Andesite

Definition

Igneous rocks with porphyritic texture composed of phenocrysts and groudmass having chemical composition of SiO₂ = 52-66 wt%. Other characters summarized below are those of general andesites, and the rock type may be inferred but cannot be determined by mineral assemblages.

Mineral composition

Mafic minerals = 30-60 vol% (Neutral), Quartz in felsic minerals = 0-20 vol%, Plagioclase in feldspar = 65-100 vol%.

Chemical composition

SiO₂ = 52-66 wt% (Neutral).

Texture

Porphyritic texture composed of phenocrysts and groudmass.

Required Geological Setting

Island arc volcanoes, Intra-oceanic arc volcanoes

Occurrence

Rock forming minerals of andesite, neutral volcanic rock formed from magma on the ground surface. Andesite distributes in island arcs of subduction zones associated with tholeiitic series (Pigeonite series) and calc-alkaline series (Hypersthene series) magma activities. Most andesite is formed by fractional crystallization of calc-alkaline magma but some andesite is formed by fractional crystallization of tholeiitic magma. 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. Recent studies discuss that andesitic magma may directly formed from mantle material. For example, adakite, a kind of andesite, is produced by melting slubs younger than 40 Ma at the back-arc of island arcs and has high Sr/Y. High-Mg andesite at the trench side of the volcanic front may also be one of examples.

Mineral Assemblages (Phenocryst)

Tholeiitic andesite: Plagioclase (Oligoclase-Labradorite), Clinopyroxene (Pigeonite, Augite), Orthopyroxene (Hypersthene), Magnetite, (Olivine, Garnet, Quartz, Cordierite)

Calc-alkaline andesite: Plagioclase (Oligoclase-Labradorite), Clinopyroxene (Augite), Orthopyroxene (Hypersthene), Hornblende, Biotite, Magnetite, (Olivine, Garnet, Quartz, Cordierite)

Mineral Assemblages (Groundmass)

Tholeiitic andesite: Glass, Anorthoclase, Phenocryst minerals

Calc-alkaline andesite: Glass, Anorthoclase, Tridymite, Cristobalite, Phenocryst minerals