Experimentelle gesteinsmetamorphose--III : Anatektische ultrametamorphose kalkhaltiger tone

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Scientific paper

Calcite-bearing illitie clays with different content of Na 2 O were hydrothermally and isochemically treated at 2000 at H 2 O pressure. At 740°C ultrametamorphism begins by formation of an anatectic melt. The melt consists initially of potassium-feldspar and quartz, to which plagioclase, more quartz and mafios join with small rise in temperature. The composition of the melts changes from aplitic to granitic and granodioritic. Comparison is made of these melts with those aplitic and granitic melts formed by ultrametamorphism of calcite-free clays. Especially, it has been found that the temperature of anatectic formation of a granitic melt can vary as much as 100°C, depending on the ratio anorthite/ albite in the metamorphic rock prior to the formation of melt. This is due to fractional melting of alkali-feldspar and plagioclase, which has been demonstrated experimentally. The amount of anatectic melt comprises up to 80 per cent of the metamorphosed clay. The unliquefied crystals are predominantly An-rich plagioclases which are accompanied by mafics, seldom by a little quartz. This leads to the suggestion that anorthosites might possibly also be looked upon as the crystalline remainder not liquefied in the course of ultrametamorphism of calcite-bearing illitic clays or shales. Separation of crystals and melt yields palingenic granite-granodiorite magma and anorthositic rocks.

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