Magmatism at Convergent Plate Margins. The wrong answer.

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Presentation transcript:

Magmatism at Convergent Plate Margins. The wrong answer

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 is calc-alkaline

Calc-Alkaline Trend Modern classification in an AFM diagram. Calc-Alkaline Trend no 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