The effect of Fe and Mg on crystallization in granitic systems

Single-step and multistep undercooling experiments using both Fe,Mg-free and Fe,Mg-bearing model granitic compositions were conducted to investigate the influence of mafic components on the crystallization of granitic melts. Crystallization of granite and granodiorite compositions in the system NaAlSi₃O₈−KAlSi₃O₈−CaAl₂Si₂O₈−SiO₂−H₂O produces assemblages containing one or more of the following phases: plagioclase, alkali feldspar, quartz, silicate liquid, and vapor. The observed phase assemblages are generally in good agreement with equilibrium data reported in the literature on the same bulk compositions.

With the addition of Fe and Mg to these bulk compositions six new phases participate in the equilibria (orthopyroxene, clinopyroxene, biotite, hornblende, epidote, and magnetite). However, crystalline assemblages produced in phase equilibrium and crystal growth experiments brought to the same final P–T–X_H2O conditions are in general not equivalent. In crys-tal-growth experiments, nucleation of the feldspars and quartz is greatly inhibited in the presence of Fe and Mg. Indeed, plagioclase is the only tectosilicate to nucleate in the granodiorite composition. Mafic phases nucleate and grow outside of their thermal stability fields as defined by the equilibrium phase diagrams. This contrast in mineral assemblages between the equilibrium and crystal growth experiments is in marked contrast to the results obtained for Fe- and Mg-free compositions. Perhaps the addition of Fe and Mg has caused a breakdown of the Si− framework in the melt, thereby promoting the more rapid nucleation of the ino-and phyllosilicates rather than the framework silicates.

Border zones of granitic plutons, commonly rich in mafic minerals, may result from the more rapid nucleation of mafic phases from the silicate liquid. These zones are thought to develop by early crystallization along the walls of the pluton. Our results suggest the mafic phases should nucleate more quickly than the feldspars and quartz and thus should enrich the early crystallization products in ferromagnesian minerals.