Alteration Rinds on Glacial Diamictite Clasts in the Gowganda Formation: Possible Indicators of Low Atmospheric Oxygen ca. 2.3 Ga

Abstract

The Coleman Member of the ea. 2.3 Ga Gowganda Formation is beautifully exposed in a glacially polished outcrop near Cobalt, Ontario. This lithologic unit is a glacial diamictite, or tillite, consisting of clasts up to 1.5 m in diameter embedded in a silty to sandy matrix. More than half of the clasts display alteration rinds. On granite clasts these consist of a pale pink outer rim and a dark inner rim, in which orthoclase has been replaced by what is now chlorite. The rinds are inferred to have initially developed as a result of chemical weathering before the clasts were deposited as part of the tillite, and to have been modified by post-depositional metamorphism. The chlorite in the rinds contains Mg and Fe in approximately the same proportion as in the unweathered clasts. The chlorite was probably generated by the low-grade metamorphism of smectites produced by the reaction of feldspars with O2-free groundwater carrying Fe+2 and Mg+2. The absence of O2 in these ground waters is consistent with the deposition of the Gowganda Formation before the rise of O2 in the atmosphere ca. 2.25 Ga.