Slide 1Slide 2Structure of an Island ArcVolcanic Rocks of Island ArcsMajor Elements and Magma SeriesSlide 6Slide 7Tholeiitic vs. Calc-alkaline differentiationSlide 9Other TrendsTrace ElementsSlide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGYLecture 15Island Arc MagmatismSlides courtesy of George Winter (http://www.whitman.edu/geology/winter/)March 2, 2009Ocean-ocean  Island Arc (IA)Ocean-continent  Continental Arc orActive 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 ArcFigure 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 VolcanicLocality 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 andesiteA = andesite, D = dacite, R = rhyoliteIsland Arc Rock TypesMajor Elements and Magma SeriesFigure 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 volcanicsFigure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.K2O is an important discriminator – 3 sub-series6 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 mixedTholeiitic vs. Calc-alkaline differentiationFigure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Tholeiitic vs. Calc-alkaline differentiationC-A shows continually increasing SiO2 and lacks dramatic Fe enrichment Tholeiitic silica in the Skaergård IntrusionNo changeNo changeOther 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 commonTrace 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 garnetFigure 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 enrichmentFigure 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 MagmasOf the many variables that can affect the isotherms in subduction zone systems, the main ones are: 1) the rate of subduction2) the age of the subduction zone3) the age of the subducting slab4) the extent to which the subducting slab induces flow in the mantle wedgeOther 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 slowerb) 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 flowFigure 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 agesSubducted CrustFigure 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 slabAdd solidi for dry and water-saturated melting of basalt and dehydration curves of likely hydrous phasesFigure 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,