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
Chapter 17 Continental Arc Magmatism

• Potential differences with respect to Island
  Arcs
• Thick sialic crust contrasts greatly with
  mantle-derived 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

10
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, 53-70 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.
11
Chapter 17 Continental Arc Magmatism
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.
12
Chapter 17 Continental Arc Magmatism
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),
López-Escobar et al. (1981), Hörmann and Pichler
(1982). Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
13
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.
14

• 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.
15

• 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.
16

• 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.
17
Petrogenesis of Island Arc Magmas

• Why is subduction zone magmatism a paradox?

18

• 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

19

• 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).
20
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).
21
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).
22

• 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

23

• 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

24
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.
25

• 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

26

• 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.
27

• 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

28

• 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.
29
Chapter 17 Continental Arc Magmatism
Figure 17-5. MORB-normalized spider diagram
(Pearce, 1983) 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 López-Escobar et al. 1981).
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
30
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.