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

ComplexlyComplexly

InterrelatedInterrelated

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

Figure 16-3. Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett., 90, 349-370.

a. Alkali vs. silicab. AFM c. FeO*/MgO vs. silica

diagrams for 1946 analyses from ~ 30 island and continental arcs with emphasis on the more primitive volcanics

Chapter 17: Continental Arc MagmatismPotential 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

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.

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), López-Escobar et al. (1981), Hörmann and Pichler (1982). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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.

• 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.

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.

• 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.

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.

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 yellow curves = mantle flow= 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).

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).

The principal source components IA magmas1. 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

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

The principal source components IA magmas

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).

• 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-temperature-time 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.