Chapter16IslandArcMagmatismMichiganTechnological16章岛弧岩浆密歇根科技精选课件

1、Chapter 16. Island Arc MagmatismArcuate volcanic island chains along subduction zonesDistinctly different from mainly basaltic provinces thus farComposition more diverse and silicicBasalt generally subordinateMore explosive Strato-volcanoes most common volcanic landform第1页，共30页。Igneous activity is r

2、elated to convergent plate situations that result in the subduction of one plate beneath another The initial petrologic model:Oceanic crust is partially meltedMelts rise through the overriding plate to form volcanoes just behind the leading plate edgeUnlimited supply of oceanic crust to melt第2页，共30页

3、。Ocean-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 P

4、etrogenesis, Allen Unwin/Kluwer.第3页，共30页。Subduction ProductsCharacteristic igneous associationsDistinctive patterns of metamorphism Orogeny and mountain belts ComplexlyInterrelated第4页，共30页。Structure of an Island ArcFigure 16-2. Schematic cross section through a typical island arc after Gill (1981),

5、Orogenic Andesites and Plate Tectonics. Springer-Verlag. HFU= heat flow unit (4.2 x 10-6 joules/cm2/sec)第5页，共30页。Volcanic Rocks of Island ArcsComplex tectonic situation and broad spectrumHigh proportion of basaltic andesite and andesiteMost andesites occur in subduction zone settings第6页，共30页。Major E

6、lements and Magma SeriesTholeiitic (MORB, OIT)Alkaline (OIA)Calc-Alkaline ( restricted to SZ)第7页，共30页。Major 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

7、for 1946 analyses from 30 island and continental arcs with emphasis on the more primitive volcanics第8页，共30页。Chapter 17: Continental Arc MagmatismPotential differences with respect to Island Arcs:Thick sialic crust contrasts greatly with mantle-derived partial melts may more pronounced effects of con

8、taminationLow density of crust may retard ascent stagnation of magmas and more potential for differentiationLow melting point of crust allows for partial melting and crustally-derived melts第9页，共30页。Chapter 17: Continental Arc MagmatismFigure 17-1. Map of western South America showing the plate tecto

9、nic 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.

10、John Wiley & Sons. New York, pp. 188-205; and Harmon et al. (1984) J. Geol. Soc. London, 141, 803-822. Winter (2019) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.第10页，共30页。Chapter 17: Continental Arc MagmatismFigure 17-2. Schematic diagram to illustrate how a shallow dip of th

11、e subducting slab can pinch out the asthenosphere from the overlying mantle wedge. Winter (2019) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.第11页，共30页。Chapter 17: Continental Arc MagmatismFigure 17-3. AFM and K2O vs. SiO2 diagrams (including Hi-K, Med.-K and Low-K types of Gi

12、ll, 1981; see Figs. 16-4 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

13、), Hickey et al. (1986), Lpez-Escobar et al. (1981), Hrmann and Pichler (1982). Winter (2019) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.第12页，共30页。Trace ElementsREEsSlope within series is similar, but height varies with FX due to removal of Ol, Plag, and Pyx(+) slope of low-

14、K DMSome even more depleted than MORBOthers have more normal slopesThus heterogeneous mantle sourcesHREE 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 refe

15、rence (from Sun and McDonough, 1989). After Gill (1981) Orogenic Andesites and Plate Tectonics. Springer-Verlag. 第13页，共30页。MORB-normalized Spider diagramsIntraplate OIB has typical humpFigure 14-3. Winter (2019) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and M

16、cDonough (1989) In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. 313-345.第14页，共30页。Figure 16-11a. MORB-normalized spider diagrams for selected island arc basalts. Using the normalization and ordering scheme of Pearce (1983) with LIL on t

17、he 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 diagramsIA: decoupled HFS - LIL (LIL are hydrophilic)What is it about subduction zone setting that causes fluid-assisted enrichment?Figure 14-3.

18、Winter (2019) 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.第15页，共30页。10Be/Betotal vs. B/Betotal diagram (Betotal 9Be sinc

19、e 10Be is so rare)Figure 16-14. 10Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88, 111-123. 第16页，共30页。Petrogenesis of Island Arc MagmasWhy is subduction zone magmatism a paradox? 第17页，共30页。Of the many variables that can affect the isotherms in subducti

20、on 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 slabfrictional heatingendothermic metamorphic reactionsmetamorph

21、ic fluid flow are now thought to play only a minor role第18页，共30页。Typical thermal model for a subduction zoneIsotherms 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 (8wt% MgO) high alum

22、ina basalts may be primary, as may some andesites, but most surface lavas have compositions too evolved to be primaryPerhaps the more common low-Mg (17wt% Al2O3) types are the result of somewhat deeper fractionation of the primary tholeiitic magma which ponds at a density equilibrium position at the

23、 base of the arc crust in more mature arcs 第27页，共30页。Fractional crystallization thus takes place at a number of levelsFigure 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 under

24、goes partial melting as amphibole (A) and phlogopite (B) dehydrate. From Tatsumi (1989), J. Geophys. Res., 94, 4697-4707 and Tatsumi and Eggins (2019). Subduction Zone Magmatism. Blackwell. Oxford.第28页，共30页。Chapter 17: Continental Arc MagmatismFigure 17-5. MORB-normalized spider diagram (Pearce, 198

25、3) 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; Lpez