Magmatism at Convergent Plate Margins. The wrong answer.
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Transcript of Magmatism at Convergent Plate Margins. The wrong answer.
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