Chapter 16. Island Arc Magmatism

Report
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
• 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. After Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer.
Subduction Products
• Characteristic igneous associations
• Distinctive patterns of metamorphism
• Orogeny and mountain belts
Complexly
Interrelated
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
• 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.
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
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
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.
• 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
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
– 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.
• 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.
10Be/Be
rare)
total
vs. B/Betotal diagram (Betotal  9Be since 10Be is so
Figure 16-14. 10Be/Be(total)
vs. B/Be for six arcs. After
Morris (1989) Carnegie Inst.
of Washington Yearb., 88,
111-123.
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).
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
• 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
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.
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
• 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.
• 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 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.
Chapter 17: Continental Arc Magmatism
Figure 17-5. MORB-normalized spider diagram (Pearce, 1983) for selected Andean volcanics. NVZ (6 samples, average SiO 2 = 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.
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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