Two kinds of structural domains can be distinguished in the Nagssugtoqidian mobile belt: "Nagssugtoqidian" shear belts, which wrap around large areas in which "pre-Nagssugtoqidian" fold structures are preserved. It has been generally held that nearly all rocks in the Nagssugtoqidian mobile belt and the pre-Nagssugtoqidian fold structures are of Archaean age. We describe field and lead-isotope evidence suggesting that (1) some of the gneisses in the Nagssugtoqidian mobile belt are indeed Archaean, but that gneisses derived from Proterozoic intrusive rocks also occur; (2) most of the supracrustal rocks in the area are probably of Proterozoic rather than Archaean age; (3) the main phase of metamorphism took place at 1880 ± 50 Ma; and (4) most of the structures recognised in the Nagssugtoqidian mobile belt, including the pre-Nagssugtoqidian fold structures, developed during a Proterozoic orogeny that can be broadly correlated with the Hudsonian orogeny in the Canadian Shield.

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