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According to the Encyclopædia Britannica, color indices, 0–50 are felsic, 50–90 are mafic, and 90–100 are ultramafic. [6] An online geology textbook provides an example of the use of another classification scheme, in which color indices 0–15 are felsic, 15–45 are intermediate, 45–85 are mafic, and 85–100 are ultramafic. [9]
Felsic magmas and lavas have lower temperatures of melting and solidification than mafic magmas and lavas. Felsic rocks are usually light in color and have specific gravities less than 3. The most common felsic rock is granite. Common felsic minerals include quartz, muscovite, orthoclase, and the sodium-rich plagioclase feldspars (albite-rich).
[5] [6] [8] Felsic volcanic rocks are rare in the early Earth and only contribute to less 20% of rocks in the Archean greenstone belts worldwide. [6] In contrast, mafic volcanic rocks (such as basalt and komatiite, silicate content <52% [3]) occupy about 50% in the greenstone belts. [6] Thus, felsic volcanic rocks are rare members in the ...
It is often separated from the others as the "alkali" or "soda" rocks, and there is a corresponding series of mafic rocks. Lastly, a small sub-group rich in olivine and without feldspar has been called the "ultramafic" rocks. They have very low percentages of silica but much iron and magnesia.
Assimilation is a popular mechanism to partly explain the felsification of ultramafic and mafic magmas as they rise through the crust: a hot primitive melt intruding into a cooler, felsic crust will melt the crust and mix with the resulting melt. [2] This then alters the composition of the primitive magma.
VMS deposits are associated with sub-equal proportions of mafic volcanic and siliciclastic rocks; felsic rocks can be a minor component; and mafic (and ultramafic) intrusive rocks are common. In metamorphic terranes may be known as or pelitic-mafic associated VMS deposits.
In mafic and ultramafic rocks they form economic nickel, copper and platinum group (PGE) deposits because these elements are chalcophile and are strongly partitioned into the sulfide melt. In rare cases, felsic rocks become sulfur saturated and form sulfide segregations. In this case, the typical result is a disseminated form of sulfide mineral ...
Their silica content can range from ultramafic (nephelinites, basanites and tephrites) to felsic . They are more likely to be generated at greater depths in the mantle than subalkaline magmas. [ 36 ] Olivine nephelinite magmas are both ultramafic and highly alkaline, and are thought to have come from much deeper in the mantle of the Earth than ...