The Lac Opasatica igneous complex: An autochthonous Abitibi greenstone basement to the Pontiac Subprovince

• Pontiac felsic volcanic rocks overlap in age with the oldest Abitibi volcanism. • Opasatica TTG orthogneiss intruded the volcanic pile between c. 2745–2690 Ma. • The Pontiac Group deposited on volcanic rock of Abitibi age, disfavoring an exotic origin. Basement underlying the Archean metasedimentary belts of the Superior Province is rarely exposed, but key to understanding the tectonic provenance of these basins. The Pontiac Subprovince is one such belt dominated by turbiditic clastic rocks and minor volumes of intrusive and extrusive igneous rock. Tectonic models for the Pontiac Subprovince propose that it represents either a paleo-accretionary wedge or an extensional basin host to authigenic volcanism. Both models consider the supracrustal sequence to overlie older tonalite-trondhjemite-granodiorite (TTG) gneisses, the Opasatica Gneiss, classically interpreted as the regional basement. This work presents new zircon U-Pb data and bulk-rock geochemical data for Opasatica TTG gneisses and newly identified felsic volcanites situated structurally above the gneisses. Four felsic volcanic samples record zircon U-Pb ages of 2755 and 2740 Ma, coeval with the oldest volcanism in the adjacent Abitibi Subprovince. Four TTG gneiss samples yield U-Pb ages as young as 2690 Ma, coeval with TTG occurrences elsewhere in the Pontiac and Abitibi. Melanocratic components of the gneiss yield older ages of c. 2745 Ma, implying a composite intrusion history, and trace element systematics resembling those of the felsic volcanics imply these rocks may have been comagmatic. The revised geochronological framework of the Pontiac suggests it shares its magmatic history with the Abitibi Subprovince. We propose that the Pontiac is an integral component of the Abitibi Subprovince, obscured beneath a thick sedimentary cover. This interpretation of the Pontiac, and its record of syn -sedimentary magmatism, are incompatible with deposition of the Pontiac Group in an accretionary wedge environment.