Potassium–Argon Age of the Michikamau Anorthositic Intrusion, Labrador

Abstract

THE Michikamau Intrusion is a mass of anorthosite and related rocks covering an area of about 900 square miles, centred at 54° 30′N. and 64°00′W. in Labrador, Newfoundland. The intrusion is one of a large number of Precambrian anorthosites which occur in the Grenville Province and north into Labrador in the Nam Province as recently defined by Stockwell1. The similarity of rock types and rock associations of anorthositic masses throughout the Grenville and Nain Provinces has created a general feeling that these great basic bodies were emplaced under similar conditions and therefore likely within a limited time-interval. Potassium–argon ages of biotites associated with anorthosite and magnetite–ilmenite deposits in anorthositic rocks described by Rose1,2 from within the Grenville Province have given dates ranging from 875 to 1,025 million years; that is, ages falling within the range usually attributed to the Grenville Orogeny. I suggest that the 1,400 m.y. age reported here for the Michikamau Intrusion provides the best available approximation to the time of emplacement of the large anorthositic masses in the eastern Canadian Shield. Under this hypothesis, younger ages from anorthositic rocks within the Grenville Province are due to the metamorphic overprint of the Grenville Orogeny.