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.
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)