Trachybasalt
{{Short description|Volcanic rock}}
File:Vesicular lava (July-August 2001 eruption of Mt. Etna, eastern Sicily) 4.jpg, Italy]]
File:Bayuda Vulkanfeld.jpg in Sudan where nepheline-rich trachybasalt lavas have been erupted during the Holocene epoch{{cite web | url=https://volcano.si.edu/volcano.cfm?vn=225060 | title=Bayuda Volcanic Field | publisher=Smithsonian Institution | work=Global Volcanism Program – Volcanoes of the World database | date=5 June 2020 | accessdate=6 August 2020}}]]
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.[http://www.webref.org/geology/t/trachybasalt.htm Trachybasalt]
Description
File:TAS-Diagramm-trachybasalt.png 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% {{chem2|Na2O}} > {{chem2|K2O}} + 2 for hawaiite.{{Cite journal|last1=Le Bas|first1=M. J.|last2=Streckeisen|first2=A. L.|title=The IUGS systematics of igneous rocks|journal=Journal of the Geological Society|volume=148|issue=5|pages=825–833|doi=10.1144/gsjgs.148.5.0825|bibcode=1991JGSoc.148..825L|year=1991|citeseerx=10.1.1.692.4446|s2cid=28548230}}{{Cite journal|date=1999|title=Rock Classification Scheme - Vol 1 - Igneous|url=http://nora.nerc.ac.uk/id/eprint/3223/1/RR99006.pdf|journal=British Geological Survey: Rock Classification Scheme|volume=1|pages=1–52}}{{cite book |last1=Philpotts |first1=Anthony R. |last2=Ague |first2=Jay J. |title=Principles of igneous and metamorphic petrology |date=2009 |publisher=Cambridge University Press |location=Cambridge, UK |isbn=9780521880060 |edition=2nd |pages=139–143}} The intrusive equivalent of trachybasalt is monzonite.{{cite journal |last1=Forsythe |first1=Nathan |last2=Spry |first2=Paul |last3=Thompson |first3=Michael |title=Petrological and Mineralogical Aspects of Epithermal Low-Sulfidation Au- and Porphyry Cu-Style Mineralization, Navilawa Caldera, Fiji |journal=Geosciences |date=15 January 2019 |volume=9 |issue=1 |pages=42 |doi=10.3390/geosciences9010042|bibcode=2019Geosc...9...42F |doi-access=free }}
Trachybasalt is not defined on the QAPF diagram, which classifies crystalline igneous rock by its relative content of feldspars and quartz. 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.{{cite web |title=Geologic units containing Trachybasalt |url=https://mrdata.usgs.gov/geology/state/sgmc-lith.php?text=trachybasalt |publisher=U.S. Geological Survey |access-date=3 June 2022}}
Occurrence
Trachybasalt is common in continental volcanism and is also found on some ocean islands.{{cite book |last1=Allaby |first1=Michael |title=A dictionary of geology and earth sciences |date=2013 |publisher=Oxford University Press |location=Oxford |isbn=9780199653065 |edition=Fourth |chapter=trachybasalt}} It is abundant at Mount Etna{{cite journal |last1=Orlando |first1=Andrea D'Orazio |last2=Armienti |first2=Pietro |last3=Borrini |first3=Daniele |title=Experimental determination of plagioclase and clinopyroxene crystal growth rates in an anhydrous trachybasalt from Mt Etna (Italy) |journal=European Journal of Mineralogy |date=29 August 2008 |volume=20 |issue=4 |pages=653–664 |doi=10.1127/0935-1221/2008/0020-1841|bibcode=2008EJMin..20..653O }} and at Mount Taylor (New Mexico).{{cite journal |last1=Goff |first1=Fraser |last2=Kelley |first2=Shari A. |last3=Goff |first3=Cathy J. |last4=McCraw |first4=David J. |last5=Osburn |first5=G. Robert |last6=Lawrence |first6=John R. |last7=Drakos |first7=Paul G. |last8=Skotnicki |first8=Steven J. |title=Geologic map of the Mount Taylor volcano area, New Mexico |journal=New Mexico Bureau of Geology and Mineral Resources Geologic Map |date=2019 |volume=80}} It has also been found on Gale crater on the planet Mars.{{cite journal |last1=Edwards |first1=Peter H. |last2=Bridges |first2=John C. |last3=Wiens |first3=Roger |last4=Anderson |first4=Ryan |last5=Dyar |first5=Darby |last6=Fisk |first6=Martin |last7=Thompson |first7=Lucy |last8=Gasda |first8=Patrick |last9=Filiberto |first9=Justin |last10=Schwenzer |first10=Susanne P. |last11=Blaney |first11=Diana|author11-link= Diana Blaney |last12=Hutchinson |first12=Ian |title=Basalt-trachybasalt samples in Gale Crater, Mars |journal=Meteoritics & Planetary Science |date=14 September 2017 |volume=52 |issue=11 |pages=2931–5310 |doi=10.1111/maps.12953|bibcode=2017M&PS...52.2931E |doi-access=free |hdl=2381/40838 |hdl-access=free }}
References
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