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.

l Igneous activity is related to convergent plate situations that result in the subduction of one plate beneath another l The initial petrologic model: F Oceanic crust is partially melted F Melts rise through the overriding plate to form volcanoes just behind the leading plate edge F Unlimited supply of oceanic crust to melt

Structure of an Island Arc 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 joules/cm 2 /sec)

Volcanic Rocks of Island Arcs l Complex tectonic situation and broad spectrum l High proportion of basaltic andesite and andesite F Most andesites occur in subduction zone settings

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

Tholeiitic vs. Calc-alkaline differentiation Figure From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Trace Elements l REEs F Slope within series is similar, but height varies with FX due to removal of Ol, Plag, and Pyx F (+) slope of low-K  DM s Some even more depleted than MORB F Others have more normal slopes F Thus heterogeneous mantle sources F 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.

l New Britain, Marianas, Aleutians, and South Sandwich volcanics plot within a surprisingly limited range of DM Isotopes Figure Nd-Sr isotopic variation in some island arc volcanics. MORB and mantle array from Figures and 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 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: F dip of the slab F frictional heating F endothermic metamorphic reactions F metamorphic fluid flow are now thought to play only a minor role

l Typical thermal model for a subduction zone l 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 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).

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

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 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).

l Left with the subducted crust and mantle wedge l The trace element and isotopic data suggest that both contribute to arc magmatism. How, and to what extent? F Dry peridotite solidus too high for melting of anhydrous mantle to occur anywhere in the thermal regime shown F LIL/HFS ratios of arc magmas  water plays a significant role in arc magmatism

Effect of adding volatiles (especially H 2 O) on melting Figure Figure Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.

l P-T-t paths for subducted crust l Based on subduction rate of 3 cm/yr (length of each curve = ~15 Ma) l Takes Ma to reach equilibrium between subduction and heating of the slab Yellow paths = various arc ages Subducted Crust Figure 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, ). Red paths = different ages of subducted slab

Add solidi for dry and water-saturated melting of basalt and dehydration curves of likely hydrous phases and dehydration curves of likely hydrous phases Figure 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 releases water in mature arcs (lithosphere > 25 Ma) 1.Dehydration D releases water in mature arcs (lithosphere > 25 Ma) No slab melting! 2. Slab melting in arcs subducting young lithosphere. 2. Slab melting M in arcs subducting young lithosphere. Dehydration of chlorite or amphibole releases water above the wet solidus  (Mg-rich) andesites directly. Dehydration of chlorite or amphibole releases water above the wet solidus  (Mg-rich) andesites directly. Subducted Crust

Mantle Wedge P-T-t Paths

l Amphibole-bearing hydrated peridotite should melt at ~ 120 km l Phlogopite-bearing hydrated peridotite should melt at ~ 200 km  second arc behind first? Crust and Mantle Wedge Figure Some calculated P-T-t paths for peridotite in the mantle wedge as it follows a path similar to the flow lines in Figure 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 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, and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.

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

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, ; 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.

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

Chapter 17: Continental Arc Magmatism Figure AFM and K 2 O vs. SiO 2 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. F Continental arcs have the same general trend as island arcs

Chapter 17: Continental Arc Magmatism Figure Chondrite-normalized REE diagram for selected Andean volcanics. NVZ (6 samples, average SiO 2 = 60.7, K 2 O = 0.66, data from Thorpe et al. 1984; Geist, pers. comm.). CVZ (10 samples, ave. SiO 2 = 54.8, K 2 O = 2.77, data from Deruelle, 1982; Davidson, pers. comm.; Thorpe et al., 1984). SVZ (49 samples, average SiO 2 = 52.1, K 2 O = 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 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.

Chapter 17: Continental Arc Magmatism Figure 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 Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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.