Geochemistry of early proterozoic tillites and argillites of the gowganda formation, Ontario, Canada

The average composition of the matrix of 16 Lower Proterozoic (Gowganda Formation) tillites from Ontario (in wt.%) is: SiO ₂, 65.14 TiO ₂, 0.60; Al ₂O ₃, 14.80; Fe ₂O ₃, 3.68; FeO, 3.69; MgO, 3.56; CaO, 1.50; Na ₂O, 4.39; K ₂O, 1.72; H ₂O, 2.73; P ₂O ₅, 0.16; MnO, 0.75. There are significant differences between the chemistry of the tillite matrix materials in the eastern and western parts of the Gowganda outcrop belt in agreement with results of regional paleocurrent studies which indicate a high percentage of Archaen volcanic and sedimentary rooks in the hinterland of the eastern region and a dominantly "granitoid" source in the west. The "varved" argillites show a remarkable compositional homogeneity. These samples contain less silica and more alumina, magnesia and potassium than the tillite samples. The Na ₂O /K ₂O ratios of the tillite samples are consistently high (> 2.0), comparable to those of the basement rocks and higher than those of the argillites. Comparison with a previous estimate of the composition of part of the Archaean Canadian Shield reveals that the tillites have a relative iron and magnesium enrichment which may be explained by preferential breakdown of mafic minerals.