{{Short description|Type of volcanic rock}} {{hatnote|This article is about the igneous rock. Basanite is also a variety of Jasper}} {{Distinguish|Bassanite}} {{Infobox rock |name=Basanite |alternative_name= |type=Igneous |image=Basanite&Dunite bomb.JPG |image_size=300 |alt= |caption=Volcanic bomb of black basanite enclosing a xenolith of green dunite from Réunion |coordinates= |composition= |composition_secondary= }} '''Basanite''' ({{IPAc-en|ˈ|b|æ|s|.|ə|ˌ|n|aɪ|t}}) is an igneous, volcanic (extrusive) rock with aphanitic to porphyritic texture. It is composed mostly of feldspathoids, pyroxenes, olivine, and calcic plagioclase and forms from magma low in silica and enriched in alkali metal oxides that solidifies rapidly close to the Earth's surface.
==Description== thumb|QAPF diagram showing the basanite/tephrite field in yellow thumb|TAS diagram with basanite/tephrite field highlighted Basanite is an aphanitic (fine-grained) igneous rock that is low in silica and enriched in alkali metals. Of its total content of quartz, feldspar, and feldspathoid (QAPF), between 10% and 60% by volume is feldspathoid and over 90% of the feldspar is plagioclase. Quartz is never present. This places basanite in the basanite/tephrite field of the QAPF diagram. Basanite is further distinguished from tephrite by having a normative olivine content greater than 10%. While the IUGS recommends classification by mineral content whenever possible, volcanic rock can be glassy or so fine-grained that this is impractical, and then the rock is classified chemically using the TAS classification. Basanite then falls into the U1 (basanite-tephrite) field of the TAS diagram. Basanite is again distinguished from tephrite by its normative olivine content and from nephelinite by a normative albite content of over 5% and a normative nepheline content under 20%.<ref>{{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}}</ref><ref>{{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}}</ref>
The mineral assembly in basanite is usually abundant feldspathoids (nepheline or leucite), plagioclase, and augite, together with olivine and lesser iron-titanium oxides such as ilmenite and magnetite-ulvospinel; minor alkali feldspar may be present. Clinopyroxene (augite) and olivine are common as phenocrysts and in the matrix.<ref name="Carracedo 2016">{{Cite book|last=Carracedo|first=J. C.|title=The Geology of the Canary Islands|date=2016|author2=V. R. Troll|isbn=978-0-12-809663-5|location=Amsterdam, Netherlands|publisher=Elsevier|oclc=951031503|doi=10.1016/C2015-0-04268-X}}</ref> The augite contains significantly greater titanium, aluminium and sodium than that in typical tholeiitic basalt.<ref>{{Cite journal|last1=Weis|first1=Franz A.|last2=Skogby|first2=Henrik|last3=Troll|first3=Valentin R.|last4=Deegan|first4=Frances M.|last5=Dahren|first5=Börje|date=2015|title=Magmatic water contents determined through clinopyroxene: Examples from the Western Canary Islands, Spain|journal=Geochemistry, Geophysics, Geosystems|language=en|volume=16|issue=7|pages=2127–2146|doi=10.1002/2015GC005800|bibcode=2015GGG....16.2127W |issn=1525-2027|doi-access=free|hdl=10553/72171|hdl-access=free}}</ref> Quartz is absent, as are orthopyroxene and pigeonite.<ref>{{cite book |last1=Blatt |first1=Harvey |last2=Tracy |first2=Robert J. |title=Petrology : igneous, sedimentary, and metamorphic. |date=1996 |publisher=W.H. Freeman |location=New York |isbn=0716724383 |pages=164–165 |edition=2nd}}</ref>
Chemically, basanites are mafic. They are low in silica (42 to 45% SiO<sub>2</sub>) and high in alkalis (3 to 5.5% Na<sub>2</sub>O and K<sub>2</sub>O) compared to basalt, which typically contains more SiO<sub>2</sub>, as evident on the diagram used for TAS classification. Nephelinite is yet richer in Na<sub>2</sub>O plus K<sub>2</sub>O compared to SiO<sub>2</sub>.
==Occurrences== [[File:Livia Drusilla Louvre Ma1233.jpg|thumb|Basanite sculpture of Livia Drusilla<ref name="LiviaBasanite">{{cite web | url=https://library.nga.gov/discovery/fulldisplay/alma991077873804896/01NGA_INST:IMAGE | title=Livia | publisher=National Gallery of Art Library | work=Image Collections Digital (NGA Exhibitions, 2008–2009) | accessdate=26 March 2021}}</ref>]] Basanite appears early in the alkaline magma series and basanites are found wherever alkaline magma is erupted.<ref>{{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=139–148}}</ref> This includes both continental and ocean island settings. Together with basalts, they are produced by hotspot volcanism, for example in the Hawaiian Islands, the Comoros Islands<ref>{{cite web|url=http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_65599.htm|title=Geochemistry and Petrology of a Miocene Trachyte-Basanite Suite from Mt. Tsaratanana, Northern Madagascar|website=gsa.confex.com|access-date=2006-04-05|archive-url=https://web.archive.org/web/20070513192113/http://gsa.confex.com/gsa/2003AM/finalprogram/abstract_65599.htm|archive-date=2007-05-13|url-status=dead}}</ref> and the Canary Islands.<ref name="Carracedo 2016"/> They are particularly common in areas of rifting.<ref>{{cite book |last1=Fisher |first1=Richard V. |last2=Schmincke |first2=H.-U. |title=Pyroclastic rocks |date=1984 |publisher=Springer-Verlag |location=Berlin |isbn=3540127569 |pages=19–20}}</ref>
During eruption of the Laacher See caldera some 12,900 years ago, the final phase of the eruption, which tapped the deepest part of the magma chamber, produced basanite lapilli mixed with phonolite lapilli.<ref>{{cite book |last1=Schmincke |first1=Hans-Ulrich |title=Volcanism |date=2003 |publisher=Springer |location=Berlin |isbn=9783540436508 |page=33}}</ref> This has been interpreted as fresh magma injected into the magma chamber that may have helped trigger the eruption.{{sfn|Schmincke|2003|p=205}}
Eruption of basanite and other alkaline magmas characterizes the late alkaline phase (rejuvenation phase)<ref>{{cite book |last1=Macdonald |first1=Gordon A. |last2=Abbott |first2=Agatin T. |last3=Peterson |first3=Frank L. |title=Volcanoes in the sea : the geology of Hawaii |date=1983 |publisher=University of Hawaii Press |location=Honolulu |isbn=0824808320 |pages=152–154 |edition=2nd}}</ref> of volcanic islands, which often comes 3 to 5 million years after the main shield-building phase.{{sfn|Schmincke|2003|pp=80-81}}
==See also== *{{annotated link|Limburgite}}
== References == {{Reflist}} {{Rock type}}
==External links== {{Commons category|Basanite|position=left}}
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Category:Aphanitic rocks Category:Porphyritic rocks Category:Mafic rocks Category:Volcanic rocks