Abstract
40Ar/ 39Ar analyses have been made on phlogopite-bearing peridotite nodules from Bultfontein and phlogopite nodules from Du Toitspan, Kimberley area, South Africa. Neither definite plateau nor isochron age could be obtained due to the occurrence of an excess 40Ar in phlogopite. However, the extrusion age of a phlogopite nodule from Du Toitspan has been estimated to be about 86 m.y. from the combination of the youngest 40Ar/ 39Ar age in the intermediate temperature fraction with Rb/Sr age data reported for this area. Excess 40Ar correlates with K-derived 39Ar in some phlogopites suggesting that it is trapped in K- or K-similar sites and has been incorporated during phlogopite formation. The occurrence of large amounts of excess 40Ar in phlogopite suggests that it was not formed at a shallow depth.
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