Figure 2.14. Haplogranite melting diagram.
A haplogranite is a "simple" granite made from just the minerals alkali feldspar and quartz. Experimental petrologists have studied haplogranites because their chemical composition is easy to make and they are a good approximation to natural granites. There is a minumum liquid temperature and composition in the alkali feldspar quartz system (see the upper portion of the NaAlSiO4-SiO2-KAlSiO4 ternary phase diagram). The temperature and pressure of the haplogranite minimum is shown here for a range of added water contents. As with the melting of other silicate rocks, the temperature at which melting begins on heating or crystallization ends on cooling for granite is strongly dependent on the the amount of available water.
Many granites originate by melting of crustal rocks heated by the effects of crustal thinkening due to continent-continent collisions. Water released by metamorphic reactions involving hydrous minerals provides the "antifreeze" to lower the melting temperatures as shown in this figure. When the granite liquid rises slowly because its density is lower than crustal rocks, the pressure is lowered, the magma degasses gradually, and the conditions may cross the solidus. This can lead to crystallization of the magma at depth, forming a plutonic rock, rather than eruption at the surface.
The diagrams drafted for this figure are based on laboratory melting experiments at pressure reported in or summarized by Makhluf and others (2017).
