Chapter 16- Island Arcs

Report
Ocean-ocean  Island Arc (IA)
Ocean-continent  Continental Arc or
Active Continental Margin (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.


Igneous activity is related to convergent
plate situations that result in the subduction
of one plate beneath another
The initial petrologic model:
 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
Structure 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)
Volcanic Rocks of Island Arcs


Complex tectonic situation and broad spectrum
High proportion of basaltic andesite and andesite

Most andesites occur in subduction zone settings
Table 16-1. Relative Proportions of Quaternary Volcanic
Island Arc Rock Types
Locality
Talasea, Papua
Little Sitkin, Aleutians
Mt. Misery, Antilles (lavas)
Ave. Antilles
Ave. Japan (lava, ash falls)
B
9
0
17
17
14
B-A
23
78
22
A
55
4
49
42
85
D
9
18
12
39
2
After Gill (1981, Table 4.4) B = basalt B-A = basaltic andesite
A = andesite, D = dacite,
R = rhyolite
R
4
0
0
2
0
Figure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Tholeiitic vs. Calc-alkaline differentiation
Figure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

REEs


Slope within series is similar, but
height varies with FX due to
removal of Ol, Plag, and Pyx
(+) slope of low-K  DM




Trace Elements
Some even more depleted than
MORB
Others have more normal slopes
Thus heterogeneous mantle
sources
HREE flat, so no deep garnet
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.
Isotopes

New Britain, Marianas, Aleutians, and South Sandwich
volcanics plot within a surprisingly limited range of DM
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).
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:
 dip of the slab
 frictional heating
 endothermic metamorphic reactions
 metamorphic fluid flow
are now thought to play only a minor role


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)
yellow curves
= mantle flow
Figure 16-15. Cross section of a
subduction zone showing
isotherms (red-after Furukawa,
1993, J. Geophys. Res., 98, 83098319) and mantle flow lines
(yellow- after Tatsumi and
Eggins, 1995, Subduction Zone
Magmatism. Blackwell. Oxford).
The principal source components  IA 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
Figure 16-15. Cross section of a
subduction zone showing
isotherms (red-after Furukawa,
1993, J. Geophys. Res., 98, 83098319) and mantle flow lines
(yellow- after Tatsumi and
Eggins, 1995, Subduction Zone
Magmatism. Blackwell. Oxford).
The principal source components  IA magmas
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
Figure 16-15. Cross section of a
subduction zone showing
isotherms (red-after Furukawa,
1993, J. Geophys. Res., 98, 83098319) and mantle flow lines
(yellow- after Tatsumi and
Eggins, 1995, Subduction Zone
Magmatism. Blackwell. Oxford).


Left with the subducted crust and mantle wedge
The trace element and isotopic data suggest that both
contribute to arc magmatism. How, and to what
extent?
 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
Effect of adding volatiles (especially H2O) on melting
Figure 10-4. Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.



P-T-t paths for subducted crust
Based on subduction rate of 3 cm/yr (length of each curve = ~15 Ma)
Takes 30-70 Ma to reach equilibrium between subduction and
heating of the slab
Yellow paths =
various arc ages
Red paths =
different ages of
subducted slab
Figure 16-16. Subducted crust
pressure-temperature-time (P-Tt) paths for various situations of
arc age (yellow curves) and age
of subducted lithosphere (red
curves, for a mature ca. 50 Ma
old arc) assuming a subduction
rate of 3 cm/yr (Peacock, 1991,
Phil. Trans. Roy. Soc. London,
335, 341-353).
Subducted Crust
Add solidi for dry and water-saturated melting of basalt
and dehydration curves of likely hydrous phases
Subducted Crust
Figure 16-16. Subducted crust
pressure-temperature-time (P-Tt) paths for various situations of
arc age (yellow curves) and age
of subducted lithosphere (red
curves, for a mature ca. 50 Ma
old arc) assuming a subduction
rate of 3 cm/yr (Peacock, 1991).
Included are some pertinent
reaction curves, including the
wet and dry basalt solidi (Figure
7-20), the dehydration of
hornblende (Lambert and
Wyllie, 1968, 1970, 1972),
chlorite + quartz (Delaney and
Helgeson, 1978). Winter (2001).
An Introduction to Igneous and
Metamorphic Petrology.
Prentice Hall.
1. Dehydration D releases water in mature arcs (lithosphere > 25 Ma)
No slab melting!
2. Slab melting M in
arcs subducting
young lithosphere.
Dehydration of
chlorite or
amphibole releases
water above the
wet solidus 
(Mg-rich) andesites
directly.
Subducted Crust
Mantle Wedge P-T-t Paths


Amphibole-bearing hydrated peridotite should melt at ~ 120 km
Phlogopite-bearing hydrated peridotite should melt at ~ 200 km
 second arc behind first?
Figure 16-18. 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 from Figures 10-5
and 10-6. 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.
Crust and
Mantle
Wedge
Island Arc Petrogenesis
Figure 16-11b. 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.
From Tatsumi (1989), J.
Geophys. Res., 94, 4697-4707
and Tatsumi and Eggins
(1995). Subduction Zone
Magmatism. Blackwell.
Oxford.
Chapter 17: Continental Arc Magmatism
Potential differences with respect to Island Arcs:



Thick sialic crust contrasts greatly with mantlederived partial melts may  more pronounced
effects of contamination
Low density of crust may retard ascent  stagnation
of magmas and more potential for differentiation
Low melting point of crust allows for partial melting
and crustally-derived melts
Chapter 17:
Continental Arc
Magmatism
Figure 17-1. Map of western South America showing
the plate tectonic framework, and the distribution of
volcanics and crustal types. NVZ, CVZ, and SVZ are
the northern, central, and southern volcanic zones.
After Thorpe and Francis (1979) Tectonophys., 57, 5370; Thorpe et al. (1982) In R. S. Thorpe (ed.), (1982).
Andesites. Orogenic Andesites and Related Rocks. John
Wiley & Sons. New York, pp. 188-205; and Harmon et
al. (1984) J. Geol. Soc. London, 141, 803-822. Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
Chapter 17:
Continental Arc
Magmatism
Active volcanic zones
restricted to steeply dipping
parts of the subducting slab
(i.e., 25-30°)
 Inactive areas have
shallower dips (10-15°)

Figure 17-2. Schematic diagram to illustrate how a
shallow dip of the subducting slab can pinch out the
asthenosphere from the overlying mantle wedge.
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
Chapter 17:
Continental Arc
Magmatism
Continental arcs have
the same general trend
as island arcs

Figure 17-3. AFM and K2O vs. SiO2 diagrams
(including Hi-K, Med.-K and Low-K types of Gill,
1981; see Figs. 16-4 and 16-6) for volcanics from the
(a) northern, (b) central and (c) southern volcanic
zones of the Andes. Open circles in the NVZ and
SVZ are alkaline rocks. Data from Thorpe et al.
(1982,1984), Geist (personal communication),
Deruelle (1982), Davidson (personal
communication), Hickey et al. (1986), LópezEscobar et al. (1981), Hörmann and Pichler (1982).
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
Chapter 17: Continental Arc Magmatism
Figure 17-4. Chondrite-normalized REE diagram for selected Andean volcanics. NVZ (6 samples, average SiO2 = 60.7, K2O = 0.66, data
from Thorpe et al. 1984; Geist, pers. comm.). CVZ (10 samples, ave. SiO2 = 54.8, K2O = 2.77, data from Deruelle, 1982; Davidson, pers.
comm.; Thorpe et al., 1984). SVZ (49 samples, average SiO2 = 52.1, K2O = 1.07, data from Hickey et al. 1986; Deruelle, 1982; LópezEscobar et al. 1981). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Chapter 17: Continental Arc Magmatism
Figure 17-6. Sr vs. Nd isotopic ratios for the three zones of the Andes. Data from James et al. (1976), Hawkesworth et al. (1979), James
(1982), Harmon et al. (1984), Frey et al. (1984), Thorpe et al. (1984), Hickey et al. (1986), Hildreth and Moorbath (1988), Geist (pers.
comm), Davidson (pers. comm.), Wörner et al. (1988), Walker et al. (1991), deSilva (1991), Kay et al. (1991), Davidson and deSilva
(1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Chapter 17: Continental Arc Magmatism
Figure 17-9. Relative frequency of rock types in the Andes vs. SW Pacific Island arcs. Data from 397 Andean and 1484 SW Pacific
analyses in Ewart (1982) In R. S. Thorpe (ed.), Andesites. Wiley. New York, pp. 25-95. Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
Chapter 17: Continental Arc Magmatism
Figure 17-23. Schematic cross section of an active continental margin subduction zone, showing the dehydration of the subducting slab,
hydration and melting of a heterogeneous mantle wedge (including enriched sub-continental lithospheric mantle), crustal underplating of
mantle-derived melts where MASH processes may occur, as well as crystallization of the underplates. Remelting of the underplate to
produce tonalitic magmas and a possible zone of crustal anatexis is also shown. As magmas pass through the continental crust they may
differentiate further and/or assimilate continental crust. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice
Hall.

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