Abstract

Three mica schists from the Pontiac Group have a La/Yb ratio of ~19, a La content of ~32 ppm, and a Th content of ~6 ppm. These values are higher than those of the average Archean upper crust (La/Yb, ~7; La, ~13 ppm; Th, ~3.5 ppm) and the mean compositions of Blake River (La/Yb, ~2.6 (184 analyses); La, ~10 ppm (184 analyses); Th, ~1.3 ppm (143 analyses)) and other subalkalic volcanic groups in the Kirkland Lake–Noranda region. Hence it is unlikely that the latter are exclusive sources of Pontiac detritus. The chemical data reveal that Timiskaming-type alkalic rocks (La/Yb, ~ 45; La, ~95 ppm; Th, ~21 ppm (means of 29 analyses)) are likely contributors to Pontiac metasediments. However, the Timiskaming Group itself is generally considered to be younger than the Pontiac Group. This implies the presence of an undetected or unpreserved terrane characterized by Timiskaming-type alkalic rocks.

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