Abstract

New U–Pb zircon, titanite, and monazite ages help constrain the history of magmatism and tectonism within the Pontiac Subprovince of western Quebec. The Pontiac Subprovince resembles other metasedimentary belts of the Superior Province; however, the stratigraphic relationships between the dominantly sedimentary rocks of the Pontiac and the adjacent, volcanic-dominated Abitibi belt to the north and west remain controversial. Volcanic rocks of the Belleterre volcanic zone in the southern part of the Pontiac Subprovince have been interpreted by other workers as klippen of Abitibi strata that were thrust southward onto the Pontiac Subprovince. However, volcanic rocks in the Belleterre zone give crystallization ages of 2689–2682 Ma, which are younger than any extrusive rocks dated thus far from the Abitibi belt. Single detrital zircon grains from Pontiac sedimentary rocks give ages as young as 2683 Ma, indicating that the sediments are similar in age, or younger than, the volcanic units. The volcanic rocks probably represent distal facies of small volcanic arcs deposited within a large turbidite basin.The Lac des Quinze tonalitic gneiss body gives U–Pb zircon and titanite ages of 2695 ± 1 Ma and 2673 ± 4 Ma, respectively. Although the gneiss may represent basement to the supracrustal units, field relationships indicate that it was tectonically juxtaposed against the supracrustal package. Alkaline intrusive rocks in the Pontiac Subprovince yield U–Pb ages that overlap with the youngest ages obtained from the volcanic units. This attests to a very short-lived cycle of sedimentation and arc magmatism, followed by late tectonic and posttectonic alkaline plutonism.

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