dykes is offset from that of the Greenland dykes by a direction and magnitude consistent with a ∼200 km displacement along a fault beneath Nares Strait in accordance with other lines of evidence such as dyke distribution and age boundaries in the bedrock The paleopole from the Canadian dykes is significantly different (p=0.05) from that of Franklin rocks elsewhere, suggesting rapid plate motion over the duration of magmatism. Also associated with Franklin magmatism are the N/S-trending Clarence Head dyke swarm, dated at 715+/-1 Ma, and the Thule sills, dated for the first time, at 712+/-2 Ma. Two Clarence Head dykes have been chemically remagnetized, likely as a result of fluids expelled by the Ellesmerian Orogeny.;Three dykes of the Melville Bugt dyke swarm were sampled, one dated at 1622+/-4 Ma. The measured pole (6.1°N, 267.7°E, dp=2.36°, dm=3.76°) indicates that Laurentia and Baltica drifted separately at this time. Though the Melville Bugt swarm's extension was not found in Canada, a possible candidate, a dyke at Dundas Harbour, was dated at 1337+/-2 Ma, a previously unknown age of dyke emplacement in North America, with a VGP of 12.1°S, 250.8°E, dp=6.7°, dm=13.4°.;This study includes the first analysis of the effects of alpha recoil in baddeleyite, which could have a strong influence on analyses of very small crystals. The effect is apparently not as significant in baddeleyite as in zircon, as the higher density of baddeleyite's crystal lattice may restrict recoil distances.