Chapter 16. Island Arc Magmatism

Transcript Chapter 16. Island Arc Magmatism

Chapter 16. Island Arc Magmatism

• Arcuate volcanic island chains along subduction zones • Distinctly different from mainly basaltic provinces thus far – Composition more diverse and silicic – Basalt generally subordinate – More explosive – Strato-volcanoes most common volcanic landform

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

Ocean-ocean  Ocean-continent  Island Arc (IA) Continental Arc or Active 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.

Subduction Products

• Characteristic igneous associations • Distinctive patterns of metamorphism • Orogeny and mountain belts Complexly Interrelated

Structure of an Island Arc

Figure 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/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

Table 16-1

. Relative Proportions of Quaternary Volcanic Island Arc Rock Types

Locality

Talasea, Papua Little Sitkin, Aleutians Mt. Misery, Antilles (lavas) Ave. Antilles Ave. Japan (lava, ash falls)

9 0 17 17 14 A = andesite, D = dacite, R = rhyolite

B-A

23 78 22 42 85

55 4 49 After Gill (1981, Table 4.4) B = basalt B-A = basaltic andesite

9 18 12 39 2

4 0 0 2 0

Major Elements and Magma Series • Tholeiitic (MORB, OIT) • Alkaline (OIA) • Calc-Alkaline (~ restricted to SZ)

Major Elements and Magma Series

Alkali vs. silica AFM FeO*/MgO vs. silica diagrams for 1946 analyses from ~ 30 island and continental arcs with emphasis on the more primitive volcanics

Figure 16-3. Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett., 90, 349-370.

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

Chapter 17: Continental Arc Magmatism

Figure 17-1. 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, 53 70; Thorpe et al. (1982) In R. S. Thorpe (ed.), (1982). Andesites. Orogenic Andesites and Related Rocks. John Wiley & Sons. New York, pp. 188-205; and Harmon et al. (1984) J. Geol. Soc. London, 141, 803-822. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Chapter 17: Continental Arc Magmatism

Figure 17-2. 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.

Chapter 17: Continental Arc Magmatism

Figure 17-3. AFM and K 2 O vs. SiO 2 diagrams (including Hi-K, Med.-K and Low-K types of Gill, 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), Hickey et al. (1986), López Escobar et al. (1981), Hörmann and Pichler (1982). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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  DM • Some even more depleted than MORB – Others have more normal slopes – Thus heterogeneous mantle sources – HREE flat, so no deep garnet

Figure 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.

• MORB-normalized Spider diagrams – Intraplate OIB has typical hump

Figure 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.

• MORB-normalized Spider diagrams – IA: decoupled HFS - LIL (LIL are hydrophilic) What is it about subduction zone setting that causes fluid-assisted enrichment?

Figure 14-3.

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.

10 Be/Be total rare) vs. B/Be total diagram (Be total  9 Be since 10 Be is so

Figure 16-14.

10 Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88, 111-123.

Petrogenesis of Island Arc Magmas

• Why is subduction zone magmatism a paradox?

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 (<50-100 Ma according to Peacock, 1991) yellow curves = mantle flow

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 1.

The crustal  IA magmas 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

Figure 16-15.

The principal source components  IA magmas 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

Figure 16-15.

• Left with the subducted crust and mantle wedge • The trace element and isotopic data suggest that both contribute to arc magmatism. How, and to what extent?

– Dry peridotite solidus too high for melting of anhydrous mantle to occur anywhere in the thermal regime shown – LIL/HFS ratios of arc magmas  significant role in arc magmatism water plays a

• The sequence of pressures and temperatures that a rock is subjected to during an interval such as burial, subduction, metamorphism, uplift, etc. is called a pressure-temperature time or P-T-t path

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, 4697-4707 and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.

Island Arc Petrogenesis

A multi-stage, multi-source process

• Dehydration of the slab provides the LIL, signatures 10 Be, B, etc. enrichments + enriched Nd, Sr, and Pb isotopic – These components, plus other dissolved silicate materials, are transferred to the wedge in a fluid phase (or melt?) • The mantle wedge provides the HFS and other depleted and compatible element characteristics

• 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, 4697-4707 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 • Some high-Mg (>8wt% MgO) high alumina basalts may be primary, as may some andesites, but most surface lavas have compositions too evolved to be primary • Perhaps the more common low-Mg (< 6 wt. % MgO), high-Al (>17wt% Al mature arcs 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

• Fractional crystallization thus takes place at a number of levels

Chapter 17: Continental Arc Magmatism

Figure 17-5. MORB-normalized spider diagram (Pearce, 1983) 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 17-23. 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.

Chapter 16. Island Arc Magmatism

Transcript Chapter 16. Island Arc Magmatism