Geochronology and radiogenic isotope geochemistry of plutonic rocks from the central Abitibi subprovince: significance to the internal subdivision and plutono-tectonic evolution of the Abitibi belt

Abstract

Nine new U-Pb ages are reported for plutons of the central granite-gneiss zone of the Abitibi belt in Quebec. The large plutonic complex along Lithoprobe seismic reflection line 28 formed by multiple intrusion over at least 40 million years, synchronous with and postdating formation of adjacent volcanic sequences. Ages for the four principal plutons within the complex are: Mistaouac at 2726 ± 2 Ma, Boivin at 2713 ± 2 Ma, Rousseau at 2703 ± 2 Ma, and Paradis at 2686 ± 2 Ma. The latter also constrains deformation within the Laberge deformation zone to be at least in part younger than 2686 Ma. Inherited zircons in the Mistaouac pluton indicate that the oldest pluton formed in significantly older crust (>2.75 Ga), not presently exposed in the area. The La Reine and Waswanapi plutons have ages of ca. 2695 Ma similar to other tonalitic plutons in the area and elsewhere in the Abitibi belt. A syenite pluton deformed within the Douay fault zone, a late fault associated with the Casa Berardi zone, has an age of 2676+6-5 Ma, similar to alkalic plutons associated with the Destor-Porcupine and Cadillac-Larder Lake deformation zones of the southern Abitibi belt. Two samples from the Lac Case pluton yielded monazite ages of 2676 ± 3 and 2660 ± 3 Ma. Nd, Pb, and Sr isotopic compositions for central Abitibi belt plutons show dominantly juvenile sources with minor contributions of older crust in the Lac Case pluton. Although geochronological data for volcanic rocks has been used to suggest that the northern zone is older and magmatic activity youngs to the south, consideration of the ages for plutonic and volcanic rocks does not support such hypothesis. The available data indicate that magmatism occurred throughout the Abitibi subprovince from 2730 to 2685 Ma, permissive of a linked tectono-magmatic evolution for the northern and southern zones.