S-type Granite

Synonym

Sedimentary source 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%. S-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

Cordierite, Muscovite, Almandine, Discrete apatite

Variety

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

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 S-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 S-type granites are ilmenite and sulfides, and deficient in magnetite. Thus most of S-type granites correspond to ilmenite series granites, although there are examples of magnetite-bearign S-type granites. S-type granites contain metasedimentary xenoliths and hornblende-free xenoliths. M-type granitoids and A-type granitoids correspond to gabbro and alkaline granite, respectively. S-type granites are reductive and rich in K, Rb, F, Li, Sn, Be, and are associated with W and Sn oxide deposit and various types of pegmatites. S-type granites distribute in the oceanic side in the Southwestern Japan (Eurasian Plate), whereas those distribute in the continental side in the Northeastern Japan (North American Plate). Representative S-type granites in Japan are Fukabe Tonalite in Hidaka belt, Kumano acidic rocks, granite bodies of Kashiwa island and Okinoshima island in Shikoku, and Osumi body.

Granite porphyry and quartz porphyry are also observed as dykes.

Mineral Assemblages

Quartz, Potassic feldspar, Plagioclase (Oligoclase), Biotite, Muscovite, Ilmenite, Apatite, Sulfides, (Cordierite, Monazite, Almandine, Zircon)