Sub-alkaline magma

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The term sub-alkaline is used in petrology to be able to consider magmas and their rocks ( plutonites such as volcanic rocks ) separately as alkaline magmas and sub-alkaline magmas (or rocks). In contrast to the alkaline magmas, sub-alkaline magmas have a lower total content of the alkali oxides sodium oxide (Na 2 O) and potassium oxide (K 2 O).

Etymology and history

The term is a word creation from the Latin “sub” , under, below ”and Arabic القلي, DMG al-qilī 'ashes of the spring ', in turn derived from Arabic قلى, DMG qalā 'toast , fry in a pan'.

The term was first introduced into the specialist literature in 1895 by Joseph Paxton Iddings .

Classification of the Magma Series

TAS diagram - in yellow the field of sub-alkaline magmas

For the classification of magma types, this represents the first essential subdivision. Since magmas have a continuous composition, this dividing line should not be viewed as a sharp separation, but rather as a border area between the two magma types.

The borderline shown in the adjacent TAS diagram comes from Macdonald (1968).

Further redefinitions of this demarcation were later made by Irvine & Baragar and Akiho Miyashiro . Miyashiro's dividing line has a curved course with overall lower Na 2 O and K 2 O values in the intermediate and acidic magma region (52 - 76 percent by weight SiO 2 ).

TAS diagram with Miyashiro's border line in red

The sub-alkaline magma series can then be divided into further magma series based on the K 2 O content:

  • High-K series
  • Middle-K series
  • Low-K series.

The high-K series and the middle-K series are combined to form the aluminum-rich, calcareous magma series . The low-K series is also known as the tholeiitic magma series .

The distinction between alkaline and sub-alkaline is of particular importance in the case of the basaltic terminal links, which are divided into alkali basalt and sub-alkaline basalt . Tholeiitic basalts are sub-alkali basalts and belong to the low-K series.

Basaltic andesites , andesites and dazites are exclusively sub-alkaline.

Individual evidence

  1. ↑ Stephen R. Langhoff, Harry Partridge, Charles W. Bauschlicher Jr .: Theoretical study of the spectroscopy of the alkali oxides LiO, NaO, and KO. In: Chemical Physics , Vol. 153, Nos. 1-2, May 15, 1991, pp. 1-12.
  2. ↑ JP Iddings: The origin of igneous rocks. In: Bulletin of the Philosophical Society of Washington. Volume 12, 1895, pp. 89-213. Differentiation between alkaline and sub-alkaline.
  3. ^ GA Macdonald: Composition and origin of Hawaiian lavas. In: Mem. Geol. Soc. At the. , Volume 116, 1968, pp. 477-522.
  4. ↑ TN Irvine, WRA Baragar: A guide to the chemical classification of the common rocks. In: Can. J. Earth Sci. , Volume 8, 1971, pp. 523-548.
  5. ↑ A. Miyashiro: Nature of alkalic volcanic rock series. In: Contrib. Mineral. Petrol. , Vol. 66, 1978, pp. 91-104

literature

  • RW Le Maitre: Igneous Rocks. A Classification and Glossary of Terms. In: Cambridge University Press , 2002, ISBN 0-521-61948-3 .
  • M. Wilson: Igneous Petrogenesis. A Global Tectonic Approach. In: Chapman & Hall , 1989, ISBN 0-412-53310-3 .