Magma evolution in a Proterozoic rifting environment.

The volcanic stratigraphy of the Cape Smith foldbelt of N Quebec preserves a geochemical record of magmatic evolution during Proterozoic continental rifting. The earlier Povungnituk volcanic rocks at the margins of the belt are monotonous subaqueous basalts and andesite interbedded with clastic sediments and represent basic magmas whose relatively evolved compositions lay along an olivine-plagioclase cotectic, apparently controlled by assimilation-accelerated crystal fractionation. The centre of the foldbelt, however, is composed of the almost entirely volcanic Chukotat group whose lower portion contains repeated sequences of picritic olivine-phyric basalt grading upwards to pyroxene basalt. The olivine-phyric basalts represent primitive unfractionated magmas, whereas the pyroxene basalts represent magmas derived from the former by the fractionation of olivine and then clinopyroxene during the waning stages of episodic eruptive cycles. The upper portion of the Chukotat group is mainly monotonous feldspar-phyric basalts, representing basic magmas whose compositions were controlled by crystal fractionation on an olivine-pyroxene-plagioclase-opaque cotectic. This succession 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, but with the eventual failure of the crust these reservoirs collapsed and magmas were able to reach the surface as the basalts of the lower Chukotat group. In the later stages of rifting, the oceanic crust developed, which again forced primitive magmas to pool and fractionate. A decrease in the Al/Si ratio and content of incompatible elements from the Povungnituk to the Chukotat basalts suggests that their mantle source became progressively depleted in time, possibly associated with an adiabatic rise in the level of magma segregation in the mantle as rifting progressed. -R.A.H.