Abstract

A U–Pb study of basement gneisses in the Thompson Belt yields the following results: a gneiss from the Thompson pit contains zircon, monazite, and titanite dated at 2772 ± 7, 1770 ± 2, and 1720 ± 5 Ma, respectively. An amphibolite boudin in the gneiss contains zircon dated at 2725 ± 9 and 1770–1760 Ma. A banded gneiss from the Manasan River area contains zircon with minimum ages of 3029–3078 and 2905–3112 Ma and monazite 1786 ± 2 Ma old. The paleosome of a lit-par-lit migmatite in the Sasagiu Rapids area contains zircon defining a discordia with intercepts at 3038 ± 6 and 1809 ± 14 Ma, whilst two types of titanite yield apparent ages of 1768 ± 5 and 1737 ± 5 Ma. Zircons from the granitic neosome define a discordia between 2655 ± 10 and 1726 ± 12 Ma.These data indicate that the Thompson Belt is underlain by two types of Archean crust: a "Pikwitonei-type" represented by 2.65–2.8 Ga old gneisses and a pre-Kenoran type older than 3000 Ma, possibly as old as 3.2–3.4 Ga. Basement reactivation lasted for 80–100 Ma and took place during the collision between the Trans-Hudson Orogen and the Superior Province.

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