Oxygen-isotope geochemistry of Archean granitoid gneisses and related rocks in the English River Subprovince, northwestern Ontario

Abstract

Whole-rock δ 18O values for Archean granitoid gneisses from the southern domain of the English River Subprovince, northwestern Ontario, increase from an average of +6.5 for 3000 Ma old tonalites in the eastern Lac Seul area to an average of +7.9 for 2750–2800 Ma old tonalites, trondhjemites and granodiorites that occur in the Kenora and eastern Lac Seul areas. Still younger granitoid plutons and dykes (2660-2560 Ma) that intrude the older gneisses also have average δ 18O = +7.9. These values lie towards the low end of the range known for normal granitoid rocks. Oxygen-isotope data for minerals from the granitoid rocks indicate minor isotopic disequilibrium, but no evidence exists for large-scale depletion in 18O subsequent to granitoid formation. Rocks rich in 18O are abundant only in the northern domain of the English River Subprovince, which is dominated by paragneiss and metasedimentary migmatite (δ 18O +9−12). Within the Kenora area of the southern domain, however, a remnant supracrustal sequence characterized by a thin metachert/iron formation unit (δ 18O = +8−10) has been recognized. The low whole-rock oxygen-isotope values indicate that pre-existing 18O-rich supracrustal rocks were not involved in the genesis of the 3000 Ma old gneisses and have made only limited, if any, contribution to the magmas that formed the younger units. This conclusion is supported by the low initial Sr-isotope ratios of these rocks (0.7009-0.7027). The oxygen- and strontium-isotope data require that most of the granitoid rocks in the southern domain of the English River Subprovince comprise juvenile or near-juvenile additions to the continental crust.