Andesitic Rocks

Andesitic Rocks - "orogenic andesites"

Weird andesites

Orogenic andesites make up the bulk of andesites erupted. The weird ones are volumetrically minor.

Three types based on K2O content:

 

Mineralogy of Andesites

Pyroxene andesites

Plag > opx, cpx > mt ol

Hornblende andesites

Plag > hornblende > mt cpx, opx

1. Plagioclase - 50-90% of phenocrysts

2. Pyroxene

3. Amphibole

Hornblende only - later phase as T decreases, H2O increases

 

Amphibole needs at least 3 wt.% H2O in melt to crystallize

4. Olivine - only in basaltic andesites Fo80-65

 

Calcalkaline versus tholeiitic

How get calcalkaline versus tholeiitic trend?

Possibly shallow vs. deep fractionation

 

How do andesites form?

  1. Differentiates from basalt derived by melting of mantle wedge. Basalt fractionates in crust to andesite.
  2. Basalts mix with rhyolites (crustal melts)
  3. Slab melts - basalt melts to form andesite, then differentiate to dacite & rhyolite (doesn't happen, need 50% melting of slab)