Upload
belinda-austin
View
224
Download
0
Tags:
Embed Size (px)
Citation preview
Classification of Granitic Rocks
~80% are monzogranites
Biotite, hornblende and px are common accessories.
Subsolvus Granite
Orthoclase withperthite (albitelamellae).
Two feldspars
Albite with orthoclase
But: Only in Alkali Granites – plagioclase causes problems!
Classification: Timing of Emplacement
• Post tectonic - after deformation
• Syntectonic - during deformation (mesozone, catazone)
• Pretectonic - before deformation (catazone)
Nature of Granitic Intrusions
Epizone - also plugs, dykes, sills andlaccoliths.
Mesozone - batholiths, stocks
Chemical (Tectonic) Types of Granite
S-type Graniteshigh 87Sr/86Sr
I-type Graniteslow 87Sr/86Sr
A-type Graniteslow 87Sr/86Sr
Chemical (Tectonic) Types of Granite
I-type Granites
S-type Granites
M-type Granites A-type
Granites
M-type Granites
Continental Arc Granitoids
• S-type granites further from subduction zone.
• 87Sr/86Sr decreases towards subduction zone
• Silica content decreases towards subduction zone (I.e. granites to qtz diorites).
• More assimilation of crust/less magmatic rock away from subduction zone.
Related Rocks: Lamprophyres
• Igneous rocks dominated by phenocrysts of hydrous minerals (biotite/phlogopite/hornblende.
• Often contain feldspathoids in groundmass
• Found as plugs and dykes associated with granitoid rocks.
• High Nb/Ti (metasomatised mantle)
• Enriched in incompatibles (low degrees of partial melting)