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)

Peralkaline

3. Amphibole -

Metaluminous

Peralkaline

4. Fe-Ti oxides

ilmenite and magnetite - solid solution

5. Micas

Emplacement levels

Granite batholiths are emplaced at a variety of levels in the crust.

  1. catazone - deeply eroded terranes. Concordant margins and strong foliation.
  2. Mesozone - range in which magmatic and metamorphic rocks more easily distinguished. Roof pendants common
  3. Epizone - within a few km of the surface. Emplaced in brittle sedimentary or volcanic rocks. Associated with contemporary volcanism. Narrow contact metamorphic zones. Economic mineral deposits.

Compositional Zoning in Granites

Granites are often concentrically zone. Why?

  1. May be successive intrusions of more differentiated magmas (from holding tank below)
  2. Crystallization in from walls
  3. Wall rock contamination

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.