Date of Award
Master of Science (MS)
D. Perkins, III
The English River subprovince of the Superior Province, Canada, is a linear, east-west trending high-grade metamorphic belt which extends from Lake Winnipeg in the west, to the James Bay lowlands in the east. It is composed of two prominent lithologic domains: a northern sedimentary gneiss-migmatite domain, and a southern plutonic domain. The northern domain consists primarily of alternating migmatized layers of garnet-biotite "wacke" and garnet-cordierite-biotite "pelitic" metasediments. The southern domain is composed mainly of intermediate granitic to trondhjemitic plutons. Bordering to the north and south are the lower grade Uchi and Wabigoon greenstone belts. Metamorphism and migmatization occurred during the Kenoran orogeny approximately 2.68 B.Y. ago.
By conducting a detailed geothermometry-geobarometry study, patterns of metamorphism were detected which further develop our understanding of the processes operating on the earth at this very early time in its history. Results from the application of geobarometers have shown that the pressures attained during metamorphism were constant throughout the 2 15000 Km eastern Lac Seul region of the English River subprovince (5 +/- 1 Kbar). There is strong evidence from garnet-orthopyroxene barometry that pressures may have been constant over the rest of the subprovince as well.
Temperatures attained during metamorphism show a trend across the subprovince, depicting a "thermal anticline" whose axis runs approximately east-west parallel to the strike of the subprovince. Temperatures ranged from 6oo0 c at the contact with the Uchi greenstone belt, 675°c for the garnet-cordierite "in" isograd, 700°c for the orthopyroxene "in" isograd, with maximum temperatures of around 750°c at the center of the subprovince.
Langford and Morin (1976), noting the similarity of the Superior Province to the Canadian Cordillera, propose a model of accreting island arcs for the Superior Province. The strong contrasts in lithologies and structure between the northern sedimentary and southern plutonic domains suggest that the southern domain could be an allochthonous terrain accreted onto the northern domain. Since geobarometry has shown that the sediments were buried to a depth of at least 20 kms, it is postulated that the southern domain was thrust onto the sediments. Erosion has cut obliquely through the thrust plane resulting in metasediments exposed in the north, and plutonics to the south.
The temperatures attained in the English River subprovince are several hundred degrees greater than can be explained by conductive heating alone. The contribution of a convective magmatic heat component must be invoked to explain the high temperatures. Block faulting and uplift with a magmatic heat source at the center of the block, combined with thermal diffusivity, explains both the high temperatures, and the "thermal anticline" of the English River subprovince.
Chipera, Steve J., "Metamorphism in the eastern Lac Seul region of the English River subprovince, Ontario" (1985). Theses and Dissertations. 55.