Tholeiitic basalt

Photomicrograph of thin section of tholeiitic basalt (in plane polarised light)

Photomicrograph of thin section of tholeiitic basalt (in cross polarised light)

Tholeiitic basalt is a type of basalt (an igneous rock) characteized by containing less (Na₂O plus K₂O) at similar SiO₂ than alkali basalt; the field for basalts plotted with these chemical variables is shown in the TAS classification, and the distinction between basalt types is discussed by Le Maitre and others (2002). Chemically, these rocks have been described as subalkaline basalts.

Like all basalt, the rock type is dominated by clinopyroxene plus plagioclase, with minor iron-titanium oxides. [1] Orthopyroxene or pigeonite may also be present in tholeiitic basalt, and olivine, if present, may be rimmed by either of these calcium-poor pyroxenes. Tridymite or quartz may be present in the fine-grained groundmass of tholeiitic basalt, and feldspathoids are absent. Tholeiitic rocks may have a fine, glassy groundmass(AGI, 41), as may other types of basalt.

The International Union of Geological Sciences recommends that "tholeiitic basalt" be used in preference to the term "tholeiite" (Le Maitre and others, 2002).

Basalt magmas are partial melts of peridotite produced by decompression melting in the Earth's mantle, a process described for igneous rocks. Tholeiitic basalts are the most common volcanic rocks on Earth, as they are produced by submarine volcanism at mid-ocean ridges and make up much of the ocean crust. MORB, the acronym for typical mid-ocean-ridge basalt, is a type of tholeiitic basalt particularly low in incompatible elements. In contrast, alkali basalt is not typical at ocean ridges, but is erupted on some oceanic islands and on continents, as also is tholeiitic basalt. (AGI, 41), [2]

See also

• Calc-alkaline
• Petrology
• Igneous rocks
• Igneous differentiation

References

• R. W. Le Maitre (editor), A. Streckeisen, B. Zanettin, M. J. Le Bas, B. Bonin, P. Bateman, G. Bellieni, A. Dudek, S. Efremova, J. Keller, J. Lamere, P. A. Sabine, R. Schmid, H. Sorensen, and A. R. Woolley, Igneous Rocks: A Classification and Glossary of Terms, Recommendations of the International Union of Geological Sciences, Subcommission of the Systematics of Igneous Rocks. Cambridge University Press, 2002. ISBN 0-521-66215-X

• "Polarized Light Microscopy Digital Image Gallery: Tholeiitic Basalt" (Accessed 4/1/06)

• American Geological Institute. Dictionary of Geological Terms. New York: Dolphin Books, 1962.