The Granite System Qz-Ab-Or-An

Abstract

In addition to the haplogranite components Qz, Ab and Or, the granite system contains also the component CaAl₂Si₂O₈ (An). Two feldspars may be present, a plagioclase (Na_1-xCa_xAl_1+xSi_3-xO₈) and an alkali feldspar (K, NaAlSi₃O₈). Due to the melting loop of plagioclase (distribution of plagioclase components between melt and coexisting plagioclase, Bowen 1913), this quarternary system is noneutectic. At a given pressure and bulk composition, there is always a temperature interval between solidus and complete melting. The beginning of melting temperature at a given pressure is different for different Ab/An ratios of the system.