Arc Magmatism - University at Buffalo

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Arc Magmatism
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Winter, Chapter 16
Island Arc Magmatism
• Activity along arcuate volcanic island chains
along subduction zones
• Distinctly different from the mainly basaltic
provinces
– Composition more diverse and silicic
– Basalt generally occurs in subordinate
quantities
– More explosive than the quiescent basalts
– 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
• 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
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. SAfter Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer.
Subduction Products
• Characteristic igneous associations
• Distinctive patterns of metamorphism
• Orogeny and mountain belts
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
Major Elements and Magma
Series
• Tholeiitic (MORB, OIT)
• Alkaline (OIA)
• Calc-Alkaline (~ restricted to SZ)
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.
Sub-series of Calc-Alkaline
• K2O is an important discriminator  3 sub-series
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).
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.
FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.
From Winter (2001)
FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.
FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.
Tholeiitic vs. Calc-alkaline
From Winter (2001)
Tholeiitic vs. Calc-alkaline
Tholeiitic silica in the
Skaergård Intrusion
No change
C-A shows continually
increasing SiO2 and lacks
dramatic Fe enrichment
Calc-alkaline differentiation
– Early crystallization of an Fe-Ti oxide phase
Probably related to the high water content of
calc-alkaline magmas in arcs, dissolves 
high fO2
– High water pressure also depresses the
plagioclase liquidus and  more An-rich
– As hydrous magma rises, DP  plagioclase
liquidus moves to higher T  crystallization of
considerable An-rich-SiO2-poor plagioclase
– The crystallization of anorthitic plagioclase and
low-silica, high-Fe hornblende is an alternative
mechanism for the observed calc-alkaline
differentiation trend
– E
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.
• REEs
Trace Elements
– Slope within series is
similar, but height varies
with FX due to removal of
Ol, Plag, and Pyx
– (+) slope of low-K  DM
• Some even more depleted than
MORB
– Others have more normal
slopes
– Thus heterogeneous mantle
sources
– HREE flat, so no deep garnet
REE diagrams for some representative Low-K (tholeiitic), MediumK (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
– Intraplate OIB has typical hump
Winter (2001) 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.
• MORB-normalized Spider diagrams
– IA: decoupled HFS - LIL (LIL are hydrophilic)
What is it about subduction zone setting that
causes 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
Cross section of a
subduction zone
showing isotherms (redafter 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
Cross section of a
subduction 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 components  IA magmas
2.
3.
4.
5.
The mantle wedge between the slab and the arc crust
The arc crust
The lithospheric mantle of the subducting plate
The asthenosphere beneath the slab
Cross section of a
subduction 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).
• 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
• 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-temperaturetime 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) source
• Thus modern opinion has swung toward the
non-melted slab for most cases
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?
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).
Crust and
Mantle
Wedge
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. From
Tatsumi (1989), J.
Geophys. Res., 94,
4697-4707 and
Tatsumi and Eggins
(1995). Subduction
Zone Magmatism.
Blackwell. Oxford.
Multi-stage, Multi-source
Process
• Dehydration of the slab provides the LIL
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?)
• The mantle wedge provides the HFS and other
depleted and compatible element characteristics
• 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
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.
• 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
• Some high-Mg (>8wt% MgO) high alumina basalts may
be primary, as may some andesites, but most surface lavas
have compositions too evolved to be primary
• 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
Fractional crystallization occurs at various
levels
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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