Magmatism at Convergent Plate Margins.

The wrong answer

Convergent Plate Margins

Convergent Plate Margins

Two possible

Convergent Plate Margins: In Detail

Magmatism

Calc-alkaline magmatism is characteristic.

Variation diagrams against silica show fractionation

Peacock index of 56-61 is calc-alkaline

Calc-Alkaline Trend

Modern classification in an AFM diagram.

Calc-Alkaline Trendno Fe-enrichment

Tholeiite Trend Fe-enrichment

Calc-Alkaline Trend

• High water partial pressure lowers the solidus of olivine, px and plag.

• High water partial pressure raises the solidus of hornblende.

• High water partial pressure raises the oxygen fugacity and stabilises iron oxides.

Ocean Island Arcs & Cordillera

Younger arcs are mainly tholeiitic,

older and cordillera calc-alkaline

Ocean Island Arcs & Cordillera

Sub-groups of Calc-Alkaline

Shoshonitic

Depth of Benioff Zone

Distribution of Calc-Alkaline

• First magmas are mainly tholeiitic, followed by calc-alkaline and finally alkaline.

• K contents increase with the depth of the benioff zone.

Nature of Calc-Alkaline Volcanism

Petrology of Calc Alkaline Rocks.

Composition of Calc-Alkaline

Large Ion Lithophiles (LILE) - high solubility in water

High Field Strength Elements (HFSE) – not soluble in water

Light Rare Earth Elements (LREE) -

Heavy Rare Earth Elements (HREE) – retained in garnet

REE Calc-Alkaline

Plag fractionation

Tholeiite like

Enriched

REE Calc-Alkaline

Has similarities to kimberlites

Isotopes

B-Be Isotopes

Altered Oceanic Crust

Black Smokers

Origins of Calc-Alkaline Magmas

From Where the Water?

Dehydration at >600C and P >18 kbar

Origins of Calc-Alkaline Magmas

Adding water to the overlying mantle wedge will cause it to melt

Where does the water go? Metasomatism

Cratons: Kimberlites

Melting of the Mantle Wedge

Rise and Fractionation