Kiglapait Geochemistry I: Systematics, Sampling, and Density |
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Authors: | MORSE S A |
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Institution: | Department of Geology/Geography, University of Massachusetts Amherst, Massachusetts 01003 |
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Abstract: | The Kiglapait intrusion affords many opportunities for evaluatingplutonic fractionation processes. Estimates of boundary conditionsinclude emplacement of anhydrous high alumina basaltic magmaat 4 kbar and about 1250 °C, initial crystallization nearthe WM buffer, and fractionation to Mg-free ferrosyenite whichcrystallized at 960°C, somewhat above the WM buffer. Thelast ferrosyenites represent fractionation to 0.01 per centof 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 intrusionis 2.93, varying between 2.87 and 3.18 on smoothed models. Crystals accumulated chiefly at the floor. As they did so, themagma depth decreased as the square root of the volume fractionof liquid. The volume fraction solidified was roughly proportionalto time. Cooling was slower than the t relation because of hotsurroundings on one side. The crystallization time was about106 yr, and the average accumulation rate was about 1 cm/yr.Average crystal concentrations of 3300 ppm are impliedfor the nucleation zone. The cooling rate corresponds to crystallizationof 2 x 107 kg/yr per km2 roof area, about 550 times slower thana lava lake. Calculated liquid densities range from 2.67 to 2.88 g/cm3 athigh temperatures. Feldspar almost surely did not sink in themagma, but nevertheless it accumulated mainly on the floor.Cooperative sinking with mafics as proposed by Coats, combinedwith oscillatory nucleation as proposed by Wager, may accountfor this paradox. Oscillatory nucleation leading to feldspar-supersaturatedliquids is supported by laboratory evidence on the feldspar-likestructure of liquids and the concave-úp plagioclase liquidusin systems involving olivine. Both lines of evidence imply highpolymerization of feldspar-rich liquids, particularly in slowprocesses. Such polymerization can help to explain the genesisof primary Eu anomalies and anorthositic magmas in additionto the floor accumulation of feldspar and rhythmic layering. |
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