KIGLAPAIT GEOCHEMISTRY .1. SYSTEMATICS, SAMPLING, AND DENSITY

The Kiglapait intrusion affords many opportunities for evaluating plutonic fractionation processes. Estimates of boundary conditions include emplacement of anhydrous high alumina basaltic magma at 4 kbar and about 1250 °C, initial crystallization near the WM buffer, and fractionation to Mg-free ferrosyenite which crystallized at 960°C, somewhat above the WM buffer. The last ferrosyenites represent fractionation to 0.01 per cent of the initial volume.Plagioclase varies from An67 to An10, olivine from Fo69 to Fo0, and augite from En73 to En0. The specific gravity of the intrusion is 2.93, varying between 2.87 and 3.18 on smoothed models.Crystals accumulated chiefly at the floor. As they did so, the magma depth decreased as the square root of the volume fraction of liquid. The volume fraction solidified was roughly proportional to time. Cooling was slower than the t2-Jan relation because of hot surroundings on one side. The crystallization time was about 106 yr, and the average accumulation rate was about 1 cm/yr. Average crystal concentrations of 3-300 ppm are implied for the nucleation zone. The cooling rate corresponds to crystallization of 2 × 107 kg/yr per km2 roof area, about 550 times slower than a lava lake.Calculated liquid densities range from 2.67 to 2.88 g/cm3 at high temperatures. Feldspar almost surely did not sink in the magma, but nevertheless it accumulated mainly on the floor. Cooperative sinking with mafics as proposed by Coats, combined with oscillatory nucleation as proposed by Wager, may account for this paradox. Oscillatory nucleation leading to feldspar-supersaturated liquids is supported by laboratory evidence on the feldspar-like structure of liquids and the concave-úp plagioclase liquidus in systems involving olivine. Both lines of evidence imply high polymerization of feldspar-rich liquids, particularly in slow processes. Such polymerization can help to explain the genesis of primary Eu anomalies and anorthositic magmas in addition to the floor accumulation of feldspar and rhythmic layering. © 1979 Oxford University Press.