Abstract

40Ar/39Ar age spectrum data are presented for amphibole and biotite from four dykes representing the diabase swarm in the Mealy Mountains Terrane, Grenville Province, Labrador. An amphibole plateau age of ca. 1215 Ma is recorded by two of the dykes. A third amphibole records a younger age (ca. 1150 Ma) through an apparent veil of excess radiogenic argon. It is suggested that the ca. 1215 Ma age may closely approximate the time of dyke intrusion; the third amphibole, from a dyke relatively close to the terrane boundary, may have been significantly overprinted by heat associated with the Grenville Orogeny. Two of the four biotite age spectra are discordant; apparent ages range from ~980 to 1060 Ma. It is suggested that Grenvillian reheating caused gas loss from all four biotites, the loss having been complete in two, including the one closest to the terrane boundary.Mealy dyke B paleopoles plot on and about the Grenville Track close to the calibrated 980 Ma point. This observation is consistent with observed magnetic "unblocking" temperatures, taken in conjunction with the extent of Grenvillian reheating as estimated from the biotite data. It is suggested that the Mealy A magnetization is much older and cannot be used to define the eastern (descending) arm of the Grenville apparent polar wander track. A plausible structural correction (to compensate for Grenvillian thrusting) could bring the A paleopoles close to the ca. 1200–1250 Ma point on the North American polar wander track; the latter is considered to be the most likely age.

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