Comparison of K-Ar and whole-rock Rb-Sr dating in New Quebec and Labrador

Abstract

The E.-W.-trending Cape Smith-Wakeham Bay belt in Ungava is bordered on the S. by paragneiss believed to be an extension of the Superior Province (Kenoran age). The belt appears to be synclinal, and consists of 2 unconformably overlying series of folded rocks: the Povungnituk and the Chukotat. On the N. side there is another extensive area of paragneiss of unknown geological relationship. K-Ar age determinations on the rocks of the Chukotat series showed values of about 1450 m.y. In the Povungnituk series, the K-Ar ages varied from 1450-1650 m.y. Basement gneiss on both sides showed K-Ar values from 1600-1800 m.y. On the other hand, whole-rock Rb-Sr age measurements on the gneisses gave values of 2300-2700 m.y. It appears therefore that the belt is a trough bordered on both sides by gneisses of Kenoran age; the sedimentation of the Povungnituk series was completed prior to 1650 m.y., and that of the Chukotat series prior to 1450 m.y. The N.-S.-trending Labrador Trough was found to be quite similar, except that K-Ar ages on the basement gneiss to the W. gave values of 2450 m.y. The rocks in the various Trough strata gave K-Ar ages from 1400-2000 m.y., but no clear evidence that the eastern gneisses were older than the Trough was obtained. Again the whole-rock Rb-Sr ages settled this point with values of 2100 m.y. and 2500 m.y. in the 2 gneiss samples tested. The belt is, therefore, truly a trough, bordered on either side by rocks of Kenoran age.