The Pontiac problem, Quebec–Ontario, in the light of gravity data

Abstract

A belt of Archean quartzose metasedimentary gneisses with minor mafic volcanic rocks (the Pontiac Group) lies south of the Blake River and older Archean mafic volcanic rocks of the Abitibi Greenstone Belt, and is separated from them by the Larder Lake – Cadillac Break. To the west of the Pontiac Group, on strike, is the Archean Larder Lake Group of turbidite conglomerate, argillite, limestone, and iron formation with abundant mafic flows and intrusions. These strata also lie south of the Larder Lake – Cadillac Break and south of the Blake River and older Archean mafic volcanic rocks. The western contact between the Pontiac and Larder Lake groups is covered by a narrow north–south strip of Proterozoic Cobalt sedimentary rocks. On the basis of gravity work that compares the Bouguer gravity anomaly gradient across the Cadillac Break with that across the west margin of the Pontiac Group, it is proposed that the Larder Lake and Pontiac groups are separated by a north–south fault and that the Pontiac Group represents a lithologically distinct uplifted block. The Pontiac block may be an Archean terrane.