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Chemical Process (Liquid-phase Growth) - Closed system under ground (Melt precipitation)


Igneous rocks with porphyritic texture composed of phenocrysts and groudmass having chemical composition of SiO2 = 45-52 wt%. Other characters summarized below are those of general basalts, and the rock type may be inferred but cannot be determined by mineral assemblages.

Mineral composition

Mafic minerals = 60-70 vol% (Mafic), Quartz in felsic minerals = 0-20 vol%, Plagioclase in feldspar = 65-100 vol%.

Chemical composition

SiO2 = 45-52 wt% (Basic).


Porphyritic texture composed of phenocrysts and groudmass.

Required Geological Setting

Island arc volcanoes, Intra-oceanic arc volcanoes, Ridges, Hot spot volcanoes, Manle plumes


Rock forming minerals of basalt, mafic (basic) volcanic rock formed from magma on the ground surface. Basalt is classified by chemical composition to low-alkaline tholeiitic basalt, high-alkaline tholeiitic basalt, high-alumina basalt, and alkaline basalt. Basalt is classified by mineral assemblages to tholeiitic basalt, alkaline basalt, and calc-alkaline basalt. Tholeiitic basalt is formed from a high-degree partial melt of the upper mantle, and alkaline basalt is from a low-degree partial melt of the upper mantle. Origin of calc-alkaline magma is under discussion. The upper mantle is composed of Lherzolite, and refractory material of the upper mantle after forming tholeiitic magma is considered as Harzburgite. In tholeiitic series, FeO increases along with differentiation of magma whereas SiO2 keeps constant. In calc-alkaline series, SiO2 increases along with differentiation of magma whereas FeO decreases. As the result, mineral assemblages of tholeiitic and calc-alkaline series rocks are different. 4 geological settings of tholeiitic basalt are known: Mid-ocean ridge basalt (MORB) on ridges, Oceanic island basalt on hot spot volcanoes, Island-arc basalt on island arcs of subduction zones, and Flood basalt by mantle plumes. Calc-alkaline basalt is observed as island-arc basalt on island arcs of subduction zones.


Distribution of basalt producing phenocryst minerals in this site. Yellow lines are volcanic fronts.

Mineral Assemblages (Phenocryst)

Tholeiitic basalt: Plagioclase (Bytownite-Anorthite), Clinopyroxene (Pigeonite, Augite), Orthopyroxene (Hypersthene), Forsterite, (Magnesio-hornblende, Phlogopite)

Calc-alkaline basalt: Plagioclase (Bytownite-Anorthite), Clinopyroxene (Augite), Orthopyroxene (Hypersthene), Forsterite, (Magnesio-hornblende, Phlogopite)

Alkaline basalt: Plagioclase (Bytownite-Anorthite), Kaersutite, Phlogopite

Mineral Assemblages (Groundmass)

Tholeiitic basalt: Glass, Clinopyroxene (Pigeonite, Augite), Other phenocryst minerals, (Magnetite, Ilmenite, Apatite)

Calc-alkaline basalt: Glass, Orthopyroxene (Hypersthene), Clinopyroxene (Augite), Other phenocryst minerals, (Magnetite, Ilmenite, Apatite)

Alkaline basalt: Glass, Phenocryst minerals, (Magnetite, Ilmenite, Apatite)


  • Hachijo Island (Lava, Anorthite)
  • Miyake Island (Lava, Anorthite)
  • Hawaii (Lava, Forsterite)
  • Toge (Lava, Kaersutite)
  • Kamisano (Dike, Diposide)

Related Occurrences

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