Reaction relations in the granite system; implications for trachytic and syenitic magmas

In order to evaluate crystallization paths of feldspathic, silica-poor rocks such as trachytes and syenites characterized by high-temperature crystallization histories (as indicated by the paucity of hydrous minerals, the presence of ternary feldspar, and the occasional presence of high-temperature minerals such as pyroxene and olivine), a detailed theoretical investigation of the sodic region of the synthetic granite system, Ab-Or-An-Qz-(H₂O), was undertaken. Calculated phase relations in the feldspar system (i.e., the system Ab-Or-An-(H₂O)) and in the synthetic granite system were used to identify and investigate the regions of complete and incomplete resorption of plagioclase (and less importantly, alkali feldspar) as a function of pressure, activity of H₂O, and silica content. The regions of complete and incomplete resorption of feldspar contract markedly with increasing H₂O and with increasing silica; they shift to higher An contents with increasing pressure. At low H₂O contents, however, the resorptional regime can be entered even when the melt is silica-saturated. Trachytic and syenitic magmas are likely to enter a region of at least incomplete resorption of plagioclase at an early stage of their crystallization history. However, it is possible that these magmas could re-enter a coprecipitation region once the activity of water and of other melt components has increased sufficiently during crystallization. This type of crystallization history could also characterize magmas more siliceous than trachytes as long as the bulk An and Or contents were low and the albitic region could be reached while the H₂O content was still low.