Abstract

Dr. R. D. Beckinsale suggested that although a single K-Ar age determination is better than no age at all. this result should be interpreted with extreme caution. In the North Atlantic Tertiary Igneous Province there are well established cases of whole rock material showing both extensive 40Ar loss and considerable levels of excess 40Ar. The range of apparent K-Ar ages at present available for this province is from c. 30 Ma to c. 270 Ma. When several K-Ar ages are available for one intrusion it is often possible to strengthen the interpretation of the data using the isochron plots 40K vs. 40Ar (radiogenic) or 40K/36Ar vs. 40Ar/36Ar. The published results for Helens Reef were mentioned and this is an example where the apparent ages are Cretaceous but if the data are plotted on an isochron diagram an early Tertiary age may be derived from the slope of the line. Finally it has been my experience that alteration of basic material may lead to K-Ar ages which are either too young or too old and it is not safe to assume that ages for altered material are necessarily minima. Although it is obviously better to work with fresh material the fact that a rock is petrographically fresh is no guarantee that 40Ar loss or excess 40Ar phenomena are absent.

Mr. R. K. Harrison congratulated the authors on their sampling and paper and expressed his interest in the reported minimum age of 70 Ma for available samples of the Blackstones complex. This

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