| Summary: | A suite of silica-undersaturated syenites outcrops along the margins of a monzogranite pluton emplaced in the Knee Lake greenstone belt at Cinder Lake, Manitoba. Alkali-feldspar syenitic pegmatite probably represents a cumulate unit derived from magma that subsequently evolved to fine-grained feldspathoid syenites. This evolution involved an increase in the degree of undersaturation from cancrinite-nepheline syenite to vishnevite syenite. Abundant calcite veinlets, showing a carbonatitic isotopic and trace-element signature, crosscut the pegmatite and are interpreted to have evolved from the syenites. The monzogranite and syenites gave similar radiometric ages (~2.72 Ga), but evidence of fenitization in the former suggests that the syenites are somewhat younger. The emplacement of these rocks was coeval with the collision of the North Caribou and North Superior superterranes during amalgamation of the Superior Province. The observed transition from granitic to alkaline magmatism is interpreted to mark the transition from a collisional to post-collisional tectonic regime.
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