Origin and Tectonics

~The Evolution of Igneous Rocks and the Significance of their Tectonic Associations~
Theories concerning the evolution of igneous rocks are some of the most important in geology. Without these theories modern ideas of plate tectonics and the history of the earth would be impossible as would our ability to interpret tectonic processes from igneous rocks.
Ideas about the evolution of igneous rocks go back to the early part of the 20th century when N.L. Bowen carried out extensive experimentation with igneous minerals and rocks and developed many of the principles of existing Phase DiagramsBowen's Reaction Series (BRS; Pdf) is a summary of the conclusions of Bowen's work. It not only describes the relationships among the rock forming minerals, it embodies predictions about the way these minerals are related under different conditions.
Recall that the minerals at the top of BRS;
  • Have the highest melting temperatures
  • Are dark in color
  • Are rich in Ca (anorthite) Fe and Mg (e.g. Olivine)
  • And have the highest specific gravity.
Minerals at the bottom of BRS;
  • Are light colored.
  • Are rich in Na (albite), K (Orthoclase) and Si (Quartz).
  • And are of low specific gravity.
As a logical extension of his work, Bowen proposed a hypothesis for the origin and evolution of igneous rocks. The hypothesis is summarized in the diagram above and in the illustration of Bowen's Reaction Series. The core idea is that a silica-rich mafic or ultramafic rock (the parent rock) gives rise to all other igneous rocks. The process occurs when the parent rock is fractionated, that is split into two fractions, each with a composition different from the parent. Fractionation may occur during crystallization of a magma, or melting of a preexisting rock. 
During fractionation the mafic parent rock selectively melts, producing two fractions. The first fraction is a melt whose composition is closer to the bottom of BRS than the original rock. This melt is intermediate in composition. The second fraction is the unmelted crystal residue with a composition more mafic (i.e. ultramafic)than the original rock. That is, its composition is higher in BRS than the original rock.
If time and conditions allow, the fractionation process can continue and the intermediate rock produced during the first fractionation can fractionate into a felsic magma, leaving behind a crystal residue more mafic than the intermediate rock. The page on Igneous Rock Evolution shows these fractionation relationships in terms of BRS.
~Crystallization, and Melting: Alternative Fractionation Processes~
Fractional Crystallization
Fractionation is the splitting of an original magma or rock into two fractions, each of different composition than the original. One fraction becomes more mafic rich on Bowen's Reaction Series, the other more felsic.
Bowen's original idea was that fractionation occurred during the crystallization process. The process begins with a magma (melt) slowly cooling. Crystallization begins with minerals highest in the reaction series. Because these minerals have the highest specific gravity they settle to the bottom of the magma chamber by gravity settling.
Also, because these first formed minerals are high in Ca, Mg, and Fe, they take these elements to the bottom with them in greater quantities than their average composition in the original melt. The remaining melt is thus depleted in Ca, Mg, and Fe, and has a composition lower in the reaction series. 
Thus, the original magma of one composition is divided into two fractions. The first fraction is a cumulate (early formed crystals which " accumulate" at the bottom of the magma chamber) collected at the bottom of the magma chamber composed of high density Ca, Mg, and Fe rich minerals from the top of BRS. The second fraction is the lower density, more Na, K, and Si rich remaining melt with a composition lower down in BRS. 
There are practical problems with fractional crystallization. For one, gravity settling is a slow process and requires more time than is normally available. And two, for gravity settling to occur the cooling magma must remain still, a difficult state for a hot, turbulent melt. Nonetheless, field evidence indicates that gravity settling does occur, at least under some conditions, but it is not as significant a mechanism for igneous rock evolution as fractional melting. 
Fractional Melting
With the development of the plate tectonic theory and the discovery of convergent and divergent boundaries, new ideas of the processes of fractionation became possible - specifically, fractional melting. The mechanisms of fractional melting occurs at both Divergent And Convergent Boundaries
At divergent boundaries convection cells bring hot, plastic, silica-rich ultramafic rock toward the surface. The fractionation and solidification of these magmas form the Ophiolite Suite, the four layers of rock that form the oceanic lithosphere. At depth the plastic, slow moving rock is under great pressure and so has a high melting point. At it moves closer to the surface the pressure diminishes and the hot rock begins to fractionally melt. A Basalt " sweats" off and rises to form the pillow Basalts of the ocean floor. The unmelted residue is olivine (Dunite) and Pyroxene rich (peridotite) ultramafics which remain in the mantle. 
Factional melting at convergent boundaries occurs above the subducting oceanic plate. Cold Basalt of the ocean floor descends into the mantle and gradually heats because of the geothermal gradient and friction of subduction. The descending slab also carries a lot of sea water with it and at about 120 km depth the water and heat lead to fractional melting. The hot melt rises toward the surface, crystallizes to form intermediate (DioriteGranodiorite, etc.) plutons. The unmelted mafic/ultramafic residue is left behind.

Sorting through the mechanisms by which igneous rock fractionation and evolution takes place is a lot of heavy duty chemistry and mathematics (thermodynamics). But we can understand it descriptively as done here, or at a slightly deeper level through the study of Phase Diagrams, or a a more abstract level through the theory of dissipative structures, and the evolution of complex systems.
Contributed by Lynn Fichter  
Tuesday, October 07, 2014
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