If a rock melts completely, the magma has the same composition
as the rock. However, rocks are made of many different
minerals, all of which melt at different temperatures. So if a
rock is slowly heated, the resulting melt or magma will first
have the composition of the first mineral that melts, and then
the first plus the second minerals that melts, and so on. If the
rock continues to melt, the magma will eventually end up
with the same composition as the starting rock, but this does
not always happen. What often happens is that the rock only
partially melts, so that the minerals with low melting temperatures
contribute to the magma, whereas the minerals with
high melting temperatures do not melt and are left as a
residue (or restite). In this way, the end magma can have a
composition different from the rock it came from.
The phrase “magmatic differentiation by partial melting”
refers to the process of forming magmas with differing
compositions through the incomplete melting of rocks. For
magmas formed in this way, the composition of the magma
depends on both the composition of the parent rock and the
percentage of melt.
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