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I-type Granite

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


Igneous source granite


Igneous rocks with equigranular texture having mineral compositions as follows: Quartz in felsic minerals = 10-60 vol%, Plagioclase in feldspar = 10-65 vol%. I-type granite is Al2O3 / (Na2O + K2O + CaO) < 1.1. 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

Hornblende, Titanite


Granite porphyry, Quartz porphyry

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%

Al2O3 / (Na2O + K2O + CaO) < 1.1


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


Rock forming minerals in I-type granites, felsic (acidic) plutonic rock formed from magma under ground. Granites distribute as batholiths and stocks in continents, and also distributes in island arcs. Granitoids are classified to S-type granites formed by patial melting of sedimentary rocks, I-type granites formed by crystallization differentiation of magma, M-type granitoids, and A-type granitoids. Accessary minerals of I-type granites are magnetite and ilmenite. I-type granites includes both magnetite-series granites and ilmenite-series granites. I-type granites contain igneous appearance xenoliths containing hornblende. M-type granitoids and A-type granitoids correspond to gabbro and alkaline granite, respectively. I-type granites are oxidative and poor in K, Rb, F, Li, Sn, Be, and are associated with Cu, Pb, Zn, and W sulfide deposits. I-type granites distribute in the continental side in the Southwestern Japan (Eurasian Plate), in the oceanic side in the Northeastern Japan (North American Plate), and in the Fossa Magna, a rift zone between the Eurasian Plate and the North American Plate.

Granite porphyry and quartz porphyry are also observed as dykes.

I-type Granite

Distribution of granites described in this site. Red is ilmenite-series granites corresponding to I-type granites or S-type granites, and blue is magnetite-series granites corresponding to a part of I-type granites.

Mineral Assemblages

Quartz, Potassic feldspar, Plagioclase (Oligoclase), Biotite, Magnesiohornblende, Magnetite, Ilmenite, Titanite, (Pyroxene, Monazite, Zircon, Apatite, Sulfides)

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