Potassium-argon dates on micas from the Nimrod - Beardmore - Axel Heiberg region, Ross Dependency, Antarctica

Abstract

Abstract Potassium-argon dates determined on micas from granitic rocks of the Nimrod - Beardmore - Axel Heiberg region range from 445 to 480 million years; these results indicate that granite emplacement occurred during the Ordovician. The measured ages are similar to those reported earlier by other workers for granitic rocks of the McMurdo Sound area, 600 km to the north, and provide evidence for extending considerably the region in which emplacement of granite is known to have occurred in the Ordovician. Potassium-argon dates on micas from gneisses of the Nimrod area are also in the same time range, and show that almost complete loss of radiogenic argon from the micas in the country rocks took place at the time of intrusion of the granites. However, one date of 630 m.y., found for a biotite from a gneiss, indicates that in some places not all the pre-existing argon was expelled, and provides a minimum age for regional metamorphism in Precambrian time prior to the intrusion of granite.