THE DIFFICULTIES OF DATING MAFIC DYKES - AN ANTARCTIC EXAMPLE

Archaean gneisses of the Vestfold Hills of East Antarctica are transected by several compositionally discrete suites of tholeiitic dykes. A representative of one of those suites, which has been dated in the present study, shows that not only Rb-Sr whole-rock isochrons, but also U-Pb zircon techniques (if not properly applied) can produce erroneous crystallisation ages. Two zircon populations were recovered from the mafic dyke itself, one of which is 2,483 +/- 9 Ma in age and clearly of xenocrystic origin. The other yields an age of 1,025 +/- 56 Ma, which is not ascribed to the magmatic crystallisation of the dyke, but rather to the time that it underwent metamorphic/metasomatic alteration as a response to high-grade tectonism in the adjacent mobile belt. It is presumed that the zircon in question formed by the breakdown of another mineral or minerals (possibly magmatic baddeleyite), due either to ingress of a siliceous fluid, or more probably by the release of silica from the breakdown of pyroxene to amphibole. A cogenetic 1-2 cm wide felsic vein, of late magmatic/early hydrothermal origin, also contains two zircon populations. Again, most of the grains therein, which are interpreted as of xenocrystic origin, grew at 2,483 +/- 9 Ma. However, a few euhedral zircons with very high U and Th contents grew at 1,248 +/- 4 Ma, which is taken to be the formation age of both the felsic vein and the enclosing mafic dyke.