Give various types of granitoids and discuss the classification based on tectonic settings. Add a note on the role of mantle in the genesis of granitoids.

Types of Granitoids

Granitoids are broadly classified based on their mineral composition, texture, and silica content. Some key types include:

• Granite: Typically contains quartz (20-60%), alkali feldspar (65%), and plagioclase feldspar.
• Granodiorite: Contains more plagioclase feldspar than granite, with less alkali feldspar.
• Diorite: Predominantly composed of plagioclase feldspar, with minor quartz and alkali feldspar.
• Tonalite: Similar to granodiorite but with a higher proportion of plagioclase.
• Syenite: Contains very little or no quartz, dominated by alkali feldspar.
• Monzogranite: Intermediate composition between granite and granodiorite.

Classification Based on Tectonic Settings

The tectonic setting plays a crucial role in the genesis of granitoids. Different tectonic environments favor the formation of specific granitoid types through distinct petrogenetic processes.

1. Volcanic Arc Granitoids

These granitoids are commonly associated with subduction zones, where an oceanic plate descends beneath a continental or another oceanic plate. The subduction process introduces water into the mantle wedge, lowering its melting point and generating magma. These magmas are typically intermediate to felsic in composition, leading to the formation of granodiorites and tonalites. Examples include the Cascade Range in North America and the Andes Mountains in South America.

2. Continental Collision Granitoids

Continental collisions, such as the formation of the Himalayas, result in significant crustal thickening and melting. The heat generated by friction and radioactive decay leads to partial melting of the lower crust, producing large volumes of granitic magma. These granitoids are often highly differentiated and enriched in incompatible elements. The Himalayan granitoids are a prime example.

3. Within-Plate Granitoids (Anorogenic Granitoids)

These granitoids form in stable continental interiors, away from plate boundaries. Their genesis is often linked to mantle plumes or lithospheric thinning. Mantle plumes can induce melting in the lower crust, generating granitic magmas. These granitoids are typically alkali-feldspar granites and are often associated with alkaline igneous provinces. The Gardar Province in Greenland is a classic example.

4. Post-Collisional Granitoids

Following a continental collision, the thickened crust undergoes extension and delamination. This process can lead to decompression melting in the mantle and lower crust, generating granitic magmas. These granitoids are often enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE).

Role of the Mantle in the Genesis of Granitoids

The mantle plays a significant, albeit indirect, role in the genesis of most granitoids. While granitoids are primarily crustal rocks, the mantle provides the initial heat source and often contributes fluids and elements necessary for their formation.

• Mantle Melting: Mantle plumes and decompression melting can directly generate basaltic magmas. These basaltic magmas can then interact with the crust, leading to fractional crystallization and the generation of more felsic magmas.
• Fluid Contributions: Water released from subducting slabs or mantle plumes lowers the melting point of the crust, facilitating partial melting and granitoid formation.
• Element Transfer: The mantle can contribute incompatible elements (e.g., Rb, K, U, Th) to the crust through magma transport and fluid transfer, enriching the crust and promoting granitoid genesis.
• Mantle-Derived Xenoliths: The presence of mantle-derived xenoliths within some granitoids provides direct evidence of mantle involvement in their petrogenesis.

The extent of mantle involvement varies depending on the tectonic setting. In volcanic arc settings, the mantle contribution is significant due to the direct influence of subduction. In within-plate settings, the mantle contribution is often more indirect, through mantle plumes or lithospheric thinning.