Important Igneous Rocks

Granite is a light colored igneous plutonic rock with large grain size that can be visible with naked eyes. Granite is mainly composed of Quartz and Feldspar with minor amount of mica, amphibole and other minerals.

This composition gives granite a red, pink, gray and white color appearance.

Kimberlite is a variety of peridotite. It is rich in mica often in form of crystals of phlogopite, other contains minerals are chrome-diopside, olivine and chromium and pyrope-rich garnet. In the groundmass of serpentine, carbonate and provskite.

It is a group of dark colored porphyritic hypabyssal igneous rocks characterized by penidiomorphic texture. A high percentage of mafic minerals (Biotite, Hbl, Pyx) forms the phenocryst and a fine grained groundmass with same mafic minerals in addition to feldspar and/or feldspathoids. Most of these rock are altered.

They are commonly associated with carbonatities.

Spilites are altered basaltic rocks that contain sodic plagioclase (albite), and chlorite, instead of Ca-plagioclase and Cpx (usually augite) typical of basalt.

The conversion of calsic to sodic plagioclase requires a source of Na, which may be sea water or hydrothermal fluids, whereas the Ca released by basalt alteration forms minerals like epidote and calcite in the resultant spilite.

It is an igneous rock consisting of > 50% carbonates. there are three main processes for it’s formation

1. Direct generation by very low partial melting of mantle & differentiation.
2. Liquid immiscibility b/w carbonate and silicate melt.
3. Peculiar, extreme crystal fractionation.

There are two important carbonate minerals that found are – Nyereite & Gregoryite

Coarse calcite carbonatite – Sovite

Coarse dolomitic carbonatite – Rauhugite

Nephaline syenite is a plutonic coarse grained rock, predominantly consists of Na-plagioclase and K-feldspar while comparatively less Ca-plagioclase. An unsaturated nephaline minerals is also present.

It’s volcanic equivalent is Phonolite.

The rock khondalite is generally medium to coarse grained with considerable variations in modal proportion of the major phases: Quartz, K-feldspar, Garnet and sillimanite indicating different protolith compositions.

Plagioclase is also minor but important constituent in some varieties.

The rock generally contains a foliation subparallel to lithological layering, suggesting their sedimentary parantage.

It is a mafic extrusive igneous rock high in both MgO & SiO₂ and low in TiO₂. It is characterized by depleted in incompatible elements.

It is thought to be usually formed in fore-arc environment, typically during early stages of subduction. Boninite is considered to be a primitive andesite derived from melting of metasomatism mantle.