The Effects of Pressure, Temperature, and XH2Oon Phase Assemblage in Four Synthetic Rock Compositions

The variation in stable phase assemblage has been determined over a wide range of temperature, pressure, and water content for four compositions within the system CaAl₂Si₂O₈-NaAlSi₃O₈-KAlSi₃O₈-SiO₂. The compositions used were based on the average analyses of (1) hornblende-biotite granite; (2) hornblende-biotite adamellite; (3) hornblende-biotite granodiorite; and (4) hornblende-biotite tonalite as compiled by Nockolds. The variable parameters studied were pressure (2-8 kb), temperature (625°-1,200°C), and H₂O content (0-20 wt %). The results are compared with recent vapor-saturated "rock-melting" experiments on similar natural materials, and the approximate position of the bivariant liquid which coexists with plagioclase, alkali feldspar, quartz, and vapor in the system CaAl₂Si₂O₈-NaAlSi₃O₈-KAlSi₃O₈-SiO₂-H₂O determined. The study suggests that quartz resorption should commonly occur during the intrusion of granitic magmas in the absence of a free vapor phase. The liquidus temperatures determined are strongly affected by changes in anorthite and water content. The order in which crystalline phases appear during crystallization will also be strongly effected by the water content of the system as well as pressure, temperature, and anhydrous bulk composition. The study of such synthetic analogs may be helpful in determining the conditions of origin, intrusion, and crystallization of granitic magmas.