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

Abstract

We present an interpretation of the petrology of two Archean sedimentary groups in the southeastern portion 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 north–south decrease in bed thickness, sand/shale ratio, and grain size, suggesting derivation from a northern source relative to their present position. Petrographic and rare earth element (REE) analysis indicates 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–mafic volcanics.In contrast to the Pontiac sediments, the Kewagama sediments were derived from acid to mafic volcanic rocks (ratio of 3:1), with minor contributions from plutonic rocks and from graded sedimentary rocks (possibly the Pontiac). The REE data indicate a source fractionated in light/heavy REE relative to the adjacent Blake River Group volcanic rocks, and it is suggested that a suitable source may be the upper Skead volcanics 60 km to the west or a volcanic assemblage similar to the upper Skead that has been eroded from a location above the Blake River Group.The Pontiac Group has characteristics similar to Phanerozoic sedimentary sequences that have accumulated at a passive continental margin, whilst the Kewagama sediments were derived from uplifted volcamc terrane.Our study indicates that the Pontiac Group is stratigraphically below the Blake River Group and that it is not a lateral deep-water facies of the Timiskaming Group. In addition, we consider the possibility that the Kewagama is younger than the Pontiac.