Tholeiite

Tholeiite in the microscope

Tholeiitic basalt (thin section, LPL): Plagioclase (light) and ilmenite (opaque) form an ophitic structure, in the interstices of which clinopyroxene, glass and conversion products can be found.

Tholeiitic Basalt (Thin Section, XPL)

Classification of basalts according to mineral composition (basalt tetrahedron)

Preshal Beag, a hardened tholeiite basalt, UK

Tholeiite (more rarely also tholeyite ) is named after the town of Tholey in Saarland and describes a dark, dense volcanite from the group of basalts with the main parts pyroxene ( orthopyroxene + clinopyroxene ) and calcium- rich plagioclase (An> 50) in the base mass . As phenocrysts plagioclase occur (An 70-95) and pyroxene. Quartz , olivine (but never next to each other!) And ore minerals can form further mixtures .

History of nomenclature

The original type locality of this basalt variant is in the area around Tholey on the Schaumberg ; however, the rock corresponds to the basalt dyke found here , which gave it its name, u. a. due to its rare earth content, it does not completely match the oceanic and continental tholeiitic basalts. Rather, it is a leucocratic, sub-volcanic monzodiorite. For this reason, among other things, according to the recommendation of the IUGS, the term tholeiite should no longer be used for the widespread group of sub-alkaline basalt rocks previously referred to by this name , but tholeiitic basalt .

classification

In the representation in the basalt tetrahedron, the tholeiites are to the right of the plane of silicon undersaturation (shown in red in the diagram), i.e. This means that tholeiitic rocks always contain so much SiO ₂ that no foid minerals but feldspars are always formed. Within this group, a further subdivision can be made by the silicon saturation (shown in the diagram by the gray level): rocks to the right of this level contain so much SiO ₂ that free quartz (Qz in the diagram) can be present in the rock. To the left of this, the SiO ₂ content does not yet allow the formation of quartz, but of orthopyroxenes (opx in the diagram) such as enstatite.

Occurrence

Tholeiitic basalts are the most common rocks in the earth's crust . They form the bulk of the oceanic crust and the Morbs ( m id o cean r idge b asalt) and also occur in the continental crust on such. B. also on Vogelsberg , in the Westerwald and in the Lausitz .

use

Like most types of basalt, tholeiites are often used as paving stones in road construction or as ballast stones on railroad tracks .

Individual evidence

1. ^ Albert Günter Herrmann and Dieter Jung: The distribution of the lanthanides in the tholeyite of Tholey (Saar) and in palatinites, pseudopegmatites and aplites of the Permian volcanism in the Saar-Nahe-Pfalz area. Contributions to Mineralogy and Petrology, Vol. 29, No. 1, pp. 33-42, 1970. doi : 10.1007 / BF00387997
2. ↑ Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks . Ferdinand Enke Verlag, Stuttgart 1985, ISBN 3-432-94671-6 , pp. 192 .
3. ↑ RW Le Maitre et al .: Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. 252 pp., Cambridge University Press 2005. ISBN 978-0-521-61948-6