The table below gives chemical analyses for seven different plutonic rocks that formed from seven magmas of different compositions. These compositions include several examples each of very silica-rich and very silica-poor rocks, and cover the range for typical magmas. The most significant variations are the amount of silica (SiO2), and the relative amounts of alkali oxides (Na2O+K2O) compared with CaO. Although compositions cover a wide spectrum, most magmas contain 40 to 75 wt % SiO2. This is because silicon and oxygen are dominant elements in the crust and mantle where magmas originate. Alkali oxide content varies from 2.70 to 14.21 %. Some rare and unusual magma types produce igneous rocks rich in nonsilicate minerals including carbonates or phosphates, but we will not consider them here.
Compositions of Some Magmas (wt% of oxides)
|Silicic magmas||Mafic magmas|
|(Relatively SiO2-rich magmas)||(Relatively SiO2-poor magmas)|
|higher — Na2O+K2O — lower||higher — Na2O+K2O — lower|
|alkali granite||granite||tonalite||alkali syenite||syenite||diorite||gabbro||peridotite|
Magmas richest in SiO2, such as alkali granite, granite, and tonalite are generally deficient in MgO. We term such magmas silicic (Si-rich), or felsic (contraction of feldspar and silica). Light-colored minerals dominate felsic rocks, so many geologists use the term felsic to refer to any light-colored igneous rock, even if the chemical composition is unknown. At the other end of the spectrum, magmas with <50 wt % SiO2 are usually rich in MgO and contain more FeO and Fe2O3 than silicic magmas. These include diorite, gabbro, and peridotite. We call them mafic (contraction of the words magnesium and ferric) and, for peridotite, ultramafic. They are usually dark in color. The term intermediate describes rocks with compositions between mafic and silicic (but no intermediate rock analyses are in the table above).
Besides distinctions between mafic, intermediate, and silicic rocks, petrologists often classify igneous rocks based on their alkali (K2O + Na2O) and CaO contents. Alkalic rocks are those with high (K2O + Na2O):CaO ratios. The table above reveals that alkalic rocks can be either silicic or mafic compositions. Overall, though, the most common alkalic rocks are silicic. We grouped alkali syenite and syenite with the other mafic magmas because alkali syenite and syenite are silica-poor compared with silicic magmas, but their most significant distinctions are their high alkali oxide contents compared with CaO content. The photo seen here (Figure 6.13) shows a syenite from Brazil. The main minerals are gray potassium feldspar and lighter colored nepheline; both minerals are alkali rich. The black mineral is hornblende.
Rocks melt in many places within Earth, and magma compositions reflect the sources. Mid-ocean ridge and ocean hot-spot magmas are mostly mafic; subduction zone magmas are generally silicic to intermediate. Continental rifts produce a variety of magma types. Rocks of different compositions have different melting temperatures because some elements combine to promote melting. Silicon and oxygen, in particular, promote melting because they form very stable molten polymers (long chains of Si and O) that persist even when melted. Silicic minerals, and SiO2-rich rocks, therefore, melt at lower temperatures than mafic minerals and SiO2-poor rocks.