Richard J. E. Parrott

Stepwise degassing experiments were performed on eight whole-rock basalt samples from Svartenhuk Peninsula (West Greenland), from Cape Dyer (Baffin Island), and from D.S.D.P. site 112 just off mid-Labrador Sea ridge structure. In addition, experiments were performed on two whole-rock trachytes from Svartenhuk Peninsula (West Greenland and on three whole-rock lamprophyres from Ubekendt Ejland (West Greenland).

Gas released throughout the heating range was often characterized by either negative or anomalously high apparent ages. Computation of isochron ages indicated that these effects were due to the presence of initial gas, that is, argon with 40Ar/36Ar ratios which differ from the atmospheric value. The best data from Svartenhuk Peninsular yields ages of 57.9+2.6 my, 58.1+3.8 my and 59.8+3.8 my (2s errors). Furthermore, the remaining basalts from Svartenhuk Peninsula, Baffin Island and the mid-Labrador Sea ridge yield ages which are indistinguishable from the above (at the 95% confidence limit). However, the best ages of the lamprophyres from Ubekendt Ejland (i.e. 30.6+6.4 my and 32.8+6.8 my, 2s errors) are significantly different. These rocks were originally thought to have only shortly postdated the other volcanics.

These new results, along with existing data for other areas in the Brito-Arctic volcanic province indicate that volcanism in all regions occurred contemporaneously and was of relatively short duration (say from 60-58 my ago). The present data have in addition confirmed the relationship of the Davis Strait volcanics to the spreading history of the Labrador Sea. Spreading about the mid-Labrador Sea ridge started ~ 80 my ago and ceased about 60 my ago. At this time there was extensive volcanism in the Brito-Arctic province and spreading in the Labrador Sea recommenced, at this time about the Ran Ridge. This second and final phase lasted until 47 my ago.

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Pages:193
Supervisors: D.B. Clarke