Magma Evolution in a Proterozoic Rifting Environment

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Abstract
The volcanic stratigraphy of the Cape Smith foldbelt of northern Québec preserves a geochemical record of magmatic evolution during the course of a Proterozoic continental rifting event. The earlier Povungnituk volcanics which comprise the margins of the foldbelt are dominated by compositionally monotonous subaqueous basalts and basaltic andesites interbedded with clastic sediments derived from the Superior craton to the South. These volcanic rocks represent basic magmas whose relatively evolved compositions lay along an olivine–plagioclase cotectic and appear to have been controlled by assimilation-accelerated crystal fractionation. The centre of the foldbelt is composed of the almost entirely volcanic, Chukotat Group. The lower, more marginal, portion of the Chukotat Group consists of repeated sequences of picritic olivine-phyric basalt grading upwards to virtually aphyric, pyroxene-phyric basalt. The olivine-phyric basalts represent primitive magmas which experienced little or no fractionation since leaving their mantle source. The pyroxene-phyric basalts represent magmas derived from these primitive magmas by the fractionation of olivine and then clinopyroxene during the waning stages of episodic eruptive cycles. The upper, more basinward, portion of the Chukotat Group is dominated by a continuous succession of compositionally monotonous, plagioclase-phyric basalts. These represent relatively evolved, basic magmas whose compositions were controlled by crystal fractionation on an olivine–pyroxene–plagioclase–opaque cotectic. The compositional succession of the Cape Smith basalts can be interpreted in terms of the ability of the Proterozoic crust to filter picritic magmas rising from the mantle. During the initial stages of rifting, continental crust forced these magmas to pool in relatively large reservoirs where they were contaminated and fractionated towards steady-state residual liquids. With the eventual failure of the crust, these reservoirs collapsed and succeeding pulses of primitive magma were able to reach the surface as the olivine-phyric basalts of the lower Chukotat Group. In the later stages of rifting, an oceanic crust developed which again forced primitive magmas to pool and fractionate to steady-state liquids in sub-crustal magma reservoirs. A decrease in Al/Si ratio and incompatible element content from the picritic parents inferred for the Povungnituk basalts to those of the Chukotat basalts suggests their mantle source(s) became progressively depleted in time, possibly associated with an adiabatic rise in the level of magma segregation in the mantle as rifting progressed.

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