Abstract

The Cordova gabbro in the Hastings Basin of the southwestern Grenville Province has been studied thermochronometrically to see if the anomalous C magnetic pole was actually acquired prior to 1200 Ma ago. Resetting of the plagioclase argon clock very strongly indicates that, in fact, the C pole was acquired during a mild Siluro-Ordovician event and does not record the existence of a pre-Grenville small ocean between a Grenville plate and Interior Laurentia. Such a hypothesized mild event is consistent with the magnetic, isotopic, and fluid inclusion record. Finally, it is noted that the age and blocking temperature points for the two hornblendes analyzed are consistent with the cooling curve derived for the Thanet gabbro, another Hastings Basin intrusive.

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