Rb–Sr isotopic study of Archean gneisses north of the Cape Smith fold belt, Ungava, Quebec

Abstract

The Cape Smith volcanic fold belt extends across the Ungava Peninsula of Quebec, and is part of the Aphebian Circum-Ungava geosyncline. The contact with the Superior Province to the south is an unconformity, but the relationship with gneisses to the north has been obscured by Proterozoic metamorphism and deformation. Rb–Sr isochrons obtained from these gneisses yield ages ranging from 2934 to 2569 Ma, in part a result of the scale of sampling. Units interpreted to be granitic intrusions and migmatites yield ages of about 2750 Ma. All initial ratios are low (0.701–0.703) but seem to define an evolutionary path consistent with generation by melting of sialic crust with a Rb/Sr ratio of 0.14.Dominantly quartz-rich metasediments occur throughout the area of reworked Archean gneisses, and are tentatively considered to be temporally equivalent to the Proterozoic Cape Smith fold belt rocks.The simplest evolutionary model for this part of the Circum-Ungava belt would consist of a deep Proterozoic sedimentary basin to the north deposited on and deformed with Archean crust. The Cape Smith rocks would represent in part rift-related volcanism at the margin of the basin. There is no requirement for the opening of a broad ocean or for a suture between colliding Archean cratons.