Experimental Petrology of the Kiglapait Intrusion: Cotectic Trace for the Lower Zone at 5 kbar in Graphite

Abstract

The inferred crystallization history of the troctolitic Lower Zone of the Kiglapait Intrusion in Labrador is tested by melting mineral mixtures from the intrusion, made to yield the observed crystal compositions on the cotectic trace of liquid, plagioclase, and olivine. Melting experiments were made in a piston-cylinder apparatus, using graphite capsules at 5 kbar. Lower Zone assemblages crystallized from 1245 C, 5% normative augite in the liquid, to 1203 C, 24% normative augite in the liquid at saturation with augite crystals. This transit is consistent with modal data and the large volume of the Lower Zone. The 1245 C cotectic composition matches the average Inner Border Zone composition. Quenched troctolitic liquid from the Upper Border Zone, and others from nearby Newark Island, plot on or near our experimental cotectic, supporting a common fractionation history. Olivine-plagioclase intergrowths from cotectic troctolitic melt show mosaic textures reflecting the differing barriers to nucleation of these two phases. The linear partitioning of X Ab in plagioclasemelt yields an intercept constant K D 0524 for these mafic melts. Observed subsolidus exchange of Ca between plagioclase and olivine elucidates the loss of Ca from plutonic olivines. The bulk composition of the intrusion is revised downward in Fo and An.