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Chapter 16. Island Arc MagmatismChapter 16. Island Arc MagmatismOceanOcean--oceanocean →→ Island ArcIsland Arc (IA); Ocean(IA); Ocean--continentcontinent →→Continental ArcContinental Arc

Active Continental MarginActive Continental Margin (ACM)(ACM)

Figure 16-1. Principal subduction zones associated with orogenic volcanism and plutonism. Triangles are on the overriding plate.

PBS = Papuan-Bismarck-Solomon-New Hebrides arc. After Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer.


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Activity alongActivity along arcuatearcuate volcanic island chains alongvolcanic island chains along

subductionsubduction zones w/deep oceanic trench on oceaniczones w/deep oceanic trench on oceanic

sideside

Distinctly different from basaltic provincesDistinctly different from basaltic provinces

Composition more diverse and silicaComposition more diverse and silica--richrich

Dominated byDominated by andesitesandesites

Basalt generally occurs in subordinate quantitiesBasalt generally occurs in subordinate quantities

More explosive than the quiescent basaltsMore explosive than the quiescent basalts

StratoStrato--volcanoes are the most common volcanic landformvolcanoes are the most common volcanic landform

Characterisitcs of Island Arc Magmatism


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Igneous activity related to convergent plateIgneous activity related to convergent plate

situationssituations results from subduction of one plate beneathresults from subduction of one plate beneath

anotheranother The initial petrologic model:The initial petrologic model:

Oceanic crust is partially melted Oceanic crust is partially melted

Melts rise through the overriding plate toMelts rise through the overriding plate to

form volcanoes just behind the leadingform volcanoes just behind the leading

plate edge plate edge

Unlimited supply of oceanic crust to meltUnlimited supply of oceanic crust to melt


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Subduction ProductsSubduction Products

CharacteristicCharacteristic igneousigneous associationsassociations

Distinctive patterns ofDistinctive patterns of metamorphismmetamorphism

OrogenyOrogeny and mountain beltsand mountain beltsIsland ArcIsland Arc VolcanicsVolcanics – – diverse suitediverse suite


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Island ArcIsland Arc VolcanicsVolcanics – – diverse suitediverse suite

BasaltBasalt

– –

andesiteandesite

– –

rhyoliterhyolite

associationsassociations

BasaltsBasalts – – tholeiitestholeiites similar to MORBsimilar to MORB --higherhigher

AlAl22OO33 (16 wt%)(16 wt%) – – called high Al basaltscalled high Al basalts

AndesitesAndesites – – not primary mantle melts; derivednot primary mantle melts; derived

from fractional crystallization of more primitivefrom fractional crystallization of more primitive

parent parent

DacitesDacites,, rhyolitesrhyolites – – moremore felsicfelsic members; derivedmembers; derivedfromfrom andesitesandesites by fractional crystallization by fractional crystallization

BoninitesBoninites – – rare, Mgrare, Mg--rich,rich, opxopx-- bearing rocks; may bearing rocks; may be primitive be primitive andesitesandesites derived by direct meltingderived by direct melting


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Structure of an Island ArcStructure of an Island Arc

Figure 16-2. Schematic cross section through a typical island arc after Gill

(1981), Orogenic Andesites and Plate Tectonics. Springer-Verlag. HFU= heat

flow unit (4.2 x 10-6 joules/cm2 /sec)

Benioff Zone – earthquake foci

describe inclined zone

(subduction zone)

which projects as deep

as 700 km

Site of convergence

marked by trench

Forearc – contains

volcanic andsedimentary rocks

derived by weathering

of arc


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Accretionary prism – uppermost sediments and seafloor brought into

trench; scraped off and accreted to inner wall of trenchVolcanic arc – behind forearc and parallel to trench; distance between

trench and arc correlates to dip angle of subducting slab

Back arc basin – behind volcanic arc; underlain by basaltic oceanic

crust; may contain secondary spreading ridge; extensional tectonics

Trench and forearc show

anomalously low heat

flow.Volcanic arc and backarc

show unusually high heat

flow.


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Volcanic Rocks of Island ArcsVolcanic Rocks of Island Arcs

Complex tectonic situation and broad spectrumComplex tectonic situation and broad spectrum High proportion ofHigh proportion of basaltic andesite basaltic andesite andand andesiteandesite

Most andesites occur in subduction zone settingsMost andesites occur in subduction zone settings

Table 16-1. Relative Proportions of Quaternary Volcanic

Locality B B-A A D RTalasea, Papua 9 23 55 9 4

Little Sitkin, Aleutians 0 78 4 18 0

Mt. Misery, Antilles (lavas) 17 22 49 12 0 Ave. Antilles 17 42 39 2

Ave. Japan (lava, ash falls) 14 85 2 0

After Gill (1981, Table 4.4) B = basalt B-A = basaltic andesite

A = andesite, D = dacite, R = rhyolite

Island Arc Rock Types


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bS b i f li f C l lk liAlk li


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SubSub--series of Calcseries of Calc--AlkalineAlkaline K K 22O is an important discriminatorO is an important discriminator →→ 33 subsub--seriesseries

LowLow--K, MediumK, Medium--K, and HighK, and High-- K K

Figure 16-4. The three

andesite series of Gill (1981)

Orogenic Andesites and Plate

Tectonics. Springer-Verlag.

Contours represent the

concentration of 2500 analyses

of andesites stored in the large

data file RKOC76 (Carnegie

Institute of Washington).


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Sub-series of Calc-Alkaline Magmas

• Low-K series – dominated by basalts and basalticandesites

Represents initial stages of mantle melting beneath an

island arc and fractionation of initial melts

Very primitive basalt magmas

Typical of young, immature and smaller arcs

Low alkalis and SiO2

• High-K series – dominated by andesite and daciteTypical of late-stage magmatic activity in larger, older

more mature arcs like Japan


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Figure 16-6. b. AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows

represent differentiation trends within a series.


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Figure 16-8. K2O-SiO2 diagram of nearly 700 analyses for Quaternary islandarc volcanics from the Sunda-Banda arc. From Wheller et al. (1987) J. Volcan. Geotherm. Res., 32, 137-160.


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Other TrendsOther Trends

SpatialSpatial

LowLow--K tholeiite near trenchK tholeiite near trench →→ calccalc--alkalinealkaline →→

alkaline as depth to seismic zone increasesalkaline as depth to seismic zone increases

TemporalTemporal

Early to later calcEarly to later calc--alkaline to latest alkaline isalkaline to latest alkaline is

commoncommon

l


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Trace ElementsTrace Elements

REEsREEs Slope within series is similarSlope within series is similar

(+) slope of low(+) slope of low--KK →→

depleted mantle sourcedepleted mantle source Some even more depletedSome even more depleted

than MORBthan MORB

Others have more normalOthers have more normalslopesslopes

Heterogeneous mantleHeterogeneous mantlesourcessources

Figure 16-10. REE diagrams for some representative Low-K

(tholeiitic), Medium-K (calc-alkaline), and High-K basaltic

andesites and andesites. An N-MORB is included for reference

(from Sun and McDonough, 1989). After Gill (1981) Orogenic

Andesites and Plate Tectonics. Springer-Verlag.

II


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New Britain, Marianas, Aleutians, and South Sandwich volcanics New Britain, Marianas, Aleutians, and South Sandwich volcanics

plot within a surprisingly limited range of depleted mantle plot within a surprisingly limited range of depleted mantle

IsotopesIsotopes

Figure 16-12. Nd-Sr

isotopic variation in some

island arc volcanics.

MORB and mantle array

from Figures 13-11 and

10-15. After Wilson(1989), Arculus and

Powell (1986), Gill

(1981), and McCulloch et

al . (1994). Atlantic

sediment data from

White et al . (1985).


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CosmogenicCosmogenic IsotopesIsotopes

1010BeBe created by cosmic rays + oxygen and nitrogen in uppercreated by cosmic rays + oxygen and nitrogen in upperatmosphereatmosphere

→→ Earth by precipitation and readilyEarth by precipitation and readily incorporated intoincorporated into clayclay--

rich oceanic sedimentsrich oceanic sediments

Half Half --life of only 1.5 Ma. After about 10 Ma,life of only 1.5 Ma. After about 10 Ma, 1010Be is noBe is no

longer detectablelonger detectable

1010Be/Be/99Be averages about 5000Be averages about 5000 xx 1010--1111 in the uppermostin the uppermost

oceanic sedimentsoceanic sediments

In mantleIn mantle--derived MORB and OIB magmas, and continentalderived MORB and OIB magmas, and continentalcrust,crust, 1010Be is below detection limits (


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BB is a stable elementis a stable element

Very brief residence time deep in subduction zonesVery brief residence time deep in subduction zones

B in recent sediments is high (50B in recent sediments is high (50--150 ppm), but has a greater150 ppm), but has a greater

affinity for altered oceanic crust (10affinity for altered oceanic crust (10--300 ppm)300 ppm)

In MORB and OIB it rarely exceeds 2In MORB and OIB it rarely exceeds 2--3 ppm3 ppm


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1010Be/BeBe/Betotaltotal vs. B/vs. B/BeBetotaltotal diagram (diagram (BeBetotaltotal ≈≈99Be sinceBe since 1010Be is so rare)Be is so rare)

Figure 16-14. 10Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88, 111-123.

Petrogenesis of Island Arc Magmas


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Petrogenesis of Island Arc Magmas

Main variables affecting isotherms inMain variables affecting isotherms in subductionsubduction zone systems are:zone systems are:

1)1) thethe raterate ofof subductionsubduction

2)2) thethe ageage of theof the subductionsubduction zonezone

3)3) thethe ageage of theof the subductingsubducting slabslab

4)4) the extent to which thethe extent to which the subductingsubducting slab inducesslab induces flow in the mantleflow in the mantle

wedgewedge

Other factors are now thought to play only a minor role:Other factors are now thought to play only a minor role:

dip of the slabdip of the slabfrictional heatingfrictional heating

endothermic metamorphic reactionsendothermic metamorphic reactions

metamorphic fluid flowmetamorphic fluid flow

i l h l d l f bd iT i l h l d l f bd i


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Typical thermal model for a subduction zoneTypical thermal model for a subduction zone

Isotherms will be higher (i.e., system hotter) ifIsotherms will be higher (i.e., system hotter) if

a)a) the convergence rate is slower the convergence rate is slower

b) b) the subducted slab is young and near the ridge (warmer)the subducted slab is young and near the ridge (warmer)

c)c) the arc is young (


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Figure 16-15. Cross section of asubduction zone showing

isotherms (red-after Furukawa,

1993, J. Geophys. Res., 98, 8309-

8319) and mantle flow lines

(yellow- after Tatsumi and

Eggins, 1995, Subduction Zone

Magmatism. Blackwell. Oxford).

The principal source componentsThe principal source components →→ island arc magmasisland arc magmas

1.1. CrustalCrustal portion of the portion of the subducted slab:subducted slab:

Altered oceanic crust (hydrated by circulating seawater, andAltered oceanic crust (hydrated by circulating seawater, andmetamorphosed in large part to greenschist facies)metamorphosed in large part to greenschist facies)

Subducted Subducted oceanic and forearc sedimentsoceanic and forearc sediments

Seawater trapped in pore spacesSeawater trapped in pore spaces

Th i i lTh i i l t i l di l d


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2.2. Mantle wedgeMantle wedge between the slab and the arc crust between the slab and the arc crust

3.3. Arc crustArc crust

4.4. LithosphericLithospheric mantle of the subducting platemantle of the subducting plate

5.5. AsthenosphereAsthenosphere beneath the slab beneath the slab

The principal source componentsThe principal source components →→ island arc magmasisland arc magmas

Figure 16-15. Cross section of asubduction zone showing

isotherms (red-after Furukawa,

1993, J. Geophys. Res., 98, 8309-

8319) and mantle flow lines

(yellow- after Tatsumi and

Eggins, 1995, Subduction Zone

Magmatism. Blackwell. Oxford).

OnlyOnly subducted crustsubducted crust (1)(1) andand mantle wedge (2)mantle wedge (2) possible possible


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OnlyO y subducted crustsubduc ed c us (1)( ) anda d mantle wedge (2)a e wedge ( ) possibleposs b e

Trace element and isotopic data suggest thatTrace element and isotopic data suggest that both both contributecontribute

to arcto arc magmatismmagmatism Incompatible element ratios of arc magmasIncompatible element ratios of arc magmas showshow water water

plays a significant role in arc magmatism plays a significant role in arc magmatism

I tibl t l t d t i di t thIncompatible trace element data indicates the


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Incompatible trace element data indicates theIncompatible trace element data indicates the

importance of slabimportance of slab--derived water and a MORBderived water and a MORB--

like mantle wedge sourcelike mantle wedge source

Most geologists favor a nonMost geologists favor a non--melted slabmelted slab

Island Arc PetrogenesisIsland Arc Petrogenesis


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Island Arc PetrogenesisIsland Arc Petrogenesis

Figure 16-11b. Aproposed model

for subduction

zone magmatism

with particular

reference to island

arcs. Dehydration

of slab crust

causes hydration

of the mantle(violet), which

undergoes partial

melting as

amphibole (A) andphlogopite (B)

dehydrate. FromTatsumi (1989), J. Geophys.

Res., 94, 4697-4707 and

Tatsumi and Eggins (1995).

Subduction Zone Magmatism. Blackwell.

Oxford.

A multiA multi--stage, multistage, multi--source processsource process


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g ,g , pp

Dehydration of the slab provides the LIL,Dehydration of the slab provides the LIL, 1010Be, B, etc. enrichments +Be, B, etc. enrichments +

enriched Nd, Sr, and Pb isotopic signaturesenriched Nd, Sr, and Pb isotopic signatures

These components, plus other dissolved silicate materials, areThese components, plus other dissolved silicate materials, are

transferred to the wedge in a fluid or melt phasetransferred to the wedge in a fluid or melt phase

The mantle wedge provides the incompatible and compatible elemenThe mantle wedge provides the incompatible and compatible elementtcharacteristicscharacteristics

The parent magma for the calcThe parent magma for the calc--alkaline series is aalkaline series is a high aluminahigh alumina


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The parent magma for the calcThe parent magma for the calc alkaline series is aalkaline series is a high aluminahigh alumina

basalt basalt

largely restricted to the subduction zone environment, and thelargely restricted to the subduction zone environment, and theorigin of which is controversialorigin of which is controversial

Some highSome high--Mg (>8 wt% MgO) high alumina basalts may be primaryMg (>8 wt% MgO) high alumina basalts may be primary

More common lowMore common low--Mg (17wt% AlAl (>17wt% Al22OO33))

basalts are the result of deeper fractional crystallization of t basalts are the result of deeper fractional crystallization of thehe primary tholeiitic magma which ponds at the base of the arc crus primary tholeiitic magma which ponds at the base of the arc crust int in

more mature arcsmore mature arcs

Fractional crystallization takes place at a number of levelsFractional crystallization takes place at a number of levels


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y py p

Figure 16-11b. A

proposed model

for subduction

zone magmatism

with particular

reference to islandarcs. Dehydration

of slab crust

causes hydration

of the mantle(violet), which

undergoes partial

melting as

amphibole (A) and

phlogopite (B)

dehydrate. FromTatsumi (1989), J. Geophys.

Res., 94, 4697-4707 and

Tatsumi and Eggins (1995).

Subduction Zone Magmatism. Blackwell.

Oxford.

Model for


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Model for

principal

features of arcmagmatism:

1. Heating and dehydration of subducted crust and sediments

2. Rise of released fluid phase + LIL elements into overlying mantle

wedge

3. Dragging hydrous peridotite in overlying mantle wedge to great

depths where it dehydrates initiating partial melting to form olivinetholeiite basalt w/1-2 wt % H2O

Model for


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principal features

of arc magmatism:

4. Ponding of tholeiitic magma at base of arc crust to form high-Al

basalt magma by fractional crystallization. Overlying arc crust meltedforming silica-rich melts that mix w/mafic melts.

5. Differentiation of tholeiitic and calc-alkaline series at higher crustal

levels to produce broad spectrum of volcanics at surface.

6. Induced mantle flow may cause convective mantle upwelling and back

arc volcanism behind the arc.

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arco de isla 3