Please explain this to me, my mind cannot grasp this geology?

Question

Explain how different minerals can be concentrated during the crystallization of a plutonic magma body

Answer 1

One thing to add to Lady Geologist's answer:

Remember that a crystallizing magma will cool over time. If it also rises to the surface, it will also depressurize over time. Thus, during different stages of the magma's life, you'll be at different locations in Bowen's Reaction series.

Further, as different minerals crystallize, they tend to remove certain elements from the magma, changing it's composition. Crystallizing olivine, for instance, will remove iron from the magma, making it less mafic. Generally iron-rich minerals will form first, such that magmas over time will become cooler, at lower pressure, and become more felsic in composition.

Answer 2

Two reasons mainly:

1) Refer to Bowen's Reaction Series. This dictates which minerals will form first during cooling (or melt first during heating).

2) However, what crystals form depends on the elemental composition of the magma to begin with. A magma low in Mg and Fe will produce less olivines, pyroxenes, and amphiboles. A magma low in sodium and calcium will produce less feldspars.

Differing magmatic compositions can be the result of partial melting of rocks in a certain area or the melting of rocks comprised of few minerals.

Answer 3

The simplest way to think about this is the following.  As you probably know, or know from the other answers, different minerals will crystallize from a magma body at different temperatures.  Cooling of a magma body takes millions of years in most situations, and therefore mafic minerals may crystallize from a magma millions of years before felsic minerals.  

So think about it this way, if you have a mafic mineral such as olivine crystallized in a magma it will be denser than its surroundings, the result is it sinks.  The cause is gravity.  Therefore you commonly find mafic cumulates which have magnesium numbers higher than that of the bulk magma.  Furthermore, as you remove the cumulate you will change the composition of the remaining magma, thus making a differerent composition or different mineral assemblage stable.  Finally, the more evolved the magma body becomes the higher the concentration of incompatible rare earth elements become as they have nowhere else to go.  These are concentrated in very felsic, volatile rich fluids and form pegmatitic type rocks, commonly with rare minerals.