AGE OF THE GRENVILLE DYKE SWARM, ONTARIO-QUEBEC - IMPLICATIONS FOR THE TIMING OF IAPETAN RIFTING

U-Pb baddeleyite and zircon ages for three diabase dykes from widely s paced localities within the Grenville dyke swarm indicate a single age of emplacement at 590(-1)(+2) Ma. The 700 km long Grenville dyke swar m, located in the southeastern part of the Canadian Shield, was emplac ed syntectonically with the development of the Ottawa graben. This gra ben may represent a plume-generated Iapetan failed arm that developed at the onset of the breakup of Laurentia. Other precisely dated Iapeta n rift-related units, such as the Callander Alkaline Complex and the T ibbit Hill Formation volcanic rocks, indicate a protracted 36 Ma perio d of rifting and magmatism prior to volcanism along this segment of th e Iapetan margin. The age of the Grenville dykes is the youngest in a progression of precisely dated mafic magmatic events from the 723 Ma F ranklin dykes and sills to the 615 Ma Long Range dykes, along the nort hern and northeastern margins of Laurentia, respectively. Thus, the ag e for these dykes represents a key time marker for continental breakup that preceded the formation of the Iapetus ocean.