Tectonic setting of 2.6 Ga carbonatites in the Slave Province, NW Canada

Abstract

An Archaean carbonatite-bearing alkaline complex, the Grays Bay alkaline complex, has been mapped in the State Province of northwestern Canada. It consists of five sequentially emplaced phases: a melanocratic syenite, a carbonatite and silico-carbonatite, a pegmatitic biotite syenite and fine-grained biotite-clinopyroxene-plagioclase gabbro dykes. The entire complex is tightly folded in concert with late Archaean regional upright folding. A minimum age of 2605.8+2.8/-2.5 Ma for the melanocratic syenite is determined by U/Pb dating of titanite, an alteration product of primary ilmenite. A single concordant zircon from the same unit, interpreted as xenocrystic, places an upper age limit of ∼2.67 Ga. Monazite from the carbonatite yields an age of 2594.8 ± 2.0 Ma, indicating that a minimum of 6.5 my passed between the intrusion of syenite and carbonatite magmas. A second carbonatite-bearing complex in the southwestern Slave Province, the Leith Lake carbonatite, was dated by U/Pb titanite at 2591.8+3.3/-2.8 Ga, within error of the Grays Bay alkaline complex. The synchroneity between the two widely separated complexes suggests that intrusion of these bodies may have been a pan-Slave Province event. The carbonatite complexes are temporally coincident with two major tectonic events in the Slave Province: (I) emplacement of voluminous, crustally derived monzogranite to granodiorite plutons between ∼2.61 and 2.59 Ga; (2) deposition of polymictic conglomerate after ∼2.60 Ga within localized fault-bounded basins. Both events are represented in the Grays Bay area. The duration of uplift and conglomerate deposition is poorly defined, but coincident deformation of conglomerates, the alkaline complexes and granitic plutons suggests a terminal compressive event after 2.59 Ga. © Oxford University Press 1998.