Abstract

Samples from a belt of gneiss in the Slave Craton have been analysed for lead isotope ratios. Lead was volatilized from whole rocks, and their constituent mineral phases, as well as from granites which intrude their upper amphibolite facies gneisses and which were responsible for their metamorphism. The most radiogenic lead (208Pb/204Pb > 50) was found in the biotite and garnet of the gneiss; the least radiogenic was found in the feldspars. The isotope ratios from the minerals of all samples, whether gneiss, granite, inclusions in the gneiss, or xenoliths in the granite, plot on a single straight line (secondary isochron) with a slope-age of 2540 ± 50 m.y. An assumed growth curve with μ = 8.4 implies that the lead developed in a three-stage system. Possible variations in a three-stage model applicable to the data and to lead systematics in general are discussed. The data support derivation of the rocks in an island arc environment from oceanic crust.

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