Fractionation in the feeder system at a Proterozoic rifted margin

Abstract

In the Proterozoic Cape Smith Foldbelt of Ungava, Quebec, basal basalts of continental affinity are succeeded upward and basinward by cyclic sequences of MgO-rich (≤ 19 wt.% MgO) to MgO-poor submarine basalts of oceanic affinity belonging to the Chukotat Group. The more primitive komatiitic basalts of the Chukotat Group evolved via fractional crystallization of olivine within a crustal feeder system that is represented by large, layered sills and discordant dyke – sill complexes. These intrusions occupy horizons of mechanical weakness such as sedimentary or hyaloclastite-rich horizons within the volcanic stratigraphy. Peridotite and peridotite – gabbro sills predominate at the base of the Chukotat volcanic pile, whereas gabbroic sills are more common higher in the stratigraphy, reflecting the progressive fractionation within the feeder system. Gravitationally controlled settling of crystals or crystal clots is thought to be the dominant process responsible for fractionation in these crustal sills. Fractional crystallization of olivine within the feeder system produced the olivine-phyric to pyroxene-phyric evolutionary trend observed in the coexisting volcanic rocks. Continuing extraction of clinopyroxene, plagioclase, and iron – titanium oxides in subcrustal sills or magma chambers is thought to have generated the MORB-like upper plagioclase-phyric Chukotat basalts. The compositional gap between the pyroxene-phyric and plagioclase-phyric basalts is a by-product of the fractional crystallization mechanism: liquids with compositions typical of the gap are so highly charged with suspended plagioclase crystals that they resist extrusion.