Abstract

Alternating greenstone–granite and metasedimentary gneiss belts are a first-order tectonic feature of the southern Superior Province. The tectonic development of the Quetico metasedimentary belt is reviewed with regard to depositional, structural, and metamorphic–plutonic history. Over its 1200 km length, the belt consists of marginal metasedimentary schists of turbiditic origin and interior metasedimentary migmatite and peraluminous leucogranite. Polyphase deformation has resulted in a steep easterly-striking foliation and regional, gently east-plunging stretching lineation. Metamorphic grade varies in a low-P facies series from greenschist at the belt margins to upper amphibolite and local granulite in the central migmatite – intrusive granite zone. Mineral assemblages in the central zone yield estimates of metamorphic pressure that increase systematically eastward over 800 km from about 250 MPa (2.5 kbar) near the Canada – United States border to 600 MPa (6 kbar) in granulites adjacent to the Kapuskasing structural zone.Geochronology suggests that sediments were deposited at approximately the same time as active volcanism in adjacent volcanic belts, although evidence of volcanic–sedimentary stratigraphic contiguity is weak as a result of later transcurrent movement parallel to major lithological boundaries. Adjacent belts are inferred to have been contiguous since common D2 deformation, 2689–2684 Ma ago. Major plutonism and associated metamorphism occurred in the Quetico Belt approximately 2670–2650 Ma ago, significantly later than major plutonism in the adjacent volcanic belts.The linear disposition of greywacke-rich sediments over 1200 km invites an analogy with modern accretionary prisms. However, the high-temperature, low-pressure metamorphism of the Quetico Belt is inconsistent with such a low-heat-flow environment, and a change in tectonic regime would be required to account for the metamorphism and intracrustal plutonism. Simple cessation of subduction beneath the thick sedimentary prism could have led to restoration of isotherms, with possible attendant crustal melting and isostatic recovery.

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