Abstract
Summary The northern segment of the early Proterozoic Circum-Superior Belt extending from the Ungava region of Quebec (Cape Smith Belt) through the islands of eastern Hudson Bay to northeastern Manitoba (Fox River and Thompson Nickel Belts) is distinguished by an extraordinary magmatic assemblage composed of two principal suites; komatiitic and tholeiitic. The older tholeiitic suite comprising basaltic flows and dolerite sills associated with a stable-shelf sedimentary sequence lies next to the Superior Craton in the Ungava-Hudson Bay region and is adjoined on its outboard side by the komatiitic suite of predominantly pillowed lavas. Their relationships are obscured by faulting but the presence of sills of the komatiitic suite at a number of levels in, and folded with, the tholeiite-sedimentary assemblage is evidence of its younger age. In northeastern Manitoba a discrete tholeiitic suite is absent and komatiitic flows and sills, in part differentiated to tholeiitic compositions, are associated directly with siliceous sediments of apparent continental provenance. The komatiitic suite differs from its Archaean counterparts in that magma compositions rarely exceed about 16–18% MgO, yet spinifex texture (albeit clinopyroxene-olivine) like that of Archaean komatiites is one of its distinctive features. It is typically developed in the upper parts of certain layered (differentiated) flows that are otherwise little distinguished from the majority of layered flows common to the komatiitic sequence. The tholeiitic suite is little differentiated but the komatiitic suite ranges from olivine- to clinopyroxene- and in places to plagioclase-phyric lavas through both long- and superimposed short-range stratigraphic intervals. Differentiation in the komatiitic suite is effectively represented by declining MgO/CaO ratios through the olivine fractionation stage (to about 1) then by increasing contents of oxides of the less refractory minerals, notably A1 2 O 3 , through clinopyroxene and clinopyroxene-plagioclase fractionation. The short-range variation is attributable to fractionation in the layered flows which are interpreted as principal surface distributors of lava and the long-range variations to major high-level sills as represented by those exposed in the Fox River Belt.
Published Version
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