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Basanite (/ˈbæs.əˌnaɪt/) is anigneous, volcanic (extrusive)rock withaphanitic toporphyritic texture. It is composed mostly offeldspathoids,pyroxenes,olivine, and calcicplagioclase and forms from magma low insilica and enriched inalkali metal oxides that solidifies rapidly close to theEarth's surface.
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 anormative olivine content greater than 10%. While theIUGS 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 theTAS 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 fromnephelinite by a normativealbite content of over 5% and a normativenepheline content under 20%.[1][2]
Themineral assembly in basanite is usually abundantfeldspathoids (nepheline orleucite),plagioclase, andaugite, together witholivine and lesser iron-titanium oxides such asilmenite andmagnetite-ulvospinel; minor alkalifeldspar may be present.Clinopyroxene (augite) andolivine are common asphenocrysts and in thematrix.[3] The augite contains significantly greatertitanium,aluminium andsodium than that in typicaltholeiitic basalt.[4]Quartz is absent, as areorthopyroxene andpigeonite.[5]
Chemically, basanites aremafic. They are low insilica (42 to 45% SiO2) and high inalkalis (3 to 5.5% Na2O and K2O) compared to basalt, which typically contains more SiO2, as evident on the diagram used for TAS classification. Nephelinite is yet richer in Na2O plus K2O compared to SiO2.
Basanite appears early in thealkaline magma series and basanites are found wherever alkaline magma is erupted.[7] This includes both continental and ocean island settings. Together with basalts, they are produced byhotspot volcanism, for example in theHawaiian Islands, theComoros Islands[8] and theCanary Islands.[3] They are particularly common in areas ofrifting.[9]
During eruption of theLaacher Seecaldera some 12,900 years ago, the final phase of the eruption, which tapped the deepest part of themagma chamber, produced basanitelapilli mixed withphonolite lapilli.[10] This has been interpreted as fresh magma injected into the magma chamber that may have helped trigger the eruption.[11]
Eruption of basanite and otheralkaline magmas characterizes the late alkaline phase (rejuvenation phase)[12] of volcanic islands, which often comes 3 to 5 million years after the mainshield-building phase.[13]
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