Abstract
In the high grade gneiss terranes of the Grenville Province in Ontario, metagabbro with silicate coronas around primary olivine and Fe-Ti oxide contains trace amounts of both baddeleyite and zircon. Single baddeleyite crystals with radiating columnar coronas of polycrystalline zircon, in places with outer rims of garnet, are analogous to the coronas around olivine and ilmenite. Single crystals of zircon and baddeleyite without zircon coronas also occur in the same rocks. Both zirconium minerals have been isotopically dated at three widely separated localities. In each case baddeleyite records an igneous crystallization age of ca.1170 Ma, 125 Ma older than the ca.1045 Ma age of the zircons, interpreted to be the age of metamorphism. These data agree well with U-Pb ages obtained from other rocks in the same region and refute the alternative interpretation that silicate coronas in these metagabbros formed during cooling following magmatic crystallization. The age does not support direct correlation with diabase dykes of either the Sudbury or Abitibi swarms northwest of the Grenville Front.
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