Andesite



  Andesite (TAS diagrams.

Classification of andesites may be refined according to the most abundant phenocryst. Example: hornblende-phyric andesite, if hornblende is the principal accessory mineral.

Andesite can be considered as the extrusive equivalent to diorite. Andesites are characteristic of subduction tectonic environments in active oceanic margins, such as the western coast of South America. The name andesite is derived from the Andes mountain range.

Genesis of andesite

Andesite is formed at accretionary plate margins. Intermediate volcanic rocks are created via several processes:

  1. Dehydration melting of peridotite and fractional crystallization
  2. Melting of subducted slab containing sediments
  3. Magma mixing between felsic basaltic magmas in an intermediate reservoir prior to emplacement or eruption.

Via fractional crystallisation

Andesitic magma in island arc regions (i.e. active oceanic margins) comes from the interplay of the subducting plate and the mantle wedge, the part of the overriding plate above the subducted plate.

Water in the subducted oceanic crust 'boils off' from the slab by dehydration of hydrated minerals such as Franciscan Facies Sequence metamorphism during subduction, they metamorphose to more stable, dehydrated forms, releasing water and volatile elements into the mantle wedge.

The slab itself, or the overlying mantle wedge, may melt. Melting subducted slab components have a sediment component from the subducted plate, which can be detected by increased adakite).

On its way to the surface, the melt stalls and cools, enabling the fractional crystallization of silica poor minerals, thus raising the silica content of the remaining melt and resulting in andesitic magma.

Via magma mixing

Basaltic magma may also mix with rhyolitic magma. This usually occurs in continental arc areas such as the Andes, where the high geothermal gradient above the subducted plate, and hydrothermal flows within the mantle wedge may create an underplate of softened, partially molten continental crust of intermediate or felsic composition. Basaltic magmas intruded into this anomalously hot zone will prompt partial melting of the crust, and may mix with these melts to produce intermediate compositions, typically andesite to trachyte in composition.

Alternatively, the basaltic melt may heat up the overlying arc, prompting partial melting, and may even assimilate sediments, previous volcanic rocks, etcetera, whilst undergoing fractional crystallisation. These rocks are subordinate due to the difficulty in assimilating sufficient cold material by magmas without cooling to a degree that they become immobile.

Ultimately, the resultant composition of andesite and intermediate magmas is the result of fractional crystallisation, assimilation, partial melting and contaminaton by the subducted slab. These may take considerable effort to resolve the individual components.

See also

References

  • Origins of the Continental Crust
  • Island arc magmatism
  • Experimental and Theoretical Constraints on Peridotite Partial Melting in the Mantle Wedge
 
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