Rb-Sr whole-rock isochron studies on volcanic and sedimentary rocks from the Belcher Fold Belt in Hudson Bay, and sedimentary rocks from the Labrador Trough and the Lake Mistassini area of Quebec, yield ages of 1800, 1870, and 1790 m.y., respectively. These age determinations substantiate previous correlations between these areas based on stratigraphic and structural similarities. Consequently, the concept of a Circum – Ungava Geosyncline appears to be valid on geochronological grounds, with geosynclinal development terminating with the onset of the Hudsonian orogeny at about 1800 m.y.The Circum – Ungava Geosyncline contains banded iron-formations exhibiting shallow-water sedimentary structures which distinguish them from older banded iron-formations. These deposits probably represent the hydrospheric response to the initial large scale introduction of oxygen to the atmosphere by evolving life forms. Because of their probable unique nature these younger banded iron-formations may serve as a valuable marker horizon for global correlations of Proterozoic strata.