Geol 285 - Petrology, Dr. Helen M. Lang, West Virginia University, Fall 2005
Generation of Subduction-related Andesites - J.K. Gill’s
Model
Different from model presented in textbook
When mantle melts beneath Mid-Ocean Ridges or at Hot-spots, uniform
tholeiitic basalt is formed

Why does melting of mantle at subduction zones produce calc-alkaline magmas,
mostly andesites
Things about subduction-related magmatism that must be explained by any
model

The magmas (and rocks) are calc-alkaline, not tholeiitic

i.e., they show no Fe-enrichment

Magmas (and rocks) are dominantly andesites with higher SiO2 than basalts

More varied magma types are produced
Generation of Subduction-related Andesites - J.K. Gill’s Model
See handout
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POAM fractionation - key to Gill’s model: fractionation of the following
minerals

P=Plagioclase
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O=Olivine and/or Orthopyroxene
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A=Augite
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M=Magnetite: fractionation of some phase with high Fe and low SiO2 is
necessary to explain SiO2 rich andesites and lack of Fe-enrichment of subductionrelated magmas (could be Mt or Hornblende or Garnet) and to lower density of
trapped melt (Mt=5.2!)
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Subduction-related volcanoes commonly have bimodal volcanism
• Cinder cones, rhyolite domes and zoned ash flows at Mt. Mazama/Crater
Lake
• Cinder cones, rhyolite domes at Mt. Shasta
• Abundant andesite lava flows at Mt. Rainier
• Explosive rhyolitic ash flows at many subduction-related volcanoes
Liquid Segregation in Shallow Chamber can lead to Bimodal Volcanism (copy
sketch)
Ash eruption could produce layered ash flows like at Crater Lake Pinnacles
Crater Lake Ash Flow - Pinnacles
Gentle gas-poor eruption could form rhyolite domes (central) and basalt cinder
cones (flanks)
Also at Crater Lake (after the big ash flows)
Bimodal Volcanism at Mount Shasta
Things about subduction-related magmatism that must be explained by any
model and Gill's explanation

The magmas (and rocks) are calc-alkaline, not tholeiitic, i.e., they show no Feenrichment

Magmas (and rocks) are dominantly andesites with higher SiO2 than basalts
Explanation: POAM fractionation at base of crust

More varied magma types are produced
Explanation: Liquid segregation in shallow chamber, variable
fractionation/contamination on way through crust