Arc Magmatism: Island Arcs and

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GEO1003 Spring 2009 Arc Magmatism
Arc Magmatism Island Arcs (and Continental Arcs)
Ref ch16 (Winter)
Note that in this course we will only study
island arcs not continental arcs
Merapi
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Arc Magmatism Introduction

• Arc magmatism is all subduction-related
  island-arcs and continental arcs
• (some arcs have characteristics of both)
• Subduction is responsible for much plutonism,
  volcanism, metamorphism and orogenesis
• Island-arcs (overiding plate is oceanic) and
  continetal arcs (where
• overiding plate is continental) clearly have the
  potential for generating
• magmas of different compositional ranges
• Most magmas are intermediate in composition
  (andesitic), but is typically
• very much more diverse in composition than MORs
  or within-plate settings
• Volcanism dominated by pyroclastic activity
• Simplest model might be that we generate
  andesites by partial melting
• of basaltic oceanic crust, but we clearly aslo
  have to consider the overlying
• mantle wedge and (in the case of continental
  arcs), contamination from
• the continental crust

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Principle subduction zones (and associated island
and continental arcs)
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Island Arc Volcanism Introduction

• Island arcs are mostly few hundred kms long and
  most have features illustrated below

• Note first melting typically at about 110km
• Note thick arc crust (typically about 30km)
  beneath many arcs and
• back-arc basin magmatism

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Subduction Basics

• Rates of subduction vary from about 1 to 12cm/yr
• Angles of subduction vary greatly from 30 degrees
  to almost vertical
• Subduction associated with inclined zone of
  earthquakes called the Benioff Zone
• goes down to about 650-700km
• Close relationship between position of volcanism
  relative to trench and depth to
• subduction zone (volcanism is closer to trench
  for steeper subduction)
• This implies strong relationship between P and T
  conditions as slab descends and
• point of initial melting
• Depth of melting is on average 110km
• Many island arcs also have back-arc extensional
  basins with volcanism often
• very similar to MORB

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Island Arc Volcanism Magma Classification

• Plots of arc (island and continental)
• Dominantly subalkaline
• Both tholeiitic and calc-alkaline, but overall
• calc-alkaline magmas dominate
• Basalts are subordinate
• Andesites and basaltic andesite dominate
• Almost all andesites on Earth occur in arcs
• Tholeiites occur in a variety of tectonic
• settings, but calc-alkaline magmas largely
• restricted to arcs

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Classification of Andesites on K20 content

• Note K content correlates often with distance
  from trench

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Andesite classification of Gill (1981) K20 vs
FeO/MgO

• Low K calc-alkaline type is uncommon, but other
  five fields are common in arcs
• (often in same arc or even same volcano)

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Island Arc Magmas Major Element Chemistry

• Island arc basalts are similar to MORB but higher
  K2O and Al2O3
• Some arc basalts are very high in alumina
  (high-alumina basalts)
• Tholeiitic arc magmas show Fe-enrichment on AFM
  plots
• Calc-alkaline magmas display much greater silica
  enrichment with
• evolution than tholeiitic magmas and also have a
  higher water content
• All island arc magmas display significant alkali
  enrichment with differentiation
• Some series start out as tholeiitic and end up as
  calc-alkaline
• Harker diagrams typcially show decreases in
  alumina, MgO, FeO and CaO indicating
• likely FC of plagioclase and mafic phases like
  olivine and pyroxene

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Island Arc Andesites Mineralogy

• Calc-alkaline andesites often have gt20 phxysts.
  
• Many phxysts are zoned and show resorption
  (disequilibrium textures)
• Plagioclase of highly variable composition (An50
  to An90) is most
• common phxyst phase
• Phxysts Cpx (augite) are also common in low-K
  andesites, and are often aluminous
• High-Ca and High-Al nature of phxysts due to
  depolymerizing nature of high water content of
  magmas (Al-O bonds not as easily broken as Si-O
  and Ab has
• more Si-O bonds than An)
• Some andesites are two-pyroxene andesites with
  both CPx and Opx phxysts
• Hornblende phxysts are common in medium to high K
  calc-alkaline andesites
• (needs gt3wt water in melt and P gt 0.1GPa)
• Common sequence in many andesites is Cpx, plag,
  then Opx

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Island Arc Volcanics Trace Element Chemistry (1)

• Andesites are not primary mantle
• melts as most trace elements support FC,
• but trace elements (and isotopes) indicate
• a mantle wedge (not crustal) source
• REE (left) of three series suggest
• Low K series indicates depleted source
• (for low silica rocks even more depleted
• than MORB)
• Higher K series are more enriched in
• LREE and many other incompatible elements
• and probably reflects lower degrees
• of partial melting of mantle
• Flat HREE for all series implies garnet
• was not involved (garnet in mantle OR
• garnet in eclogite, ie ocean crust at 110km)

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Island Arc Volcanics Trace Element Chemistry (2)

• High LIL/HFS ratios are typical. LIL elements
  are very soluble in water
• Water is sourced from sediments and dehydration
  of crust
• Water-rich fluids are very important in
  initiating melting and in petrogenesis of arc
• magmas

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Island Arc Volcanics Isotopes

• Complex due to heterogeneity of mantle source etc
• Isotopes suggest important source is similar
  depleted mantle to MORB (eg Sr87/Sr86 ratio
• often below 0.704), but EM1, EM2, PREMA and HIMU
  source can all be recognized
• Remember that HIMU probably indicates sedimentary
  crustal source
• Mixing between sources is common, especially DM
  plus one or more other reservoirs
• Many arc magmas can be explained by mixing of DM
  (and PREMA) plus continental sediment
• Isotopes indicate either mantle wedge is
  initially heterogeneous AND/OR is made so by
• LIL and Sr-enriched solutions from underlying
  oceanic crust and sediments

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Petrogenesis of Island Arc Magmas

• Descending slab is relatively cool (compared to
  surroundings) so basalt wont melt until
• about 200km
• Five possible sources for arc magmas crustal
  slab (altered crust, sediments and
• seawater) mantle wedge, arc crust lithosphere
  beneath mantle wedge asthenosphere
• beneath mantle wedge
• Last three are not likely lithosheric mantle is
  too depleted (MORB melts at MORs
• from this) and wont melt at 110km asthenosphere
  also heats up little arc crust is a
• product of arc magmatism so cant be a fundamental
  source
• Mantle wedge and crustal slab sediments and
  altered crust are most important (as
• trace elements and isotopes suggest)
• Altered crust at 11okm would be eclogite, but
  most arc magmas have flat HREE, so
• most favor mantle wedge as source of partial
  melts

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• Note water released does not cause melting in
  descending slab as Ts are too low, but alters
  overlying
• mantle and lowers its melting point. Amphibole
  forms in mantle and phlogopite at greater depths
• Note also flow in mantle wedge which drags
  altered mantle down (to higher Ts)
• Water released carries LIL, Sr and other
  incompatible elements
• Increasing K from trench reflects phlogopite
  breakdown and/or smaller degrees of partial
  melting
• (less water coming off slab deeper down)
• Closed cell flow of wedge explains depletion of
  some arc magmas
• FC of the mantle wedge derived melts (and magma
  mixing) at several level including the base of
  arc
• crust generates the dominant andesites