Lower Temperature Terminations of the Three-phase Region Plagioclase-Alkali Feldspar-Liquid

Geological and experimental evidence indicate that the three-phasefield, plagioclase-alkalifeldspar-liquid, may terminate in severaldifferent ways. The possible terminations have been developedfrom Schreinemakers' rules governing the disappearance of three-phasefields. In igncous rocks, these different terminations may arisefrom variations in the relative amounts of additional componentsin magmas, or from changes of total pressure, or from structuralchanges that effect the extent of solid solution in feldspar. The three-phase region originates from the intersection of thesolidus and the feldspar solvus. The available evidence regardingthis intersection is reviewed, as is the evidence for the existenceand form of the boundary curve on the feldspar liquidus. Thedata are used to project a series of isobaric polythermal andisobaric isothermal diagrams for each possible termination.Subsequent discussions relate the theoretical arguments to thenatural evidence, suggest the more probable geological environmentsof some of the terminations, and indicate characteristic featuresof each termination. It may be possible to identify the type of termination involvedin the crystallization of some rocks. The necessary data arethe amounts and compositions of both kinds of coexisting feldsparsand of the feldspar components of the coexisting liquid at oneor more stages of the crystallization process. Volcanic rocksare most suitable for such studies. The terminations explain many of the compositional relationshipspossible between feldspar crystals and silicate melt under geologicaland experimental conditions. These compositional relationshipsare fundamental to understanding the crystallization of igneousrocks and the formation of melts by partial fusion. Other geologicalapplications include a simple explanation for some resorbedfeldspars, the separation, correlation, and comparison of porphyriticrock units, interpretation of compositional changes of successivezones of zoned feldspars, and mantling of one feldspar by another.With all types of termination, extensive fractionation yieldsa liquid rich in Or and Ab that may crystallize to alkali feldsparor feldspars.