Paleomagnetism and proterozoic dyke swarms continental configurations and evolution of the earth's core

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Mafic dykes, the remains of magma-filled fissures that once fed basaltic lava to the Earth's surface, frequently preserve a remanent or "fossil" magnetism that formed at the time of magma solidification and cooling, and from which the magnetic pole position and latitude of the dyke at the time of cooling can be calculated.  The age of the dyke, and hence its magnetization, can be obtained by precise radioactive (U-Pb) dating of primary U-bearing minerals. Proterozoic dykes of many ages between 600 and 3000 million years old, criss-cross Precambrian shields in abundance. A shield is largely composed of ancient granitic rocks  surrounded by younger mountain-belts or orogens. By virtue of their toughness the granites have helped to buttress the dykes from the effects of these orogens. The dykes, despite their great age, are therefore unusually pristine, especially in the interiors of the shields, and preserve their original mineralogy, and paleomagnetic signature. Therefore they represent the cornerstone upon which Precambrian paleomagnetic apparent polar wander paths for drifting continents are defined. Paleomagnetism  represents the only definitive way to assemble continental fragments in the Precambrian. The question of super-continent cyclicity through geological time can thus be addressed  and links made to presently dispersed continental fragments that may have economic applications, if known mineral provinces on one continent can be extended into adjoining ones. Super-continents are now  at the forefront of Earth Science enquiry with UNESCO International Geoscience Projects (Nos .417 and 509) on the existence of Proterozoic super-continents. Also an upcoming NUNA conference will highlight the role of mafic magmatism, and dykes in particular, in providing a U-Pb "bar-code" for each Precambrian fragment that may help in fitting them together (Bleeker, 2004). The proposed paleomagnetic research on Precambrian dyke swarms will contribute precisely dated reliable paleomagnetic pole positions from Canada, Greenland, India and China and thus will help in this wider global context, and also provide solutions to more local problems relating to the extent that shields have responded to the orogenic events around their margins.