Trachybasalt Explained

Trachybasalt is a volcanic rock with a composition between trachyte and basalt. It resembles basalt but has a high content of alkali metal oxides. Minerals in trachybasalt include alkali feldspar, calcic plagioclase, olivine, clinopyroxene and likely very small amounts of leucite or analcime.[1]

Description

An aphanitic (fine-grained) igneous rock is classified as trachybasalt when it has a silica content of about 49% and a total alkali metal oxide content of about 6%. This places trachybasalt in the S1 field of the TAS diagram. Trachybasalt is further divided into sodium-rich hawaiite and potassium-rich potassic trachybasalt, with wt% > + 2 for hawaiite.[2] [3] [4] The intrusive equivalent of trachybasalt is monzonite.[5]

Trachybasalt is not defined on the QAPF diagram, which classifies crystalline igneous rock by its relative content of feldspars and quartz.[2] [3] [4] However, the U.S. Geological Survey defines trachybasalt as a mafic volcanic rock (composed of over 35% mafic minerals) in which the quartz-feldspar-feldspathoid fraction of the rock is less than 20% quartz and less than 10% feldspathoid, and in which plagioclase is between 65% and 90% of the total feldspar content.[6]

Occurrence

Trachybasalt is common in continental volcanism and is also found on some ocean islands.[7] It is abundant at Mount Etna[8] and at Mount Taylor (New Mexico).[9] It has also been found on Gale crater on the planet Mars.[10]

Notes and References

  1. http://www.webref.org/geology/t/trachybasalt.htm Trachybasalt
  2. Le Bas. M. J.. Streckeisen. A. L.. The IUGS systematics of igneous rocks. Journal of the Geological Society. 148. 5. 825–833. 10.1144/gsjgs.148.5.0825. 1991JGSoc.148..825L. 1991. 10.1.1.692.4446. 28548230.
  3. 1999. Rock Classification Scheme - Vol 1 - Igneous. British Geological Survey: Rock Classification Scheme. 1. 1–52.
  4. Book: Philpotts . Anthony R. . Ague . Jay J. . Principles of igneous and metamorphic petrology . 2009 . Cambridge University Press . Cambridge, UK . 9780521880060 . 2nd . 139–143.
  5. Forsythe . Nathan . Spry . Paul . Thompson . Michael . Petrological and Mineralogical Aspects of Epithermal Low-Sulfidation Au- and Porphyry Cu-Style Mineralization, Navilawa Caldera, Fiji . Geosciences . 15 January 2019 . 9 . 1 . 42 . 10.3390/geosciences9010042. 2019Geosc...9...42F . free .
  6. Web site: Geologic units containing Trachybasalt . U.S. Geological Survey . 3 June 2022.
  7. Book: Allaby . Michael . A dictionary of geology and earth sciences . 2013 . Oxford University Press . Oxford . 9780199653065 . Fourth . trachybasalt.
  8. Orlando . Andrea D'Orazio . Armienti . Pietro . Borrini . Daniele . Experimental determination of plagioclase and clinopyroxene crystal growth rates in an anhydrous trachybasalt from Mt Etna (Italy) . European Journal of Mineralogy . 29 August 2008 . 20 . 4 . 653–664 . 10.1127/0935-1221/2008/0020-1841. 2008EJMin..20..653O .
  9. Goff . Fraser . Kelley . Shari A. . Goff . Cathy J. . McCraw . David J. . Osburn . G. Robert . Lawrence . John R. . Drakos . Paul G. . Skotnicki . Steven J. . Geologic map of the Mount Taylor volcano area, New Mexico . New Mexico Bureau of Geology and Mineral Resources Geologic Map . 2019 . 80.
  10. Edwards . Peter H. . Bridges . John C. . Wiens . Roger . Anderson . Ryan . Dyar . Darby . Fisk . Martin . Thompson . Lucy . Gasda . Patrick . Filiberto . Justin . Schwenzer . Susanne P. . Blaney . Diana. Diana Blaney . Hutchinson . Ian . Basalt-trachybasalt samples in Gale Crater, Mars . Meteoritics & Planetary Science . 14 September 2017 . 52 . 11 . 2931–5310 . 10.1111/maps.12953. 2017M&PS...52.2931E . free . 2381/40838 . free .

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