40 Ar‐ 39 Ar incremental heating studies on the Tudor Gabbro, Grenville Province, Ontario: its bearing on the North American apparent polar wander path in late Proterozoic times

Summary.40 Ar‐ 39 Ar incremental heating studies have been carried out on samples taken from the Tudor Gabbro, Grenville Province, Ontario. In an earlier K‐Ar study, these rocks have yielded an isochron age of ∼700 Ma together with very high initial argon ratios. Age spectrum plots on whole‐rock samples, in general, display a saddle‐shaped character, with two of them exhibiting minima close to 700 Ma. No clear plateaus are observed for these rocks. A hornblende separate records the time ∼1110 Ma at which the stock finally cooled through the ∼590°C isotherm. The Tudor Gabbro was probably intruded into an area undergoing middle‐amphibolite facies meta‐morphism about 1180 Ma ago. The age spectra of two whole‐rock samples together with that of their plagioclase separates, suggest that the stock cooled to ∼200–250°C at about 720 Ma. Slow cooling, averaging about 1°C Ma −1 is indicated for this section of the Grenville Province for the period 1100–700 Ma. If the age of the Tudor Gabbro's palaeomagnetic pole position is taken to be 720 Ma, the Hadrynian Track Hypothesis leads to very high polar wander rates of > 20cm a −1 for the period 820–720 Ma. If this hypothesis is rejected, the average drift rate for this period would be ∼4 cm a −1 , in much better agreement with published values of ∼5 cm a −1 for the period 1400–820 Ma.