Abstract

Two ages of magmatism have been determined from zircon in felsic flows and plutons in the Churchill Province of Manitoba. A rhyolite flow from the Lynn Lake metavolcanic belt has a U–Pb age of graphic, and a rhyolite flow from the adjacent Rusty Lake metavolcanic belt has an age of graphic. Tonalite and quartz diorite from two composite plutons emplaced into the volcanic rocks at Lynn Lake have ages of graphic and graphic, indistinguishable from the age of the Rusty Lake belt rhyolite. The arcuate domain of metavolcanic rocks that includes the Rusty Lake belt in the southeast, the Lynn Lake belt in the north, and the La Ronge belt (Saskatchewan) in the southwest has previously been considered a single structural sub-province with similar ages throughout. Our results and published U–Pb ages from Saskatchewan indicate that an older magmatism is represented by volcanic rocks in the Lynn Lake belt; a younger magmatism, by volcanic rocks in the Rusty Lake and La Ronge belts and plutons in the Lynn Lake belt. At Lynn Lake the older magmatism (1910 Ma) produced mafic, intermediate, and felsic volcanic rocks and synvolcanic plutons. The volcanic rocks are geochemically similar to Cenozoic island-arc magmatic sequences. These rocks were isoclinally folded and subsequently intruded by the 1876 Ma plutons. The younger, dominantly subaerial, volcanism (1878 Ma) at Rusty Lake was predominantly felsic, and the coeval plutons were granitoid. The distribution of ages and the 8 km thickness of the younger volcanic sequence suggest that the older rock served as basement during the younger magmatism.

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