Abstract
Basalt and its intrusive equivalents (dolerite and gabbro) are by far the most abundant igneous rocks on the surface of the Earth and other rocky planets and satellites in the solar system. By definition, basalt is a fine-grained basic igneous rock composed of calcic plagioclase and augite, with or without olivine. It forms part of a compositional continuum that extends to lower silica contents and includes the feldspar-free rock types nephelinite and melilitite. Basaltic magma originates by partial melting of the mantle (peridotite with variable amounts of pyroxenite) and is parental to most other igneous rocks through fractional crystallization. Igneous rocks are classified on the basis of silica-saturation into subalkaline (saturated to oversaturated) and alkaline (undersaturated). Subalkaline rocks are subdivided into tholeiitic and calc-alkaline suites. Tholeiitic basalt is formed at mid-ocean ridges, in large igneous provinces, and during the shield-forming stage of some ocean islands. Calc-alkaline magmas form at subduction zones where silica-rich rocks dominate, although basalt is relatively abundant in oceanic island arcs. Alkaline basaltic rocks are mostly restricted in occurrence to continental rift valleys and other intraplate settings such as ocean islands. The range of composition of basaltic rocks reflects the conditions of mantle melting and hence plate tectonic environment. Shallow, large-degree partial melting of volatile-poor mantle at mid-ocean ridges and in large igneous provinces leads to silica-rich, alkali-poor tholeiitic magmas. Increasing pressure accompanied by lower degrees of partial melting beneath rift valleys and most ocean islands produces alkaline basaltic magma with lower silica and higher alkali contents. Mantle melting in the presence of water at subduction zones leads to relatively silica-rich basaltic magma.
Published Version
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