Granite: An Overview Of Its Nature And Origin

Granite is an igneous rock formed from magma below the earth’s surface due to pressure and heat. Granite is found at a depth usually greater than 1.5 km and up to 50 km within thick continental crust of the Earth.

Geochemical origins of Granite
Granitoids are a ubiquitous component of the Earth’s crust. They have crystallized from magmas at its eutectic point due to heat and igneous differentiation. Alkali feldspar (rich in potassium) and quartz (SiO2), are two of the major defining constituents of granite.
Though the generation of granite is still into question among geologists, the composition and origin of the magma which differentiates into granite, leaves certain geochemical and mineral evidence as to what the granite's parental rock was. For instance, granite which is formed from melted sediments may have more alkali feldspar, whereas a granite derived from melted basalt may be richer in plagioclase feldspar.

Alphabet soup classification
· The 'alphabet soup' scheme of Chappell & White divides granites into I-type granite and S-type granite. Both of these types of granite are formed by melting of high grade metamorphic rocks.
· M-type or mantle derived granites are those granites which are sourced from crystallized mafic magmas that are of unique kinds.
· A-type or anorogenic granites are formed above volcanic "hot spot" activity. The rhyolites of the Yellowstone caldera are examples of volcanic equivalents of A-type granite.


Granitization
Granitization states that - “granite is formed by extreme metasomatism of fluids bringing in elements like potassium and removing others like calcium to transform the metamorphic rock into granite”.
In-situ granitisation or melting by metamorphism is recognized where leucosome and melanosome textures are present in gneisses. In such cases the solid rock melting requires high temperature. At the same time essential presence of water as well as other volatiles serves to be a catalyst.

Ascent and emplacement
Geologists have debated the ascent and emplacement of granite bulk that occurs within the upper continental crust.
There are two major hypotheses for the ascent of magma through the crust:
1. Stokes Diapir
2. Fracture Propagation
The basic idea of the Stokes diapir hypotheses is that magma will rise through the crust as a single mass through buoyancy. With its ascent it heats the wall rocks. This causes it to behave as a power-law fluid. Nonetheless since rocks and the upper crust of the earth do not deform hence this with the passage of time cools and solidifies.

Nowadays fracture propagation is the mechanism preferred by many geologists as it largely eliminates the major problems of moving a huge mass of magma through cold brittle crust.

Mechanisms for emplacing large batholiths to make room for Granitic magma:

· Stoping - where the granite cracks the wall rocks and pushes upwards as it removes blocks of the overlying crust.
· Assimilation - where the granite melts its way up into the crust and removes overlying material in its way.
· Inflation - where the granite body inflates under pressure and is injected into position.