Alkaline rocks

Alkaline rocks occur in all tectonic environments, including the ocean basins. Alkaline rocks are generally considered to have more alkalis than can be accommodated by feldspars alone. The excess alkalis then appear in feldspathoids, sodic pyroxenes/amphiboles or other alkali-rich phases.

• Alkaline rocks are deficient in SiO₂ with respect to Na₂O, K₂O,and CaO to the extent that they become “critically undersaturated” in SiO₂ and nepheline or acmite appears in the norm.
• On the other hand some rocks may be deficient in Al₂O₃ (not necessary SiO₂ ) so that Al₂O₃ may not be able to accommodate the alkalis in normative feldspar. Such rocks are called “peralkaline rocks” and may be either silica saturated or undersaturated.

Alkaline rocks generally occurs in three principal settings :-

1. Continental rifts
2. Continental and Oceanic intraplate settings with no clear tectonic control
3. Subduction zones, particularly in back arc settings or in the waning stages of activity

Some important igneous Alkaline rocks are as follows –

• Basanite :- Feldspathoid bearing basalt. Usually contains nepheline, but may have leucite and olivine
• Tephrite :- Olivine free basanite
• Nephelinite :- Volcanic rock that contains leucite + clinopyroxene + Olivine. It typically lacks feldspar
• Ijolite :- Plutonic nepheline-pyroxene rock with 30 – 70% nepheline
• Shoshonite :- K-rich basalt with k-feldspar +/- leucite
• Phonolite :- Felsic alkaline volcanic with alkali feldspar + nepheline (plutonic = nepheline syenite)
• Lamproite :- A group of peralkaline, volatile rich, potassic, volcanic to hypabyssal rock (olivine, phlogopite, leucite, clinopyroxene, sanidine)
• Lamprophyre :- A diverse group of dark, porphyritic, mafic to ultramafic hypabyssal highly potassic rocks (K>Al). They are normally rich in alkali volatiles Sr, Ba, Ti with biotite-phlogopite and/ or amphibole phenocrysts. They typically occurs as sills, dykes, plugs or stocks.
• Carbonatite :- An igneous rock composed of carbonate (calcite, ankerite, dolomite etc.) and commonly with clinopyroxene, alkali amphibole, biotite, apatite etc. The Mg-Ca rich carbonatites are technically not alkaline but are associated with them.
• Kimberlite:- A complex, hybrid volatile (Co₂), potassic, ultramafic rocks with a fine grained matrix and macrocrysts of olivine and several of the minerals like Ilmenite, Garnet, Diopside, Phlogopite, Enstatite, Chromite, Xenocrysts and Xenoliths.
  • Group 1 kimberlite :- Typically CO₂ rich and less potassic than group 2 kimberlite
  • Group 2 kimberlite (orangeite) :- Typically H₂O rich and has a mica-rich matrix (also with calcite, diopside and apatite)