IGNEOUS ROCK EVOLUTION

One of the most important ideas geology has discovered is that igneous rocks evolve. That the earth began with a composition similar to that of the moon, that is, composed mostly of an mafic/ultramafic parent rock, and from that simple beginning all the other rocks have evolved through a sequence of fractionation processes. It is a core concept essential to understanding the Earth's evolution that is explored in the Wilson Cycle and a Plate Tectonic Rock Cycle.

     The core idea is that an original rock, the parent rock, present when the earth formed gave rise not only to all other igneous rocks, but all rocks including sedimentary and metamorphic. 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 fractional melting, for example the mafic parent rock selectively melts producing two fractions. The first fraction is a melt whose composition is closer to the bottom of Bowen's Reaction Series than the original rock. This melt is intermediate in composition. The second fraction is the unmelted crystal residue with a composition more mafic 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 (granite), leaving behind a crystal residue more mafic than the intermediate rock. The fractionation process continues until everything that can be fractionated out of the original composition has been removed. The process is then at an end.

---

 Click image for larger version.      Igneous rock evolution does not just happen anywhere. It takes special plate tectonic conditions. A basic understanding of plate tectonics is necessary here, and if you are unfamiliar with the ideas go to this link for a quick, simple introduction.

     In terms of earth processes, fractionation occurs at two main locations, divergent plate boundaries, and convergent plate boundaries. Divergent plate boundaries (drawing or definition) are mostly under water so we do not easily see them working. But here magma rises up from deep in the earth and oozes out onto the ocean floor to form new oceanic lithosphere (simple drawing; or a more detailed drawing). In the process the parent rock of the earth's interior fractionates to form mafic igneous rocks, e.g. basalt and, at depth, gabbro.
     At convergent plate boundaries (drawing or definition) part of the ocean lithosphere (created at divergent plate boundaries) descends into the earth again, where it heats up and fractionally melts (drawing). This generates intermediate rocks at first, such as diorite, but may eventually create felsic rocks such as granite.

     On an earth time scale, igneous fractionation is responsible for the formation of all the world's volcanic arcs and continents, the implication being, the earth began without continents, and the total size of the continents has grown with geologic time. Contemplate the earth with no continents. It is easy to appreciate the importance of igneous fractionation to just about everything about the earth.

     A final outcome of all this is that different igneous rocks are found in different places on the earth, and all these different distributions are related to plate tectonic processes, and to the history of the earth. At its simplest, continents are made of felsic igneous rocks (such as granite), ocean basins made of mafic igneous rocks (such as basalt and gabbro), and volcanic arcs of intermediate igneous rocks (such as diorite and andesite.)