Granitic ROcks

Granitic Rocks

• encompasses diorite, granodiorite and granite (“granitoids”)

• coarse-grained analogues of andesite,dacite & rhyolite, respectively 

• occur in various tectonic settings: orogenic & collision zones, intraplate, small volumes in MORs and ophiolite complexes

• associated hydrothermal mineralization & ore deposits

Common Minerals in Granitic Rocks

• quartz

• alkali feldspar

• plagioclase

• pyroxene

• amphibole (hornblende)

• biotite

• muscovite

• tourmaline

Mineralogy of Granitic Rocks

• diorite: plg + mafic mineral

• tonalite: plg + qtz

• granodiorite: sodic plg + qtz + alk fsp (plg > alk fsp)

• granite: alk fsp + qtz + sodic plg (alk fsp > plg)

• alkali granite: alk fsp + qtz

Associated Rocks

Feldspars in Plutonic Rocks

• simple twinning → sanidine

• tartan twinning → microcline

• exsolution texture → perthite

• polysynthetic twinning → plagioclase

“Granitic” Rocks

• charnockites and mangerites

  • opx-bearing granites

• opx replaces hbl, minor biotite is the only hydrous phase

• associated with Proterozoic massif-type anorthosite complexes

Classification of Granitoid Intrusions

Classification of Granitoid Intrusions

• I-type granitoids

  • partial melting of igneous crustal rocks

  • metaluminous

  • hornblende and biotite

• S-type granitoids

  • pelitic rocks

  • peraluminous

  • muscovite

• A-type granitoids

  • continental rifts

  • metaluminous or peralkaline

  • mafic minerals

Alkali Rocks

• constitute <1% of all igneous rocks exposed on the Earth’s surface

• SiO2 deficiency leads to the various feldspathoids in place of feldspars

• magmas formed by partial melting at higher P compared to subalkaline magmas 

• high concentrations of incompatible elements in many alkali magmas

Nomenclature of Fine-grained Alkali Roks

• diversity of feldspathoids: complex nomenclature of alkali rocks

• sodic rocks - intraplate and constructive margins (hotspots and divergent)

• mildly potassic rocks – island arc and collisional environments (convergent)

Associated Rocks

• carbonatite

  • igneous rock consisting mainly of carbonate minerals (e.g. calcite, dolomite, siderite, ankerite) 

  • crystallized from a carbonate-dominated melt 

  • occurrence: intrusive, effusive or pyroclastic 

  • primarily related to intraplate magmatism; very rare reports from orogenic zones and oceanic basins 

  • rare minerals and metals - REEs, Nb

• kimberlite 

  •  ultramafic rock consisting of large anhedral olivine (“macrocrysts”) and other mafic minerals in a finer-grained matrix 

  • crystallized from a volatile and chiefly CO2 rich magma 

  • restricted to ancient continental cratons

  • contains mantle xenoliths – subcontinental lithospheric mantle (SCLM) 

  • economic interest: Diamond xenocrysts (e.g. Africa)