Petrology of the Archean Pontiac and Kewagama sediments and implications for the stratigraphy of the southern Abitibi belt

An interpretation is presented of the petrology of two Archaean sedimentary groups in the SE part of the Abitibi greenstone belt: 1) the Pontiac sediments which represent an assemblage of quartz-rich sandstones, and 2) the Kewagama sediments, which are quartz-poor and rich in volcanic fragments. The Pontiac sediments display a N-S decrease in bed thickness, sand/shale ratio, and grain size, suggesting derivation from a northern source. Petrographic and REE analyses indicate that three provenances contributed to the Pontiac: 1) a mature craton, 2) light/heavy REE fractionated igneous rocks, probably of tonalite-trondhjemite composition and 3) ultramafic volcanic rocks. In contrast, the Kewagama sediments were derived from acid to mafic volcanic rocks. The REE data indicate a source fractionated in light/heavy REE relative to the adjacent Blake River group volcanic rocks. A suitable source may be the upper Skead volcanic rocks 60 km to the W. The Pontiac group resembles Phanerozoic sedimentary sequences that accumulated on a passive continental margin, while the Kewagama sediments were derived from an uplifted volcanic terrain. The study indicates that the Pontiac group is stratigraphically below the Blake River group and not a lateral deep-water facies of the Timiskaming group. The possibility that the Kewagama is younger than the Pontiac is considered. Geochemical data for 9 Pontiac and Kewagama pelites, with average abundances of REE in the pelites, and variation in fractionation and enrichment factors for possible source regions for 7 volcanic and 3 plutonic rocks are presented.