GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 15

Island Arc Magmatism

Slides courtesy of George Winter (http://www.whitman.edu/geology/winter/)

March 2, 2009

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.

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

settingsTable 16-1. Relative Proportions of Quaternary Volcanic

Locality B B-A A D RTalasea, Papua 9 23 55 9 4Little Sitkin, Aleutians 0 78 4 18 0Mt. Misery, Antilles (lavas) 17 22 49 12 0Ave. Antilles 17 42 39 2Ave. Japan (lava, ash falls) 14 85 2 0After Gill (1981, Table 4.4) B = basalt B-A = basaltic andesite

A = andesite, D = dacite, R = rhyolite

Island Arc Rock Types

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

Figure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

K2O is an important discriminator – 3 sub-series

6 sub-series if combine tholeiite and C-A (some are rare)

May choose 3 most common:

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).

Low-K tholeiitic

Med-K C-A

Hi-K mixed

Tholeiitic vs. Calc-alkaline differentiation

Figure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Tholeiitic vs. Calc-alkaline differentiation

C-A shows continually increasing SiO2 and lacks dramatic Fe

enrichment

Tholeiitic silica in the Skaergård Intrusion

No

chan

geN

o ch

ange

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

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 Depleted Mantle (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.

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 Large Ion Lithophiles (LIL - are hydrophilic) –

Evidence for 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.

Why is subduction zone magmatism a paradox?

Petrogenesis of Island Arc Magmas

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).

P-T-t paths for subducted crust Based on subduction rate of 3 cm/yr (length of each curve = ~15 Ma)

Yellow paths = Yellow paths = various arc agesvarious arc ages

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, Phil. Trans. Roy. Soc. London, 335, 341-353).

Red paths = Red paths = different ages of different ages of subducted slabsubducted slab

Add solidi for dry and water-saturated melting of basalt

and dehydration curves of likely hydrous phases

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.

Subducted Crust

1. Dehydration D releases water in mature arcs (lithosphere > 25 Ma)

No slab melting!

2.2. Slab Slab meltingmelting M in in arcs subducting arcs subducting young lithosphere.young lithosphere.

Dehydration of Dehydration of chlorite or chlorite or amphibole releases amphibole releases water water aboveabove the the wet solidus wet solidus (Mg- (Mg-rich) andesites rich) andesites directly. directly.

Subducted Crust

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.

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.

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.

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.

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