A-type Granite

Synonym

Alkali granite, Alkaline granite

Definition

Igneous rocks with equigranular texture having mineral compositions as follows: Quartz in felsic minerals = 10-60 vol%, Plagioclase in feldspar = 10-65 vol%. A-type granite is determined by high (Na₂O + K₂O) / Al₂O₃. Other characters summarized below are those of general granites, and the rock type may be inferred but cannot be determined by mafic mineral assemblages.

Index Minerals

Annite, Hedenbergite, Hastingsite, Fayalite, Aegirine, Arfvedsonite, Riebeckite

Mineral composition

Mafic minerals < 30 vol% (Felsic), Quartz in felsic minerals = 10-60 vol%, Plagioclase in feldspar = 10-65 vol%.

Chemical composition

SiO₂ > 66 wt% (Acidic), Na₂O < 3.2 wt%

High (Na₂O + K₂O) / Al₂O₃, High Fe / Mg, and rich in F.

Texture

Granite: Equigranular texture composed of grains more than a few mm.

Granite porphyry, Quartz porphyry: Equigranular texture composed of grains more than a few mm, associated with larger euhedral phenocrysts.

Required Geological Setting

Plutonic rocks

Occurrence

Rock forming minerals in A-type granites, felsic (acidic) plutonic rock formed from magma under ground. A-type Granites are anorogenic, and distribute as stocks in continental rift zones and hot spots. A-type Granites are included in ilmenite series. Distribution of A-type granites in Japan is very limited locations such as Cape Ashizuri (Kochi pref.), Iwaki island (Ehime pref.), and Lake Aoki (Nagano pref.).

Mineral Assemblages

Quartz, Perthitic potassic feldspar, Plagioclase, Annite (Green), Hedenbergite, Hastingsite, Fayalite, Aegirine, Arfvedsonite, Riebeckite, Astrophyllite