Experimental Studies of Igneous Rock Series: The Kungnat Syenite Complex of Southwest Greenland

Abstract

The melting relationships of five rocks from the alkaline igneous complex at Kungnat, southwest Greenland, were determined in the presence of 25 wt percent water at pressures to 3 kbar. Run durations varied between 4 and 27 days. The rocks studied are samples from the two layered syenite bodies comprising the complex and from late granite sheets intruding the syenites. The experimental results are consistent with the petrogenetic hypothesis that there were four magma portions tapped from an already differentiated magma chamber, with each subsequently differentiating along independent paths after emplacement. They suggest that the magmas were emplaced at low water pressures at temperatures about 1,000°C, with water pressure reaching about 1.5 kbar in the stages of crystallization when amphibole was precipitated as reaction rims around pyroxenes. The phase relationships in the syenites differ from those previously reported for calc-alkaline plutonic series from tonalite to granite mainly by (1) the wide temperature interval for crystallization of two feldspars (feldspars did not reach equilibrium compositions) and (2) the absence of amphibole through the melting interval because of the low dissociation temperature of the amphibole. All rocks investigated are at least 75 percent melted at temperatures 100°C above the solidus.

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