Basanite is a fine-grained, dark-colored volcanic or extrusive igneous rock. Fine-grained means its grains are too small to see without a microscope.
Compositionally, basanite is a mafic rock with 45-52% silica. Its main minerals are calcic plagioclase, augite, nepheline, and olivine. Also, it has other minor minerals, including alkali feldspar.
How does it form? Basanite rocks are formed from the fast cooling of alkalic mafic magmas near or on the Earth’s surface. This fast cooling results in the fine-grained texture we have already mentioned.
Note: This rock is not the same as bassanite, a mineral or a black chert known as basanite chert.
Quick overview and properties
- Name: Basanite
- Rock type: Igneous
- Origin: Extrusive or volcanic
- Texture: Aphanitic or fine-grained to porphyritic
- Colors: Usually, gray to black, but can be other colors
- Chemical composition: Mafic
- Silica content: 42-49%
- Mohs hardness scale: 6-7
- Tectonic settings: Convergent plate boundaries (continental margin and island arc), hotspots, divergent plate boundaries (rift zones)
What does basanite look like?
In hand specimens, color and texture may help you identify this rock. Basanite is usually a gray to black mafic rock. However, it can sometimes be greenish, brownish, or other colors. Its typical color index ‘M’ is greater than 35. The high mafic content makes this rock have dark colors.
The texture of basanite is aphanitic or fine-grained, i.e., it has tiny mineral crystals less than 1/16 mm not visible to the naked eye. These crystals are mostly holocrystalline to a bit hypocrystalline (partially glassy).
However, some varieties may have a porphyritic texture. Such a rock will have large crystals (phenocrysts) in fine-grained groundmass with or without glass.
usually, a porphyritic basanite may have olivine, plagioclase, augite, and olivine phenocrysts set in a fine-grained groundmass. The groundmass has feldspathoids, plagioclase, augite, and accessory minerals.
Lastly, some basanite, alkaline basalt, and nephelinite may host ultramafic nodules or inclusion. These inclusions include garnet-bearing lherzolites and spinel-bearing peridotite.
Basanite composition
Basanite is a silica-undersaturated acidic rock with at least 35% mafic minerals. Here is more on its chemical and mineral composition.
1. Chemical composition
Basanite is a silica-poor (< 46 wt %) rock rich in alkali metal oxides (Na2O and K2O) than basalt (usually 3 to 5.5 wt%) and has a higher Mg/Fe ratio.
It is a silica-undersaturated rock relative to alkali oxides. Thus, it will have modal feldspathoids like nepheline or leucite. Other common silica-undersaturated rocks are tephrite, phonotephrite, tephriphonolite, phonolite, nephelinite, and melilitite.
Based on data by Le Maitre (1976) from 165 sample analyses, the average weight percentage of basanite is SiO2: 45.16%, TiO2: 2.56%. Al2O3: 14.99%, Fe2O3: 4.02%, FeO: 7.65%, MnO: 0.16%, MgO: 8.71%, CaO: 10.39%, Na2O: 3.62%, K2O: 2.00% and P2O5: 0.75%.
2. Mineral composition
Basanite rock is high in feldspathoids, calcic plagioclase, olivine, and pyroxenes with minor (< 10% by volume of total feldspar) to no alkali feldspar. Some rocks may have alkali amphibole.
Accessory minerals present in this rock may include ilmenite, magnetite-ulvospinel, apatite, and sometimes perovskite (calcium-titanium oxide).
The felsic minerals are mainly calcic-plagioclase and nepheline, with less than 10% alkali felspar. Calcic plagioclase varies from labradorite to bytownite, whose phenocrysts, when present, are zoned, notes Bowes (1989).
On the other hand, nepheline is the common feldspathoid. It may occur with or without analcite or leucite and less often nosean.
The mafic minerals in this rock are olivine, titanium-bearing augite, and sometimes alkali amphibole. It doesn’t have orthopyroxenes or pigeonite.
On the QAPF diagram, basanite is a volcanic rock in which feldspathoids make 10-60% of QAPF content by volume, with plagioclase over 90% of total feldspar. It doesn’t have quartz and plots in the same place with tephrite on the QAPF diagram.
While mineral IUGS recommends classifying igneous rocks by their mineral composition, this rock has a fine-grained texture with tiny crystals. Thus, it is practically impossible to determine mineral content. Instead, we use TAS (Total alkali silica) weight % content.
On the TAS, basanite lies above picrobasalt. Its silica content is about 42-49 wt%, and alkali oxides (Na2O + K2O) are 3 to 9 wt%. Again, it plots the same spot as tephrite.
Lastly, reducing the feldspar proportion grades this rock into leucitite and nephelinite.
Varieties and related rocks
- Ankaramite: Ankaramite is a porphyritic dark-colored or melanocratic basanite variety named after Ankaramy in Madagascar, its type locality. This volcanic rock has mainly olivine and pyroxene phenocrysts with minor plagioclase. Accessory minerals may include apatite and oxides of iron.
- Limburgite (hyalo-nepheline basanite). Limburgite is a dark-colored extrusive igneous rock named after its type locality, Limberg or Limburg in Germany. It resembles basalt and has abundant olivine and titanian augite in a brownish hyaline or glassy groundmass. The common accessory minerals in this rock are apatite, ilmenite, and titanian magnetite.
How is basanite formed?
Basanite rock forms when silica-poor basaltic magma quickly cools on or near the Earth’s surface. The fast cooling impedes the growth of large mineral crystals. Thus, this rock has a fine-grained texture.
Magma is from fractionation of more primitive magmas than tholeiitic. Such magma’s origin is likely from partially melting the enriched mantle, probably the asthenosphere.
For instance, partially melting mantle rocks like peridotites (kimberlite or dunite) rich in volatiles, especially CO2 and water, can generate such magma.
Evidence of the magma origin includes ultramafic nodules and some high-pressure minerals. These two imply that magma travels fast and has a deeper origin than basalts. Also, the association of basanitic magma with carbonatites and chemical signatures is further evidence. Blatt & Tracy (2006) and Winter (2014) have more insight.
Lastly, basanite rocks are emplaced mainly as Hawaiian-style effusive eruptions characterized by lava flow. However, it may erupt explosively with lava fountains and form volcanic cones.
Where is basanite found?
Basanite rocks occur on continental arc margins, island arcs, hotspots, and rift zones but in less volume than alkali basalts.
It may occur with basalt and other silica undersaturated. Such rocks include alkali basalt, tephrite, phonolite, tephriphonolite, nephelinite, and melanephenelinite. Some emplacements may also have intrusive alkali gabbro, ijolite, and theralite.
To specific locations, basanite is found in hotspots like Hawaii Island, Canary Islands, Comoros Island, Fernando de Noronha archipelago, and Tristan da Cunha. More hotspots are Santo Antão (Cape Verde), Society hotspot (South Pacific Ocean)
Also, rift zones, such as the East African Rift system, the Baikal Rift in Siberia, the Rhine Graben and the Rio Grande Rift in the United States, are dominated by alkali basalts, basanites, and nephelinites, notes Rogers & Hawkesworth (2000, p.122).
Volcanic arcs like Campanian volcanoes near Naples (Italy), Mohéli (Comoros), and Vitória-Trindade Ridge, including on its seamounts, have basanite.
More places where basanite is found include Tahiti Island, Bohemian Mountain Range (Czech), Auvergne (France), Monte Somma and Vesuvius (Italy), the Laacher See area, and Kaiserstuhl volcanic (Germany).
That is not all. This rock also occurs in volcanic fields like Madagascar (Ankaratra and Ambohitra), and San Fransico (USA).
Lastly, the North Arch volcanic field In Oahu Island (Hawaii, USA) has deep submarine sheet flows of undersaturated alkali basalts, nephelinites, and basanites.
What is it used for?
Basanite rock uses include aggregate for the roading industry, concrete, and other construction. Also, it can work as landscaping or riprap stones, and historically, it made sculptures like Livia Drusilla and served as a touchstone.
Frequently Asked Questions (FAQs)
Basanite is a silica-undersaturated rock higher in alkali oxides (Na2O and K2O) and feldspathoids (20% or more). It has more than 5% nepheline and has no quartz. In contrast, basalt rocks tend to have more silica, less than 10% feldspathoids, and up to 20% quartz, with nepheline less than 5%.
The difference between basanite and tephrite is the olivine content in the CIPW norm. If normative olivine exceeds 10%, you will have a basanite. In contrast, you have tephrite if the normative olivine is less than 10% to none.
Basanite differs from nephelinite since it has less than 20% normative nepheline and more than 5% normative albite. In contrast, nephelinite has > 20% normative nepheline and less than 5% normative albite.
Basanites are more silica undersaturated and have more alkali and volatiles than alkali basalts. Also, they have a higher nepheline amount.
References
- Winter, J. D. (2014). Principles of igneous and Metamorphic Petrology. Pearson Education.
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- Bowes, D. R. (1989). Tephrite, basanite. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology (p. 457). New York: Van Nostrand Reinhold.
- Blatt, H., Tracy, R. J., & Owens, B. E. (2006). Petrology: Igneous, sedimentary, and metamorphic (3rd ed.). W.H. Freeman and Company.
- Rogers, N. & Hawkesworth, C. (2000). composition of magmas. In Sigurdsson, H. (ed.) Encyclopedia of volcanoes. (1st ed. p. 157). San Diego: Academic Press.