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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 > 66 wt%, and plagioclase in feldspar = 65 - 100 vol%. Other characters summarized below are those of general dacites, and the rock type may be inferred but cannot be determined by mineral assemblages.

Mineral composition

Mafic minerals < 30 vol% (Felsic), Quartz in felsic minerals = 20-60 vol%, Plagioclase in feldspar = 65-100 vol%.

Chemical composition

SiO2 > 66 wt% (Acidic).


Porphyritic texture composed of phenocrysts and groudmass.

Required Geological Setting

Island arc volcanoes, Intra-oceanic arc volcanoes


Rock forming minerals of dacite, felsic (acidic) volcanic rock formed from magma on the ground surface. Dacite distributes in island arcs of subduction zones associated with tholeiitic series (Pigeonite series) and calc-alkaline series (Hypersthene series) magma activities. Most dacite belongs to calc-alkaline series. 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.


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

Mineral Assemblages (Phenocryst)

Quartz (β-quartz), Plagioclase (Albite-Oligoclase), Biotite, Ferrohornblende, Magnetite, (Olivine (Fayalite), Garnet)

Mineral Assemblages (Groundmass)

Glass, Quartz, Plagioclase (Albite-Oligoclase), Potassic feldspar


  • Mount Akagi (Lava, Osumilite)
  • Nagahama (Dike, Almandine)

Related Occurrences

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