Trachybasalt

The term Trachybasalts was originally used by Boricky in 1974, to describe nepheline-nosean-bearing basaltic dyke rocks the had a rough appearance. In 1921 the committee on British Petrographic Nomenclature suggested that the term Trachybasalts should be used to describe "intermediate potash-rich rocks containing basic plagioclase together with potash feldspar".

Trachybasalts are fine-grained basic igneous rocks containing plagioclase and alkali feldspar, and pyroxene with no feldspathoids, in which 10-40% of feldspar is alkali feldspar. Trachybasalts are subdivided into hawaiite (andesine, anorthoclase, olivine, augite and biotite), mugearite (oligoclase, orthoclase, olivine, augite and biotite), and benmoreite (anorthoclase, olivine, augite and biotite). Despite their broad mineralogical differences these sub-types can only be properly distinguished on the basis of their compositions. The coarse-grained equivalent of trachybasalt is syenogabbro. The trachybasalts represent part of an important crystal fractionation series between alkali basalt and trachyte. They are most commonly associated with ocean island and continental rift magmatism, but are found in a wide range of settings.

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Olivine (altered by Iddingsite) and clinopyroxene crystals embedded in groundmass made by leucite and clinopyroxene. Trachybasalt from Latera volcano (Italy). PPL image, 2x (Field of view = 7mm)	Olivine (altered by Iddingsite) and clinopyroxene crystals embedded in groundmass made by leucite and clinopyroxene. Trachybasalt from Latera volcano (Italy). XPL image, 2x (Field of view = 7mm)	Olivine (altered by Iddingsite) and clinopyroxene crystals embedded in groundmass made by leucite and clinopyroxene. Trachybasalt from Latera volcano (Italy). PPL image, 2x (Field of view = 7mm)
Olivine (altered by Iddingsite) and clinopyroxene crystals embedded in groundmass made by leucite and clinopyroxene. Trachybasalt from Latera volcano (Italy). XPL image, 2x (Field of view = 7mm)	Olivine (altered by Iddingsite) and clinopyroxene crystals embedded in groundmass made by leucite and clinopyroxene. Trachybasalt from Latera volcano (Italy). PPL image, 2x (Field of view = 7mm)	Olivine (altered by Iddingsite) and clinopyroxene crystals embedded in groundmass made by leucite and clinopyroxene. Trachybasalt from Latera volcano (Italy). PPL image, 2x (Field of view = 7mm)
Olivine (altered by Iddingsite) and clinopyroxene crystals embedded in groundmass made by leucite and clinopyroxene. Trachybasalt from Latera volcano (Italy). XPL image, 2x (Field of view = 7mm)	Olivine (altered by Iddingsite) and clinopyroxene crystals embedded in groundmass made by leucite and clinopyroxene. Trachybasalt from Latera volcano (Italy). PPL image, 2x (Field of view = 7mm)	Olivine (altered by Iddingsite) and clinopyroxene crystals embedded in groundmass made by leucite and clinopyroxene. Trachybasalt from Latera volcano (Italy). XPL image, 2x (Field of view = 7mm)