The calculated individual effects of pressure and water content on phase equilibria in the granite system.

The revised quasi-crystalline model was used to calculate phase relations in the granite (An-Ab-Or-Q-(H₂O)) system and bounding binary and ternary subsystems for P and water contents of relevance to felsic magma crystallization. The predicted phase relations have been compared with available H₂O-saturated experimental phase equilibrium data on the subsystems Ab-Or-H₂O, An-Ab-H₂O and An-Or-H₂O and indicate good agreement. Comparisons of experimental data against calculated phase relations for the boundary subsystems Ab-Or-Q-H₂O, Ab-An-Q-H₂O, Ab-Or-An-H₂O and An-Or-Q-H₂O also indicate favourable agreement, particularly for the An-poor regions. Use of the model in a predictive capacity to calculate H₂O-undersaturated phase relations for the granite system and relevant subsystems, indicates that the effect of decreasing P or increasing H₂O content is the differential contraction of the Q + L field and concomitant shift of the feldspar/quartz cotectics towards more Q-rich melt compositions. Although H₂O- saturated phase relations also indicate a shift of the feldspar/quartz cotectics toward the Q apex with decreasing P, the magnitude of this shift is less than for H₂O-undersaturated conditions.-J.M.H.