Felsic Plutonic Rocks (granitoids)
Characteristics: - Si saturated to oversaturated
Most common batholiths are quartz diorite, tonalite, granodiorite + granite
Principal minerals - feldspar, quartz, hornblende, micas. Cumulate textures are rare.
Principal textures - Generally massive (hypidiomorphic granular), sometimes porphyritic.
Normative calculations - usually 90% quartz, kspar, plag
Mineralogy - depends on alumina saturation (remember definitions)
1. Feldspars - Kspar (orthoclase or microcline), plagioclase (more albitic)
2. Pyroxene -
Metaluminous (px is rare)
- Cpx - similar to andesites - low Fe/Mg
- Opx - also similar to andesites
- Cpx - aegerine - augite (brightly colored)
- No opx in peralkaline rocks
3. Amphibole -
- Typically hornblende
4. Fe-Ti oxides
ilmenite and magnetite - solid solution
Granite batholiths are emplaced at a variety of levels in the crust.
Compositional Zoning in Granites
Granites are often concentrically zone. Why?
Best way to study this is by looking at both eruptive products and batholiths. Eruptive rocks are instantaneous view of magma chamber. Zoned magma chamber may explain why volcanics are such a jumble of compositions.
Source of Granitoids
Fractionation - If all granites are derived from basalts by fractionation, need giant cumulate residues somewhere.
Australians and British believe that granites are solely derived by melting of the crust
Melting of crust - igneous or sedimentary
I & S type granites
I -type: igneous rock source. Typically metaluminous
S -type: sedimentary rock source. Typically peraluminous.