Tectonothermal history in the Mattawa Area, Ontario, Canada, deduced from paleomagnetism and 40Ar/ 39Ar dating of a Grenville Dike

The paleomagnetic ambient temperature has been determined from samples at various distances from a Grenville diabase dike cutting late Proterozoic tonalitic gneiss near Mattawa in the Grenville Province, Canada. Using the 40Ar/39Ar technique, the intrusion age of the dike is estimated to be 570 ± 3 Ma from a 0.5‐mm‐diameter chilled margin chip. The paleo‐ambient temperature of the country rock at the time of dike intrusion, 184° ± 40°C, was calculated from Jaeger's [1964] one‐dimensional heat conduction model. The burial depth of the presently exposed rocks is estimated to be 6.5 ±1.7 km, assuming a surface temperature of 15°C and a geothermal gradient of 26°C/km. This depth at 570 Ma is discordant with the presence of Ordovician shallow‐water (<100 m) limestones about 200 km distant, suggesting that differential uplift may have occurred between the two areas,