Subduction-related Igneous activity - Island Arcs

(Chapter 16)

last update:10/11/06

Activity along arcuate volcanic island chains along subduction zones i.e. island arcs

Distinctly different from the mainly basaltic provinces thus far

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Composition more diverse and silicic

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Basalt generally occurs in subordinate quantities

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Also more explosive than the quiescent basalts

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Strato-volcanoes are the most common volcanic landform

Igneous activity is related to convergent plate situations that result in the subduction of one plate beneath another (subduction of ocean crust only under oceanic or continental crust)

Ocean-ocean ® Island Arc (IA) - (this chapter)

Ocean-continent ® Continental Arc or Active Continental Margin (ACM) - (next chapter)

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The initial simple model (and largely incorrect):

Oceanic crust is partially melted

Melts rise through the overriding plate to form volcanoes just behind the leading plate edge

Unlimited supply of oceanic crust to melt

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Subduction Products

bulletCharacteristic igneous associations
bulletDistinctive patterns of metamorphism
bulletOrogeny and mountain belts

Structure of an Ocean Island Arc

subduction dip angles = 30-90° (45° ave)

the depth to plate below arc is generally constant at 110 km no matter the dip angle i.e. horizontal distance of arc from trench dependent on dip

Volcanism accounts for ~10% of heat at arc

Arcs are commonly segmented - likely related to fracture zone offsets and different dips

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Volcanic Rocks of Island Arcs

Complex tectonic situation and broad spectrum of volcanic compositions

        i.e. basalts to rhyolites = orogenic suite

High proportion of basaltic andesite and andesite

Most andesites occur in subduction zone settings

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Major Elements and Magma Series

bulletTholeiitic (MORB, OIT)
bulletAlkaline (OIA)
bulletCalc-Alkaline (~ restricted to subduction zones)

 

Major Elements and Magma Series

a. Alkali vs. silica - minor alkaline magmas

b. AFM - both tholeiites and calc-alkaline magmas exist

c. FeO*/MgO vs. silica - both tholeiites (more in this diagram) and calc-alkaline magmas exist

diagrams for 1946 analyses from ~ 30 island and continental arcs with emphasis on the more primitive volcanics

 

Sub-series of Calc-Alkaline magmas

K2O is an important discriminator ® 3 sub-series

The three andesite series of Gill (1981) Orogenic Andesites and Plate Tectonics.  Contours represent the concentration of 2500 analyses of andesites

 

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Major element chemistry of individual island arc systems

Calc-alkaline basalts are commonly high-alumina basalts (17-21% Al2O3)

Examples of actual island arc magma series

bulletTonga-Kemedic: Low-K tholeiitic
bulletGuatemala: Medium-K calc-alkaline
bulletPapu New Guinea: High-K calc-alkaline

Variations are controlled by fractional crystallization and possible magma mixing
K2O-SiO2 diagram distinguishing high-K, medium-K and low-K series. Large squares = high-K, stars = med.-K, diamonds = low-K series from Table 16-2. Smaller symbols are identified in the caption. Differentiation within a series (presumably dominated by fractional crystallization) is indicated by the arrow. Different primary magmas (to the left) are distinguished by vertical variations in K2O at low SiO2. After Gill, 1981, Orogenic Andesites and Plate Tectonics. Springer-Verlag.

 

AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows represent differentiation trends within a series. Generally, the higher the K2O, the less the Fe-enrichment

 

FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.

 

Tholeiitic vs. Calc-alkaline differentiation
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Decrease in Al2O3, MgO, FeO and CaO are consistent with fractional crystallization of plagioclase and cpx or olivine

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Decrease in TiO2 due to fractionation of Fe-Ti oxides

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

Spatial

General "K-h" relation i.e. amount of K in volcanic with depth:  

low-K tholeiite near trench ® C-A ® alkaline as depth to seismic zone increases - many exceptions

 

Temporal

Early tholeiitic ® later C-A and often latest alkaline is common

 

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Petrography of island arc volcanics

Major phenocryst mineralogy.

Changes with K-type.

Plagioclase is the most common phenocryst (An50-70) - likely related to high H2O depolymerizing the magma and favors An plag.

Mafic minerals (cpx, opx or ol) are generally Mg-rich, and cpx is Al-rich

Hornblende is common in med-high K andesites - stable only at elevated H2O contents in the melt (may undergo later dehydration due to sudden loss of H2O or magma mixing)

Also, biotite in more evolved magma.

Disequilibrium textures are common

 

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Trace Elements

Even the most primitive arc basalts have low Ni (750-150 ppm), Cr and V (200-400 ppm) - too low to be primary mantle melts

REEs

Slope within series is similar, but height varies with degree of fractionation due to removal of Ol, Plag, and Px

(+) slope of low-K ® DM

bulletSome even more depleted mantle than MORB
bulletOthers have more normal slopes
bulletThus heterogeneous mantle sources

HREE flat, so no deep garnet peridotite or eclogite source

REE diagrams for some representative Low-K (tholeiitic), Medium-K (calc-alkaline), and High-K basaltic andesites and andesites. - they are not related by deep fractionation process

 

MORB-normalized Spider diagrams

Island arc basalts: distinctive decoupled HFS - LIL (LIL are hydrophilic - i.e. H2O fluids)

MORB-normalized spider diagrams for selected island arc basalts. Using the normalization and ordering scheme of Pearce (1983) with LIL on the left and HFS on the right and compatibility increasing outward from Ba-Th. Data from BVTP. Composite OIB from Fig 14-3 in yellow.

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10Be created by cosmic rays + oxygen and nitrogen in upper atmosphere

It falls to Earth by precipitation & readily incorporates into clay-rich oceanic sediments

Half-life of only 1.5 Ma (long enough to be subducted, but quickly lost to mantle systems). After about 10 Ma 10Be is no longer detectable

10Be/9Be averages about 5000 x 10-11 in the uppermost oceanic sediments

In mantle-derived MORB and OIB magmas, & continental crust, 10Be is below detection limits (<1 x 106 atom/g) and 10Be/9Be is <5 x 10-14

B is a stable element

Very brief residence time deep in subduction zones

B in recent sediments is high (50-150 ppm), but has a greater affinity for altered oceanic crust (10-300 ppm)

In MORB and OIB it rarely exceeds 2-3 ppm

Conclusion: participation of young sediments and altered oceanic crust

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Petrogenesis of Island Arc Magmas

Of the many variables that can affect the isotherms in subduction zone systems, the main ones are:

1) the rate of subduction
2) the age of the subduction zone
3) the age of the subducting slab
4) the extent to which the subducting slab induces flow in the mantle wedge

Other factors, such as:

bulletdip of the slab
bulletfrictional heating
bulletendothermic metamorphic reactions
bulletmetamorphic fluid flow

are now thought to play only a minor role

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Typical thermal model for a subduction zone

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

a) the convergence rate is slower
b) the subducted slab is young and near the ridge (warmer)
c) the arc is young (<50-100 Ma according to Peacock, 1991)

The principal source components that may contribute to island arc magmas

1. The crustal portion of the subducted slab

1a. Altered oceanic crust (hydrated by circulating seawater, and metamorphosed in large part to greenschist facies)

1b. Subducted oceanic and forearc sediments

1c. Seawater trapped in pore spaces

2. The mantle wedge between the slab and the arc crust

3. The arc crust

4. The lithospheric mantle of the subducting plate

5. The asthenosphere beneath the slab

The trace element and isotopic data suggest that both the subducted crust and mantle wedge contribute to arc magmatism.

Dry peridotite solidus too high for melting of anhydrous mantle to occur anywhere in the thermal regime shown

LIL/HFS ratios of arc magmas ® water plays a significant role in arc magmatism

The sequence of pressures and temperatures that a rock is subjected to during an interval such as burial, subduction, metamorphism, uplift, etc. is called a pressure-temperature-time or P-T-t path

 

The LIL/HFS trace element data underscore the importance of slab-derived water and a MORB-like mantle wedge source

The flat HREE pattern argues against a garnet-bearing (eclogite or garnet peridotite) source

Thus modern opinion has swung toward the non-melted slab for most cases

Amphibole-bearing hydrated peridotite should melt at ~ 120 km

Phlogopite-bearing hydrated peridotite should melt at ~ 200 km

bullet leads to a second arc behind first?

Some calculated P-T-t paths for peridotite in the mantle wedge as it follows a path similar to the flow lines in Figure 16-15. Included are some P-T-t path range for the subducted crust in a mature arc, and the wet and dry solidi for peridotite. The subducted crust dehydrates, and water is transferred to the wedge (arrow). After Peacock (1991), Tatsumi and Eggins (1995). Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

 

Island Arc Petrogenesis

A proposed 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) and phlogopite (B) dehydrate.

 

A multi-stage, multi-source process

1. Dehydration of the slab provides the LIL, 10Be, B, etc. enrichments + enriched Nd, Sr, and Pb isotopic signatures

These components, plus other dissolved silicate materials, are transferred to the wedge in a fluid phase (or melt?)

2. The mantle wedge provides the HFS and other depleted and compatible element characteristics

3. Phlogopite is stable in ultramafic rocks beyond the conditions at which amphibole breaks down

4. P-T-t paths for the wedge reach the phlogopite-2-pyroxene dehydration reaction at about 200 km depth

5. The parent magma for the calc-alkaline series is a high alumina basalt, a type of basalt that is largely restricted to the subduction zone environment, and the origin of which is controversial