Abstract

Selected rock units of an Archean volcanic–plutonic assemblage from the Kakagi Lake area, northwestern Ontario, were sampled for geochronological and geochemical studies. The units sampled were felsic metavolcanic rocks from the upper third of the sequence (Middle Felsic Group) and a felsic pluton (the Stephen Lake stock) intruded into these metavolcanics. A Rb/Sr whole rock isochron from the Stephen Lake stock yielded an age of 2534 ± 96 Ma with an initial 87Sr/86Sr ratio of 0.7015. This date is interpreted as a minimum age for the end of volcanism in the Kakagi Lake area, and the low initial ratio indicates that older sialic material was not a significant contributor to the Stephen Lake magma.Major and trace element (Rb, Sr, Ba, Zr, Y, Ce) compositions show that the Kakagi Lake felsic metavolcanics are similar to high-alkali andesite and depleted siliceous volcanics as recognized by Condie. Geochemical indicators such as K/Rb, the potash index, and the Rb/Sr thickness grid index suggest that the Stephen Lake pluton is not simply the intrusive equivalent of the felsic metavolcanics, but that different magma sources are probably required.

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