ocean-ocean island arc (ia) ocean-continent continental arc or active continental margin (acm)...

Ocean-ocean Ocean-ocean Island ArcIsland Arc (IA) (IA)Ocean-continent Ocean-continent Continental Arc Continental Arc oror

Active Continental MarginActive Continental Margin (ACM) (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.

Igneous activity is related to convergent Igneous activity is related to convergent plate situations that result in the subduction plate situations that result in the subduction of one plate beneath another of one plate beneath another

The The initialinitial petrologic model: petrologic model: Oceanic crust is partially meltedOceanic crust is partially melted Melts rise through the overriding plate to Melts rise through the overriding plate to

form volcanoes just behind the leading form volcanoes just behind the leading plate edgeplate edge

Unlimited supply of oceanic crust to meltUnlimited supply of oceanic crust to melt

Structure of an Island ArcStructure 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 ArcsVolcanic Rocks of Island Arcs

Complex tectonic situation and broad spectrumComplex tectonic situation and broad spectrum High proportion of High proportion of basaltic andesitebasaltic andesite and and andesiteandesite

Most andesites occur in subduction zone settingsMost andesites occur in subduction zone settings

Table 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

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

Tholeiitic vs. Calc-alkaline differentiationTholeiitic vs. Calc-alkaline differentiation

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

Trace ElementsTrace Elements REEsREEs

Slope within series is similar, but Slope within series is similar, but height varies with FX due to height varies with FX due to removal of Ol, Plag, and Pyxremoval of Ol, Plag, and Pyx

(+) slope of low-K (+) slope of low-K DMDM Some even more depleted than Some even more depleted than

MORBMORB Others have more normal slopesOthers have more normal slopes Thus Thus heterogeneous mantleheterogeneous mantle

sourcessources HREE flat, so HREE flat, so no deep garnetno 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.

New Britain, Marianas, Aleutians, and South Sandwich New Britain, Marianas, Aleutians, and South Sandwich volcanics plot within a surprisingly limited range of DMvolcanics plot within a surprisingly limited range of DM

IsotopesIsotopes

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).

Of the many variables that can affect the isotherms in Of the many variables that can affect the isotherms in subduction zone systems, the main ones are: subduction zone systems, the main ones are:

1)1) the the raterate of subduction of subduction

2)2) the the ageage of the subduction of the subduction zonezone

3)3) the the ageage of the subducting of the subducting slabslab

4)4) the extent to which the subducting slab induces the extent to which the subducting slab induces flow in the mantle wedgeflow in the mantle wedge

Other factors, such as:Other factors, such as: dip of the slabdip of the slab frictional heatingfrictional heating endothermic metamorphic reactionsendothermic metamorphic reactions metamorphic fluid flow metamorphic fluid flow

are now thought to play only a minor roleare now thought to play only a minor role

Typical thermal model for a subduction zoneTypical thermal model for a subduction zone Isotherms will be higher (i.e. the system will be hotter) if Isotherms will be higher (i.e. the system will be hotter) if

a)a) the convergence rate is slowerthe convergence rate is slower

b)b) the subducted slab is young and near the ridge (warmer) the subducted slab is young and near the ridge (warmer)

c)c) the arc is young (<50-100 Ma according to Peacock, 1991) 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).


The principal source components The principal source components IA magmas IA magmas1.1. The The crustalcrustal portion of the portion of the subducted slabsubducted slab

1a1a Altered oceanic crust (hydrated by circulating seawater, Altered oceanic crust (hydrated by circulating seawater, and metamorphosed in large part to greenschist facies)and metamorphosed in large part to greenschist facies)

1b1b Subducted oceanic and forearc sediments Subducted oceanic and forearc sediments

1c1c Seawater trapped in pore spaces Seawater trapped in pore spaces

2.2. The The mantle wedgemantle wedge between the slab and the arc crust between the slab and the arc crust

3.3. The The arc crustarc crust

4.4. The The lithosphericlithospheric mantle of the subducting plate mantle of the subducting plate

5.5. The The asthenosphereasthenosphere beneath the slab beneath the slab

The principal source components The principal source components IA magmas IA magmas


Left with the Left with the subducted crustsubducted crust and and mantle wedgemantle wedge The trace element and isotopic data suggest that The trace element and isotopic data suggest that bothboth

contribute to arc magmatism. How, and to what contribute to arc magmatism. How, and to what extent?extent? DryDry peridotite solidus too high for melting of peridotite solidus too high for melting of

anhydrous mantle to occur anywhere in the anhydrous mantle to occur anywhere in the thermal regime shownthermal regime shown

LIL/HFS ratios of arc magmas LIL/HFS ratios of arc magmas waterwater plays a plays a significant role in arc magmatismsignificant role in arc magmatism

Effect of Effect of adding volatilesadding volatiles (especially (especially HH22OO) on melting) on melting

Figure 10-4. Figure 10-4. Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.

P-T-t paths for P-T-t paths for subducted crustsubducted crust Based on subduction rate of 3 cm/yr (length of each curve = ~15 Ma)Based on subduction rate of 3 cm/yr (length of each curve = ~15 Ma) Takes 30-70 Ma to reach equilibrium between subduction and heating Takes 30-70 Ma to reach equilibrium between subduction and heating

of the slabof the slab

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 Add solidisolidi for dry and water-saturated melting of basalt for dry and water-saturated melting of basalt

and and dehydration curvesdehydration curves of likely hydrous phases 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

1.1. DehydrationDehydration D releases water in mature arcs (lithosphere > 25 Ma) releases water in mature arcs (lithosphere > 25 Ma)

No slab melting!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

Mantle Wedge P-T-t PathsMantle Wedge P-T-t Paths

AmphiboleAmphibole-bearing hydrated peridotite should -bearing hydrated peridotite should meltmelt at ~ 120 km at ~ 120 km PhlogopitePhlogopite-bearing hydrated peridotite should -bearing hydrated peridotite should meltmelt at ~ 200 km at ~ 200 km

second arcsecond arc behind first? 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 PetrogenesisIsland 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.

Chapter 17: Continental Arc MagmatismChapter 17: Continental Arc MagmatismPotential differences with respect to Island Arcs:Potential differences with respect to Island Arcs:

Thick sialic crust contrasts greatly with mantle-Thick sialic crust contrasts greatly with mantle-derived partial melts may derived partial melts may more pronounced more pronounced effects of contaminationeffects of contamination

Low density of crust may retard ascent Low density of crust may retard ascent stagnation stagnation of magmas and more potential for differentiationof magmas and more potential for differentiation

Low melting point of crust allows for partial melting Low melting point of crust allows for partial melting and crustally-derived meltsand crustally-derived melts

Chapter 17: Chapter 17: Continental Arc Continental Arc

MagmatismMagmatism

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.


MagmatismMagmatism

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.

Active volcanic zones Active volcanic zones restricted to steeply dipping restricted to steeply dipping parts of the subducting slab parts of the subducting slab (i.e., 25-30°)(i.e., 25-30°) Inactive areas have Inactive areas have shallower dips (10-15°)shallower dips (10-15°)


MagmatismMagmatism

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.

Continental arcs have Continental arcs have the same general trend the same general trend as island arcsas island arcs

Chapter 17: Continental Arc MagmatismChapter 17: Continental Arc Magmatism

Figure 17-4. Chondrite-normalized REE diagram 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.


Figure 17-6. Sr vs. Nd isotopic ratios for the three zones of the Andes. Data from James et al. (1976), Hawkesworth et al. (1979), James (1982), Harmon et al. (1984), Frey et al. (1984), Thorpe et al. (1984), Hickey et al. (1986), Hildreth and Moorbath (1988), Geist (pers. comm), Davidson (pers. comm.), Wörner et al. (1988), Walker et al. (1991), deSilva (1991), Kay et al. (1991), Davidson and deSilva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


Figure 17-9. Relative frequency of rock types in the Andes vs. SW Pacific Island arcs. Data from 397 Andean and 1484 SW Pacific analyses in Ewart (1982) In R. S. Thorpe (ed.), Andesites. Wiley. New York, pp. 25-95. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


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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