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. The initial petrologic model: Oceanic crust is partially melted
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
Partial melts should be less mafic than their parent
Ultramafic mantle  mafic basalt
Basaltic oceanic crust  intermediate andesites

3. Ocean-ocean  Island Arc (IA) Ocean-continent  Continental Arc or
Active Continental Margin (ACM)
Figure 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
Orogenic and Subduction-related synonyms when referring to the common association of basalts, basaltic andesites, andesites, dacites, and rhyolites produced at subduction zones
= “orogenic suite”

5. Structure of an Island Arc
Note mantle flow directions (induced drag), isolated wedge, and upwelling to  back-arc basin spreading system
Benioff-Wadati seismic zone (x x x x)
Volcanic Front
h is relatively constant  depth is important
Figure 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)
All three series occur in SZ setting, yet something about SZ is different that  CALC-ALKALINE
Calc-alkaline magma series is used as yet another synonym to orogenic suite by some workers
Since other magma series can occur at subduction zones, I recommend that we use the term calc-alkaline strictly to denote a chemical magma series, not a tectonic association

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 Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett., 90,

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 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 ; and Harmon et al. (1984) J. Geol. Soc. London, 141, Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

11. Chapter 17: Continental Arc Magmatism
Figure 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 AFM and K2O vs. SiO2 diagrams (including Hi-K, Med.-K and Low-K types of Gill, 1981; see Figs 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 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 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

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 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
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.

16. 10Be/Betotal vs. B/Betotal diagram (Betotal  9Be since 10Be is so rare)
Figure Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88,
Each arc studied formed linear arrays, each arc having a unique slope
Also shown are other known reservoirs, including typical mantle (virtually no 10Be or B), hydrated and altered oceanic crust (high B, low 10Be), and young pelagic oceanic sediments (low B and 10Be/Be extending off the diagram up to 2000)
The simplest explanation: each arc represents a mixing line between a mantle reservoir (near the origin) and a fluid (or melt) reservoir, that is specific for each arc and itself a mixture of slab crust and sediment
Hypothetical fields for each arc are illustrated, but the exact location along the extrapolated line is unknown

17. Petrogenesis of Island Arc Magmas
Why is subduction zone magmatism a paradox?
Paradox: Great quantities of magma are generated in regions where cool lithosphere is being subducted into the mantle and isotherms are depressed, not elevated
No adequate petrogenetic model can be derived without considering the thermal regime in subduction zones

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 (< Ma according to Peacock, 1991)
yellow curves = mantle flow
This is sufficiently representative
Figure Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, ) 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 Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, ) 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
The last three sources on the list are unlikely to play much of a role
Lithospheric mantle of subducting plate (4) is already refractory, due to the extraction of MORB at the ridge, and it heats very little in the upper 200 km of the subduction zone
Asthenosphere beneath the slab (5) flows with it, but does not heat much at all, since the isotherms are essentially parallel to the flow lines at these depths
Arc crust: isotherms at the base of the (predominantly andesitic) crust  the temperature is too low for melting, even under hydrous conditions
The overriding crustal component is considered to be minor in island arcs, but is much more important in active continental margins
Figure Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res., 98, ) 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
Since we know what the general composition of the constituents in Fig are, it is a matter of combining this information with the other information in the figure showing us the pressure-temperature conditions to which the constituents will be subjected as they move through the subduction zone, and considering the consequences

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
Oceanic crust, as it subducts, will begin to heat at about km depth, and will continue to heat with rising pressure, although slowly due to the depressed isotherms
Mantle wedge material will follow the path of drag-induced flow, also illustrated in Fig This is less well known, but should follow a path (like the arrows beginning in the center of the right edge of the figure) of initial cooling from about 1100oC to about 800oC at nearly constant pressure, and then heat up toward 1000oC as pressure increases

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, and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.
Altered oceanic crust begins to dehydrate at depths ~ 50 km or less, as chlorite, phengite, and other hydrous phyllosilicates decompose
Further dehydration takes place at greater depths as other hydrous phases become unstable, including amphibole at about 3 GPa.
The slab crust is successively converted to blueschist, amphibolite, and finally anhydrous eclogite as it reaches about km depth
In most (mature) arcs, the temperature in the subducted crust is below the wet solidus for basalt, so the released water cannot cause melting, and most of the water is believed to rise into the overlying mantle wedge, where it reacts with the lherzolite to form a pargasitic amphibole and probably phlogopite (yellowish area)
Slightly hydrous mantle immediately above the slab is carried downward by induced convective flow where it heats up, dehydrates, and melts at A (120 km)

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
Mass-balance calculations suggest that the contribution of the subducted sediments is only a few percent in most arc systems, but the result is the high LIL/HFS patterns
The nearly closed-cell induced flow in the wedge may result in progressive depletion of the wedge as arc magmas are extracted. This provides an explanation of some HREE and more compatible trace element data, which in many cases is more depleted than MORB

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
If this occurs above the wet peridotite solidus, a second phase of melting will occur at B a position appropriate for the secondary volcanic chain that exists behind the primary chain in several island arcs
The P-T-t paths are nearly parallel to the solidus, and may be above it or below it. Thus dehydration may or may not be accompanied by melting, so that the development of a second arc will depend critically upon the thermal and flow regime of a particular arc
Melting initiated by the breakdown of the potassium-rich mica will probably be more potassic, as is true in most secondary arc occurrences.
The K-h relationship probably more complex, reflecting the decreasing quantity of H2O with depth and thus the degree of partial melting, as well as the depth of melting (which becomes more alkaline with depth), and perhaps the vertical length of the rising melt diapir column within the mantle wedge
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, 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 (>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
Here fractional crystallization of olivine and augite in the presence of water can produce the low-Mg high-alumina basalts and basaltic andesites observed
Further rise of these hydrous basalts  volatile loss & partial crystallization
Magmas that do reach the surface are therefore highly phyric basaltic andesites and andesites. More evolved liquids are probably the products of fractional crystallization in shallow chambers

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, and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.
In the shallower chambers the calc-alkaline fractionation trend takes place in a hydrous magma with the fractionation of magnetite, hornblende, and/or a highly anorthitic plagioclase, as discussed previously
The restriction of calc-alkaline magmas to subduction zones may thus result from the uniquely high water content or the thickened arc crust that causes the primary tholeiites to pond and fractionate
The SZ environment is a complex one, and the generation of arc magmas reflects a number of sources and stages
The wedge may be heterogeneous and variably depleted/enriched, the subducted material has a variety of crustal and sedimentary constituents
The thermal and flow patterns are variable, and the nature of the fluid is poorly constrained and probably variable as well

29. Chapter 17: Continental Arc Magmatism
Figure 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 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.