Melting in the Crust: Granitoids Yosemite National Park.

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Presentation transcript:

Melting in the Crust: Granitoids Yosemite National Park

Granitoid Intrusions Several 10,000 cubic km 3

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

Granophyres Graphic texture

Classification: Depth of Emplacement

Classification: Hypersolvus Granite Orthoclase with perthite (albite lamellae). One feldspar

Subsolvus Granite Orthoclase with perthite (albite lamellae). 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 and laccoliths. Mesozone - batholiths, stocks

Emplacement - The Space Problem

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 Granites high 87 Sr/ 86 Sr I-type Granites low 87 Sr/ 86 Sr A-type Granites low 87 Sr/ 86 Sr

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. 87 Sr/ 86 Sr 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

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?