Abstract

The Malartic Composite Block of the southern Abitibi belt underwent at least two phases of ductile deformation over a period of 25 Ma during the late Archean. It is divided into seven tectono-stratigraphic domains on the basis of their lithologicai content, structural styles, and geochemical affinities of the volcanic and plutonic rocks. The Malartic Composite Block is bounded to the south by the sedimentary rocks of the Kewagama Group that have been also affected by deformation. The crosscutting relationships between the structural elements, along with precise geochronological data, provide a basis for evaluation of the evolution of the deformation in this part of the southern Abitibi. The D1 tectonic event produced ductile thrust faults and folds in the mafic and ultramafic domains of the Malartic Composite Block. The calc-alkalic rocks of the Val-d'Or Domain (2705 Ma) were erupted on top of these deformed domains, thus establishing the minimum age for D1. The D2 event is divided into three increments that are linked to the same progressive deformation. D2.1 produced northwest-trending folds in the rocks of the Kewagama Group, but did not affect the rocks of the Malartic Composite Block. The age of the youngest detrital zircon in the Kewagama indicates that D2.1 was active after 2687 Ma. D2.2 developed a consistent east–west regional foliation that overprints all the rocks of the area. Radiometric data on related metamorphic minerals indicate an age of 2680 Ma for this event. Locally, S2.2 is modified by Fo2.3 folds of asymmetric Z shape associated with a late dextral transcurrent shearing. A minor D3 event produced small, local conjugate kink folds recording an east–west shortening.

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