Earth:Trachybasalt

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Short description: Volcanic rock
Potassic trachybasalt from the July–August 2001 eruption of Mount Etna, Italy
Satellite image of Bayuda volcanic field in Sudan where nepheline-rich trachybasalt lavas have been erupted during the Holocene epoch[1]

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.[2]

Description

TAS diagram highlighting the trachybasalt field

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% Na
2O > K
2O + 2 for hawaiite.[3][4][5] The intrusive equivalent of trachybasalt is monzonite.[6]

Trachybasalt is not defined on the QAPF diagram, which classifies crystalline igneous rock by its relative content of feldspars and quartz.[3][4][5] 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.[7]

Occurrence

Trachybasalt is common in continental volcanism and is also found on some ocean islands.[8] It is abundant at Mount Etna[9] and at Mount Taylor (New Mexico).[10] It has also been found on Gale crater on the planet Mars.[11]

References

  1. ↑ "Bayuda Volcanic Field". Global Volcanism Program – Volcanoes of the World database. Smithsonian Institution. 5 June 2020. https://volcano.si.edu/volcano.cfm?vn=225060. Retrieved 6 August 2020. 
  2. ↑ Trachybasalt
  3. ↑ 3.0 3.1 Le Bas, M. J.; Streckeisen, A. L. (1991). "The IUGS systematics of igneous rocks". Journal of the Geological Society 148 (5): 825–833. doi:10.1144/gsjgs.148.5.0825. Bibcode: 1991JGSoc.148..825L. 
  4. ↑ 4.0 4.1 "Rock Classification Scheme - Vol 1 - Igneous". British Geological Survey: Rock Classification Scheme 1: 1–52. 1999. http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf. 
  5. ↑ 5.0 5.1 Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 139–143. ISBN 9780521880060. 
  6. ↑ Forsythe, Nathan; Spry, Paul; Thompson, Michael (15 January 2019). "Petrological and Mineralogical Aspects of Epithermal Low-Sulfidation Au- and Porphyry Cu-Style Mineralization, Navilawa Caldera, Fiji". Geosciences 9 (1): 42. doi:10.3390/geosciences9010042. Bibcode: 2019Geosc...9...42F. 
  7. ↑ "Geologic units containing Trachybasalt". U.S. Geological Survey. https://mrdata.usgs.gov/geology/state/sgmc-lith.php?text=trachybasalt. 
  8. ↑ Allaby, Michael (2013). "trachybasalt". A dictionary of geology and earth sciences (Fourth ed.). Oxford: Oxford University Press. ISBN 9780199653065. 
  9. ↑ Orlando, Andrea D'Orazio; Armienti, Pietro; Borrini, Daniele (29 August 2008). "Experimental determination of plagioclase and clinopyroxene crystal growth rates in an anhydrous trachybasalt from Mt Etna (Italy)". European Journal of Mineralogy 20 (4): 653–664. doi:10.1127/0935-1221/2008/0020-1841. Bibcode: 2008EJMin..20..653O. 
  10. ↑ Goff, Fraser; Kelley, Shari A.; Goff, Cathy J.; McCraw, David J.; Osburn, G. Robert; Lawrence, John R.; Drakos, Paul G.; Skotnicki, Steven J. (2019). "Geologic map of the Mount Taylor volcano area, New Mexico". New Mexico Bureau of Geology and Mineral Resources Geologic Map 80. 
  11. ↑ Edwards, Peter H.; Bridges, John C.; Wiens, Roger; Anderson, Ryan; Dyar, Darby; Fisk, Martin; Thompson, Lucy; Gasda, Patrick et al. (14 September 2017). "Basalt-trachybasalt samples in Gale Crater, Mars". Meteoritics & Planetary Science 52 (11): 2931–5310. doi:10.1111/maps.12953. Bibcode: 2017M&PS...52.2931E. 



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