Question

difference between congruent and incongruent melting point ?

Answer 1

In congruent melting of rocks, the minerals simply melt and join the liquid. 
In incongruent melting, a new solid minerals precipitates from the melt. 
For example of a gneiss containing quartz, biotite and sillimanite is melted, then the result is a liquid plus the new solid mineral cordierite (magnesium iron aluminum cyclosilicate)

The principal difference between congruent and incongruent phase transformations is that for congruent nocompositional changes occur with any of the phases that are involved in the transformation.For incongruent therewill be compositional alterations of the phases