Geochronological constraints on the tectonic evolution of a late Archaean greenstone belt, Wabigoon Subprovince, Northwest Ontario, Canada

Abstract

U-Pb isotopic age analyses have been carried out on zircon and titanite from rocks near the contact between the Kakagi Lake-Savant Lake volcanic belt (KSVB) within the Wabigoon Subprovince, and the English River Subprovince in northwest Ontario, Canada. Two tonalite gneisses intruded by mafic dykes at Tannis Lake and Caribou Lake have minimum zircon ages of 3051 Ma and 3075 Ma, respectively. This contrasts with a 2775 ± 1 Ma age on zircon from a felsic tuff located within a lower, predominantly mafic volcanic sequence in the interior of KSVB. These data, combined with previous studies, indicate that KSVB is presently flanked by older sialic crust. In the Sioux Lookout and Kashaweogama Lake areas, lower mafic volcanic sequences are unconformably overlain by sedimentary units that are in sheared contact with overlying calc-alkaline volcanic units. Titanite and zircon analyses from clasts within a basal conglomerate indicate a maximum age for deposition of 2698 ± 4 Ma. This late age is confirmed by a 2703 ± 2 Ma zircon age for an unconformably underlying porphyry intrusion. One clast gives a zircon age of 2903 ± 16 Ma, suggesting that the sediments were at least partly derived from older material in the English River Subprovince. Data from previous studies show that the structurally overlying calc-alkaline volcanic sequences are at least 30 Ma older than the sediments. The absence of zircon inheritance in all but the youngest KSVB sequences as well as geological and geochemical arguments suggest that the greenstone belt evolved largely in an ensimatic environment, in isolation from older sialic material. The intervening oceanic crust must therefore have been consumed by subduction, which led to arc magmatism and eventual collision between the arc (KSVB) and continental material (English River Subprovince). The sediments unconformably overlying the lowermost mafic volcanic sequences were deposited in a partly subaerial, continental environment. Since chemical and textural evidence suggests the lower mafic volcanic sequences were deposited in an oceanic environment, they must have been obducted onto the edge of a continental block. This was perhaps followed by nappe and thrust emplacement on top of the sediments of calc-alkaline sequences derived from earlier arc magmatism. Continued compression resulted in isoclinal folding of the supracrustal sequences into their present vertical positions.