Andesitic Rocks - "orogenic andesites"
- Hypersthene normative, typically calcalkaline
- 53-63 wt.% SiO2
- Do not contain quartz and alkali feldspar
- Low K/Na ratios, low Ti (< 1.75%)
- Icelandites - hypersthene normative volcanics, Fe-rich
and high Ti. Differentiates of tholeiites
- Hawaiites - differentiates of alkali olivine basalts.
Intermediate K/Na ratio. Ocean island setting.
- Trachyandesites - also differentiates of alkali olivine
basalts, higher K/Na
Orogenic andesites make up the bulk
of andesites erupted. The weird ones are volumetrically minor.
Three types based on K2O content:
Mineralogy of Andesites
Plag > opx, cpx > mt ± ol
Plag > hornblende > mt ± cpx, opx
1. Plagioclase - 50-90% of phenocrysts
- PH2O affects liquidus (as PH2O increases, plag stability
- PH2O affects An content (higher than normal)
- Zoning and resorption in plag represents magma mixing or
- Adiabatic ascent - normal zoning with resorbed Ca rich
- cpx (diopside)
- opx (hypersthene)
Hornblende only - later phase as T decreases, H2O
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?
- Differentiates from basalt derived by melting of mantle
wedge. Basalt fractionates in crust to andesite.
- Basalts mix with rhyolites (crustal melts)
- Slab melts - basalt melts to form andesite, then
differentiate to dacite & rhyolite (doesn't happen,
need 50% melting of slab)