Melting relations of undersaturated alkaline Rocks from South Greenland.

Abstract

Sixteen volcanic and plutonic undersaturated alkaline rocks of diverse compositions from Greenland, Africa and Canada have been examined petrographically and their melting relations determined at 1 atmosphere pressure in air; 4 samples have been melted in an argon (non-oxidizing) environment and 6 samples at water vapour pressures up to 2070 bars. At 1 atmosphere pressure, liquidus temperatures range from 1120° to 1230° C; in the non-oxidizing atmosphere, liquidus temperatures are similar but iron-bearing minerals remain in the same oxidation state as in the unheated rock. Water vapour pressures lower liquidus temperatures by as much as 300° C with the initial 1000 bars pressure being most effective.

For the majority of samples the sequence of crystallization is nepheline, feldspar and pyroxene at 1 atmosphere pressure. Argon atmosphere and water vapour pressure alters this sequence for some rocks. There is a direct relationship between the melting intervals and chemistry of the rocks. Rocks showing agpaitic trends (USSING, 1912) and high volatile contents have much longer melting intervals. A comparison of the melting data of the rocks studied with data from pertinent synthetic systems indicates high liquidus temperatures for many of these rocks when compared to synthetic systems at 1 atmosphere, but good agreement was found between the hydrothermally melted rocks and the synthetic systems at equivalent pressures. For some of the specimens, tie-lines between coexisting nephelines and feldspars have been determined for the unheated rocks and for rocks heated to 975° C. When plotted on the system KaA-SiO_4-KAlSiO_4-SiO₂ , the slopes of the tie-lines compare favourably with those suggested by TILLEY (1958) for rocks from plutonic and volcanic environments. Relationships between Jiquidus temperatures, chemistry and petrography are discussed.