Potassium-argon dates determined on hornblendes from amphibolites in the regionally metamorphosed gneisses and schists of the Nimrod Group at the head of the Nimrod Glacier range in age from 500 to 1,050 m.y. The younger dates agree with previously determined K—Ar ages on micas and probably indicate reheating of the metamorphic terrane during emplacement of the Granite Harbour Intrusives in the Ordovician. Precambrian ages clustered between 1,000 and 1,050 m.y. probably date the end of the regional metamorphism that is termed Nimrod Orogeny and is the earliest event known to have affected the Transantarctic Mountains. An event at about the same time has been recognised on the continental margin of east Antarctica between 90° E and 130° E and in western Queen Maud Land. Elsewhere it has been overprinted with loss of radiogenic argon and strontium by later events, especially the 450–520 m.y. Ross Orogeny.