GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY Lecture 15 Island Arc Magmatism Slides courtesy of George Winter (http://www.whitman.edu/geology/winter/) March 2, 2009

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GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 15
Island Arc Magmatism
Slides courtesy of George Winter
(http://www.whitman.edu/geology/winter/)
March 2, 2009
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.
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
Major Elements and Magma
Series
a. Alkali vs. silica
b. AFM
c. FeO*/MgO vs. silica
diagrams for 1946 analyses from ~
30 island and continental arcs
with emphasis on the more
primitive volcanics
Figure 16-3. Data compiled by Terry
Plank (Plank and Langmuir, 1988)
Earth Planet. Sci. Lett., 90, 349-370.
K2O is an important discriminator – 3 sub-series
Figure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
6 sub-series if combine tholeiite and C-A (some are rare)
May choose 3 most common:

Low-K tholeiitic

Med-K C-A

Hi-K mixed
Figure 16-5. Combined K2O - FeO*/MgO diagram in which the Low-K to High-K series are combined with the tholeiitic vs. calcalkaline types, resulting in six andesite series, after Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. The
points represent the analyses in the appendix of Gill (1981).
Tholeiitic vs. Calc-alkaline differentiation
Figure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Tholeiitic vs. Calc-alkaline differentiation
Tholeiitic silica in the
Skaergård Intrusion
No change
C-A shows continually increasing
SiO2 and lacks dramatic Fe
enrichment
Other Trends

Spatial
“K-h”: low-K tholeiite near trench  C-A  alkaline as
depth to seismic zone increases
 Some along-arc as well

Antilles  more alkaline N  S
Aleutians is segmented with C-A
prevalent in segments and tholeiite
prevalent at ends


Temporal

Early tholeiitic  later C-A and often latest alkaline is
common
Trace Elements

REEs
Slope within series is similar,
but height varies with FX due to
removal of Ol, Plag, and Pyx
 (+) slope of low-K  Depleted
Mantle (DM)


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.

MORB-normalized Spider diagrams

Large Ion Lithophiles (LIL - are hydrophilic) – Evidence
for fluid assisted enrichment
Figure 14-3. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall. Data from Sun
and McDonough (1989) In A. D. Saunders and M. J. Norry
(eds.), Magmatism in the Ocean Basins. Geol. Soc. London
Spec. Publ., 42. pp. 313-345.
Figure 16-11a. 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.
Petrogenesis of Island Arc Magmas

Why is subduction zone magmatism a paradox?
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).

P-T-t paths for subducted crust

Based on subduction rate of 3 cm/yr (length of each curve = ~15 Ma)
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

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.


Phlogopite is stable in ultramafic rocks beyond the conditions at which
amphibole breaks down
P-T-t paths for the wedge reach the phlogopite-2-pyroxene dehydration
reaction at about 200 km depth
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.
 Perhaps
the more common low-Mg (< 6 wt. %
MgO), high-Al (>17wt% Al2O3) types are the result
of somewhat deeper fractionation of the primary
tholeiitic magma which ponds at a density
equilibrium position at the base of the arc crust in
more mature arcs
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

Fractional
crystallization
thus takes
place at a
number of
levels
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.

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