PowerPoint PresentationSlide 2Structure of an Island ArcVolcanic Rocks of Island ArcsSlide 5Tholeiitic vs. Calc-alkaline differentiationTrace ElementsSlide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Mantle Wedge P-T-t PathsSlide 19Slide 20Chapter 17: Continental Arc MagmatismSlide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Ocean-ocean Ocean-ocean Island ArcIsland Arc (IA) (IA)Ocean-continent Ocean-continent Continental Arc Continental Arc ororActive 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 meltedMelts 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 edgeUnlimited supply of oceanic crust to meltUnlimited supply of oceanic crust to meltStructure of an Island ArcStructure 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 ArcsVolcanic Rocks of Island ArcsComplex tectonic situation and broad spectrumComplex tectonic situation and broad spectrumHigh proportion of High proportion of basaltic andesitebasaltic andesite and and andesiteandesiteMost andesites occur in subduction zone settingsMost 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 TypesFigure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Tholeiitic vs. Calc-alkaline differentiationTholeiitic vs. Calc-alkaline differentiationFigure 16-6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Trace ElementsTrace ElementsREEsREEsSlope 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 DMDMSome even more depleted than Some even more depleted than MORBMORBOthers have more normal slopesOthers have more normal slopesThus Thus heterogeneous mantleheterogeneous mantle sourcessourcesHREE flat, so HREE flat, so no deep garnetno 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.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 DMIsotopesIsotopesFigure 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 subduction2)2) the the ageage of the subduction of the subduction zonezone3)3) the the ageage of the subducting of the subducting slabslab4)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 wedgeOther factors, such as:Other factors, such as:dip of the slabdip of the slabfrictional heatingfrictional heatingendothermic metamorphic reactionsendothermic metamorphic reactionsmetamorphic fluid flow metamorphic fluid flow are now thought to play only a minor roleare now thought to play only a minor roleTypical thermal model for a subduction zoneTypical thermal model for a subduction zoneIsotherms 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 slowerb)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 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).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 slab1a1a 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 sediments1c1c Seawater trapped in pore spaces Seawater trapped in pore spaces2.2. The The
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