Abstract

U–Pb ages for zircon, monazite, and titanite from samples collected from two transects across the Opatica tonalite-gneiss belt in the Superior Province of Quebec indicate that the belt contains rocks that are significantly older than those in the adjacent low-grade northern Abitibi belt. Tonalite and tonalite gneiss, which make up most of the belt, formed over an interval of 100 Ma, from pre-2800 to 2702 Ma. Five samples have ages of ca. 2807 ± 13, 2773 ± 23, graphic, graphic, and 2702 ± 3 Ma. Zircon growth at 2718–2721 Ma in the two oldest samples may record an early period of high-grade metamorphism in the belt. Hornblende diorite, monzodiorite, and tonalite plutons were intruded at 2693–2696 Ma, particularly along the southern boundary with the Abitibi belt. These include the Canet pluton at graphicgraphic Ma, the Lac Ouescapis pluton at 2693 ± 2 Ma, and the Barlow pluton at 2696 ± 3 Ma. Pink granite plutons and dykes are the youngest intrusive rocks in the belt; three samples have yielded zircon ages of 2690 ± 2 and 2686 ± 4 Ma and a monazite age of 2678 ± 2 Ma. The timing of D1 deformation is bracketed by the age of the youngest gneiss sample containing the D1 structures, at 2702 ± 3 Ma, and the 2690 ± 2 Ma age of a granite dyke that cuts D1 structures. The south-vergent D2 event is recorded in the 2693–2696 Ma plutons and must have occurred synchronously with or after this plutonism. D3 dextral strike-slip movement on the Nottoway River shear zone occurred after 2686 ± 4 Ma, and may be associated with post-regional metamorphic titanite growth at 2672 and 2657 Ma. Titanite ages cluster at 2678–2681 Ma along the Matagami transect and at 2665 Ma north of Chibougamau, and record the time of cooling of the belt below the titanite closure temperature.

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