Melting in the Crust: Granitoids

Yosemite National Park

Granitoid Intrusions

Several 10,000 cubic km3

Classification of Granitic Rocks

~80% are monzogranites

Biotite, hornblende and px are common accessories.

Classification: Porphyrys

Common in shallow intrusions

Associated with Mo and Cu mineralisation

Aplites

Aplite - “unspectacular”

Pegmatites

Pegmatites

Granophyres

Graphic texture

Classification: Depth of Emplacement

Classification: Hypersolvus Granite

Orthoclase withperthite (albitelamellae).

One feldspar

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

Emplacement - The Space Problem

External Contacts

External Contacts

migmatite

Melting in the Crust

Small degrees of partial melting of andesitic crust will lead to monzogranite

Melting in the Crust

Melting related to the dehydration of biotite and muscovite

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.

Heat Sources

Heat Sources

Heat Sources

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)

Lamprophyre Mantle Melts

Lamprophyres late-stage mantle melts?