Experimental crystallization of anhydrous granite at high pressures and temperatures

Pressures up to 40 kb at temperatures of 950°C and 1100°C have been applied to a glass of granitic composition (adamellite). The experiments at 950°C demonstrate that the adamellite crystallizes to quartz + plagioclase + alkali feldspar + orthopyroxene ± clinopyroxene at pressures less than 13¾ kb, but at higher pressures garnet and clinopyroxene form at the expense of orthopyroxene and plagioclase. However, even at pressures sufficiently high (above 20 kb at 950°C) to eliminate plagioclase and produce an eclogite mineralogy in basaltic rocks, the adamellite contains plagioclase as a major phase. At higher pressures a quartz + sanidine + Jadeite + garnet assemblage is formed. The incoming of garnet at moderate pressures could cause increasing seismic velocity with depth in the continental crust. The data indicate the probable mineralogy of acid gneisses or granulites under conditions of eclogite facies metamorphism.