G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 15 Island Arc Magmatism Slides courtesy of George Winter ( March 2, 2009

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.

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 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 Figure Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett., 90, 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 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. K 2 O 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 Combined K 2 O - 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 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Tholeiitic vs. Calc-alkaline differentiation C-A shows continually increasing SiO 2 and lacks dramatic Fe enrichment Tholeiitic silica in the Skaergård Intrusion No change

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 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 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

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 (< 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).

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

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 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.

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, and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.

Perhaps the more common low-Mg ( 17wt% Al 2 O 3 ) 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, 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