Abstract

Five rocks have been dated from the Kakagi Lake area of the Wabigoon Subprovince by means of U–Pb analysis of zircons. Using the techniques of air abrasion and high gradient magnetic separation, zircon fractions from four of the samples have been made concordant.Stratigraphy in the Kakagi Lake area consists of tholeiitic basalts of the Snake Bay and Katimiagamak Lake Formations overlain by mainly calc-alkalic pyroclastic rocks of the Kakagi Lake Group. A felsic tuff collected from the top of the Kakagi Lake Group is dated at graphic. This group is intruded by differentiated ultramafic to mafic sills. The age for a gabbro pegmatite from the lowermost sill near the base of the group is graphic. The Katimiagamak Lake Formation is intruded by tonalite of the Sabaskong batholith, which gives an age of graphic. The tonalite is flanked by the Phinney–Dash Lakes Complex of subvolcanic stocks and dacite to rhyolite volcanic rocks that intrude and overlie the Katimiagamak Lake Formation. A dacite from the complex gives an age of 2727.7 ± 1.1 Ma. A porphyry complex to the north, the Berry Creek Complex, is separated from the other rocks by the Pipestone – Cameron Lakes Fault and gives an age of graphic on a quartz porphyry.The predominantly mafic to intermediate pyroclastic rocks of the Kakagi Lake Group are interpreted to be approximately contemporaneous with the Kakagi sills and to have evolved from the basalt magmatism. Tonalitic rocks of the Sabaskong batholith and the Phinney–Dash Lakes Complex were derived from partial melting of the hydrous lower basalts during the early stage of regional granitoid diapirism. Because of the large age difference between the lowermost sill and the felsic tuff from the top of the Kakagi Lake Group, it is suggested that this formation is not part of the group. It and the Berry Creek Complex were formed from felsic melts separating from rising granitoid gneiss domes during a slightly later stage of regional granitoid diapirism that may have resulted from the reactivation of a predominantly sialic basement by the accumulation of heat over and adjacent to the mantle sources of the basalt.

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