Basanite is an igneous, volcanic (extrusive) rock with aphanitic (fine-grained) 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.
Classification and Composition
Basanite is a silica-undersaturated mafic rock. 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. On the QAPF diagram, basanite plots in the same field as tephrite. It is distinguished from tephrite by having a normative olivine content greater than 10% (as calculated by the CIPW norm). When classified chemically using the Total Alkali-Silica (TAS) diagram, basanite falls into the U1 (basanite-tephrite) field, with silica content of approximately 42–45% SiO₂ and total alkali content (Na₂O + K₂O) of 3–5.5%. It is distinguished from nephelinite by a normative albite content of over 5% and a normative nepheline content under 20%.
The mineral assemblage typically includes abundant feldspathoids (nepheline or leucite), calcic plagioclase (labradorite to bytownite), titanium-bearing augite, and olivine, with lesser amounts of iron-titanium oxides such as ilmenite and magnetite-ulvospinel. Minor alkali feldspar may be present. Orthopyroxene and pigeonite are absent. Accessory minerals may include apatite, biotite, and perovskite.
Occurrence
Basanite appears early in the alkaline magma series and is found wherever alkaline magma is erupted, including both continental and ocean island settings. Together with basalts, it is produced by hotspot volcanism in locations such as the Hawaiian Islands, the Comoros Islands, and the Canary Islands. It is particularly common in areas of rifting, such as the East African Rift system, the Baikal Rift in Siberia, the Rhine Graben, and the Rio Grande Rift in the United States. Basanite also occurs in volcanic arcs, such as the Campanian volcanoes near Naples, Italy.
Eruption of basanite and other alkaline magmas characterizes the late alkaline phase (rejuvenation phase) of volcanic islands, which often occurs 3 to 5 million years after the main shield-building phase.
Varieties
- Ankaramite: A porphyritic, melanocratic variety of basanite, named after Ankaramy in Madagascar, characterized by abundant olivine and pyroxene phenocrysts with minor plagioclase.
- Limburgite (hyalo-nepheline basanite): A dark-colored extrusive rock named after Limberg (Limburg) in Germany, with abundant olivine and titanian augite in a glassy groundmass.
Uses
Basanite is used as aggregate for road construction, concrete, and other construction applications. It is also used as landscaping or riprap stone. Historically, it was used for sculptures (e.g., a sculpture of Livia Drusilla) and served as a touchstone. It is also used for stone casting and as a facing material.
Distinction from Similar Rocks
- Basanite vs. Basalt: Basanite is more silica-undersaturated, higher in alkali oxides and feldspathoids (20% or more), and contains no quartz. Basalt has higher silica content, less than 10% feldspathoids, and may contain up to 20% quartz.
- Basanite vs. Tephrite: Distinguished by olivine content in the CIPW norm — basanite has normative olivine exceeding 10%, while tephrite has less than 10%.
- Basanite vs. Nephelinite: Basanite has less than 20% normative nepheline and more than 5% normative albite; nephelinite has >20% normative nepheline and <5% normative albite.
- Basanite vs. Alkali Basalt: Basanites are more silica-undersaturated and have higher alkali and volatile content, with a higher nepheline amount.
