CHRONOSTRATIGRAPHIC CONSTRAINTS ON THE GENESIS OF ARCHEAN GREENSTONE BELTS, NORTHWESTERN SUPERIOR PROVINCE, ONTARIO, CANADA

The paper reports new U-Pb zircon ages for supracrustal assemblages of the Red Lake, McInnes Lake, Hornby Lake, North Spirit Lake and Favour able Lake greenstone belts of northwestern Superior Province and evalu ates their significance within the regional stratigraphic and tectonic context. The new data confirm and refine the existing picture of a mu ltistage evolution lasting ca 300 m.y., initiated at ca 3000 Ma by per iods of coeval calc-alkalic, tholeiitic and komatiitic magmatic activi ty, followed by the eventual accretion of a microcontinental nucleus b y ca 2900-2800 Ma, and concluded by an intense period of extensive tho leiitic and calc-alkalic volcanism, and by widespread plutonism and te ctonism between 2750 and 2680 Ma. Volcanic sequences in the McInnes La ke and Hornby Lake greenstone belts of the Berens River Subprovince yi eld ages of 2974+/-2, 2969+/-3, 2928+/-2 and 2901+/-2 Ma, showing a co rrelation with the early stages of greenstone belt development in the region. Similarly, a sedimentary unit in the northern Red Lake greenst one belt was probably deposited at ca 2900 Ma, as indicated by a range of detrital zircon ages between 2989 and 2916 Ma, and the absence of younger components. By contrast, a volcanic unit in the center of the Red Lake greenstone belt, previously thought to be 2830 Ma, yields an age of 2745+7/-4 Ma, which correlates with some of the latest volcanic episodes in the Uchi Subprovince. A detrital zircon population in a s edimentary unit of the western Favourable Lake greenstone belt of the Sachigo Subprovince displays single grain ages ranging from 2843 to 27 27 Ma, the youngest age corresponding to that of an unconformably unde rlying tonalite gneiss. The geological and geochronological relationsh ips suggest that the North Spirit Lake and Favourable Lake greenstone belts represent back-are basins that evolved between ca 2750 and 2730 Ma in a transtensional setting within the older North Caribou terrane; sedimentation was initiated during opening and deepening of the basin s and accompanied most of the concluding compression and imbrication s tages. The period between 2750 and 2680 Ma was also characterized by w idespread granitoid plutonism that enveloped the McInnes Lake and Horn by Lake greenstone belts and older tonalites and formed the bulk of th e Berens River Subprovince. (C) 1998 Elsevier Science B.V. All rights reserved.