Chapter 16' Island Arc Magmatism

1
Chapter 16. Island Arc Magmatism

• Arcuate volcanic island chains along subduction
  zones
• Distinctly different from mainly basaltic
  provinces thus far
• Composition more diverse and silicic
• Basalt generally subordinate
• More explosive
• Strato-volcanoes most common volcanic landform

2

• 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

3

• 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.
4
Subduction Products

• Characteristic igneous associations
• Distinctive patterns of metamorphism
• Orogeny and mountain belts

Complexly Interrelated
5
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)
6
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

7
Major Elements and Magma Series

• Tholeiitic (MORB, OIT)
• Alkaline (OIA)
• Calc-Alkaline ( restricted to SZ)

8
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.
9
Sub-series of Calc-Alkaline

• K2O is an important discriminator ? 3 sub-series

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).
10
Figure 16-6. a. 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.
11
Figure 16-6. b. AFM diagram distinguishing
tholeiitic and calc-alkaline series. Arrows
represent differentiation trends within a series.
12
Figure 16-6. c. FeO/MgO vs. SiO2 diagram
distinguishing tholeiitic and calc-alkaline
series.
13
Figure 16-6. From Winter (2001) An Introduction
to Igneous and Metamorphic Petrology. Prentice
Hall.
14
Figure 16-6. c. FeO/MgO vs. SiO2 diagram
distinguishing tholeiitic and calc-alkaline
series.
15
Figure 16-6. c. FeO/MgO vs. SiO2 diagram
distinguishing tholeiitic and calc-alkaline
series.
16

• 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. calc-alkaline 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).
17
Tholeiitic vs. Calc-alkaline differentiation
Figure 16-6. From Winter (2001) An Introduction
to Igneous and Metamorphic Petrology. Prentice
Hall.
18
Tholeiitic vs. Calc-alkaline differentiation

• C-A shows continually increasing SiO2 and lacks
  dramatic Fe enrichment

Tholeiitic silica in the Skaergård Intrusion
No change
19
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

20
Figure 16-8. 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.
21
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

22
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 ? 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.
23

• MORB-normalized Spider diagrams
• Intraplate OIB has typical hump

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.
24

• MORB-normalized Spider diagrams
• IA decoupled HFS - LIL (LIL are hydrophilic)

What is it about subduction zone setting that
causes fluid-assisted enrichment?
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.
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.
25
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).
26
Figure 16-13. Variation in 207Pb/204Pb vs.
206Pb/204Pb for oceanic island arc volcanics.
Included are the isotopic reservoirs and the
Northern Hemisphere Reference Line (NHRL)
proposed in Chapter 14. The geochron represents
the mutual evolution of 207Pb/204Pb and
206Pb/204Pb in a single-stage homogeneous
reservoir. Data sources listed in Wilson (1989).
27

• 10Be created by cosmic rays oxygen and nitrogen
  in upper atmos.
• ? Earth by precipitation readily ? clay-rich
  oceanic seds
• 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
  (lt1 x 106 atom/g) and 10Be/9Be is lt5 x 10-14

28

• 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

29

• 10Be/Betotal vs. B/Betotal diagram (Betotal ?
  9Be since 10Be 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.
30
Petrogenesis of Island Arc Magmas

• Why is subduction zone magmatism a paradox?

31

• 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

32

• 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 (lt50-100 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, 8309-8319) and mantle flow
lines (yellow- after Tatsumi and Eggins, 1995,
Subduction Zone Magmatism. Blackwell. Oxford).
33
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, 8309-8319) and mantle flow
lines (yellow- after Tatsumi and Eggins, 1995,
Subduction Zone Magmatism. Blackwell. Oxford).
34
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, 8309-8319) and mantle flow
lines (yellow- after Tatsumi and Eggins, 1995,
Subduction Zone Magmatism. Blackwell. Oxford).
35

• 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

36

• 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

37

• 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
Subducted Crust
Red paths different ages of subducted slab
Figure 16-16. Subducted crust pressure-temperature
-time (P-T-t) 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).
38
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-T-t) 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.
39

• Dehydration D releases water in mature arcs
  (lithosphere gt 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
40

• 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

41
Mantle Wedge P-T-t Paths
42

• Amphibole-bearing hydrated peridotite should melt
  at 120 km
• Phlogopite-bearing hydrated peridotite should
  melt at 200 km
• ? second arc behind first?

Crust and Mantle Wedge
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.
43
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.
44

• A multi-stage, multi-source process
• 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?)
• The mantle wedge provides the HFS and other
  depleted and compatible element characteristics

45

• 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.
46

• 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 (gt8wt 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 (lt 6 wt.  MgO),
  high-Al (gt17wt 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

47

• 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.
48
Figures not used
Figure 16-9. Major phenocryst mineralogy of the
low-K tholeiitic, medium-K calc-alkaline, and
high-K calc-alkaline magma series. B basalt, BA
basaltic andesite, A andesite, D dacite, R
rhyolite. Solid lines indicate a dominant
phase, whereas dashes indicate only sporadic
development. From Wilson (1989) Igneous
Petrogenesis, Allen-Unwin/Kluwer.
49
Figures not used
Figure 16-11b. MORB-normalized spider diagrams
for selected island arc basalts. Using the
normalization and ordering scheme of Sun and
McDonough (1989) with increasing compatibility to
the right. Data from BVTP. OIB 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.