Chapter 16- Island Arcs - Stellenbosch University

Chapter 16- Island Arcs - Stellenbosch University

Subduction zone magmatism Activity along arcuate volcanic island chains along subduction zones Distinctly different from the mainly basaltic provinces thus far

Composition more diverse and silicic Basalt generally occurs in subordinate quantities Also more explosive than the quiescent basalts Strato-volcanoes are the most common volcanic landform

Economic geology Gold, copper, etc. as hydrothermal deposits around plutons (cf. Andes Chile) Submarine alteration of volcanic/volcanoclastic rocks occasionally precipitates (or concentrates) Cu Zn Pb

Ocean-ocean Island Arc (IA) Ocean-continent 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. SAfter Wilson (1989) Igneous Petrogenesis, Allen Unwin/Kluwer. Subduction Products

Characteristic igneous associations Distinctive patterns of metamorphism Orogeny and mountain belts Complexly Interrelated Island vs. Continental arc:

Continental arcs have Thicker lithosphere (deeper melting?/melting of slightly different mantle?) Thicker crust: possible interactions with preexisting

crust/lithosphere Island arcs are simpler as they allow to focus on the primary processes 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/cm2/sec) Location of

the volcanic arc Whatever the dip of the Benioff plane, the (main) arc is 100 km above the slab 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) B 9 0 17 17 14

B-A 23 78 22 42 85 A 55 4

49 D 9 18 0 39 2 after Gill (1981, Table 4.4) B = basalt B-A = basaltic andesite A = andesite, D = dacite,

R = rhyolite R 4 0 0 2 0 Major Elements and Magma Series

Tholeiitic (MORB, OIT) Alkaline (OIA) Calc-Alkaline (~ restricted to subduction zones) Arc alkaline series

Arc calc-alkaline (B-BA-A-D-R) Arc tholeites Island-arc subalkaline series Fresh Andesite, note black color, and fracturing

Oregon Andesite, note amp -120 cleavage, biotite - brown, augite green, plag zoned Andesite subhedral phenocryst of plag and pyroxene in fine grained Matrix Zoned plag in andesite Dacite, with zoned plag, quartz (untwinned), in fine grained matrix

Perlitic cracks in rhyolite, magnetite, and alkaline feldspar Rhyolite in glass alkaline phenocrysts with glass inclusions, mag crystals Perlitic cracks. Flow texture in rhyolite brown color due to devitrification Welded tuff Devitrification in rhyolite, spherulites

Island arc alkaline series Trachyte, alkaline felspar, no twinning, in fine matrix, gas vesicles dark patches Trachytic texture (aligned feldspars caused flow in a viscose melt) Trachyte, K-spar untwinned 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 Major Elements and Magma Series 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 16-3. Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett., 90, 349-370. Sub-series of Calc-Alkaline K2O is an important discriminator 3 sub-series Figure 16-4. The three andesite series of Gill (1981)

Orogenic Andesites and Plate Tectonics. Springer-Verlag. Contours represent the concentration of 2500 analyses of andesites stored in the large data file RKOC76 (Carnegie Institute of Washington). Figure 16-6. a. K2O-SiO2 diagram distinguishing high-K, medium-K and low-K series. Large squares = high-K, stars = med.-K, diamonds = low-K series from Table 16-2. Smaller symbols are identified in the caption. Differentiation within a series (presumably dominated by fractional crystallization) is indicated by the arrow. Different primary magmas (to the left) are distinguished by

vertical variations in K2O at low SiO2. After Gill, 1981, Orogenic Andesites and Plate Tectonics. Springer-Verlag. Figure 16-6. b. AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows represent differentiation trends within a series. Figure 16-6. c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series. Figure 16-6. c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series. Figure 16-6. c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.

6 sub-series if combine tholeiite and C-A (some are rare) May choose 3 most common: Low-K tholeiitic Med-K C-A Hi-K mixed Figure 16-5. Combined K2O - FeO*/MgO diagram in which the Low-K to High-K series are combined with the tholeiitic vs. calcalkaline 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). Figure 16-9. Major phenocryst

mineralogy of the low-K tholeiitic, medium-K calc-alkaline, and high-K calc-alkaline magma series. B = basalt, BA = basaltic andesite, A = andesite, D = dacite, R = rhyolite. Solid lines indicate a dominant phase, whereas dashes indicate only sporadic development. From Wilson (1989) Igneous Petrogenesis, Allen-Unwin/Kluwer.

Trace elements Decoupling of LIL and HFS (compare OIB) Nb-Ta anomaly No fractionnation

MREE/HREE 1) Role of fluids (as opposed to unifromally enriched source) 2) Nb-Ta rich phases in the residuum (Ti-oxides: rutile) 3) No Garnet in the residuum Volatile rich andesite, Oregon Bombs in Andesite Isotopes

New Britain, Marianas, Aleutians, and South Sandwich volcanics plot within a surprisingly limited range of DM Figure 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). Be created by cosmic rays + oxygen and nitrogen in upper atmos. Earth by precipitation & readily clay-rich oceanic seds 10

Half-life of only 1.5 Ma (long enough to be subducted, but quickly lost to mantle systems). After about 10 Ma 10Be is no longer detectable 10

In mantle-derived MORB and OIB magmas, & continental crust, 10Be is below detection limits (<1 x 106 atom/g) and 10Be/9Be is <5 x 10-14 Be/9Be averages about 5000 x 10-11 in the uppermost oceanic sediments B is a stable element Very brief residence time deep in subduction zones

B in recent sediments is high (50-150 ppm), but has a greater affinity for altered oceanic crust (10-300 ppm) In MORB and OIB it rarely exceeds 2-3 ppm Be/Betotal vs. B/Betotal diagram (Betotal 9Be since 10Be is so rare)

10 Figure 16-14. 10Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88, 111-123. In summary

Role of fluids (LIL/HFS) Role of subducted matter (Be/B) Multiple sites of melting! (diversity of series) No garnet but rutile in the residuum Thermal structure of subduction zones Possible sources?

Arc crust Unlikely (too thin in island arcs anyway) Mantle Unlikely (solidus too high + role of water)

Subducted crust Mantle + subducted fluids Possible?

Can the subducted slab melt? P-T path along the subducted slab Subducted Crust Figure 16-16. Subducted crust pressure-temperature-time (P-Tt) 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). Subduction zone magmatism (part II) Island arc magmas:

Arc tholeites (low K, high Fe/Mg) Calc-alkaline (med K, low Fe/Mg) alkaline (high K) In summary

Role of fluids (LIL/HFS) Role of subducted matter (Be/B) Multiple sites of melting! (diversity of series) No garnet but rutile in the residuum Slab vs. Mantle melting 1. Dehydration D and liberation of water takes place (mature arcs with lithosphere > 25 Ma old)

2. Slab melting M occurs arcs subducting young lithosphere, as dehydration of chlorite or amphibole release water above the wet solidus to form Mgrich andesites directly.

3. BUT slab melting occurs (when it occurs) in garnet stability field Subducted Crust Gt-in Garnet stability in mafic rocks

From a dozen of experimental studies Well-constrained grt-in line at about 10-12 kbar

The LIL/HFS trace element data underscore the importance of slab-derived water and a MORB-like mantle wedge source The flat HREE pattern argues against a garnet-bearing (eclogite) source Thus modern opinion has swung toward the

non-melted slab for most cases although thermal modelling suggests that slab can melt in specific case (cf. adakites) Amphibole-bearing hydrated peridotite should melt at ~ 120 km Phlogopite-bearing hydrated peridotite should melt at ~ 200 km second arc behind first? (K-richer)

Figure 16-18. Some calculated P-T-t paths for peridotite in the mantle wedge as it follows a path similar to the flow lines in Figure 16-15. 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 10-6. 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. Crust and Mantle Wedge 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, 4697-4707

and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford. A multi-stage, multi-source process Dehydration of the slab provides the LIL, 10Be, B,

etc. enrichments + enriched Nd, Sr, and Pb isotopic signatures 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 Continental Arc Magmatism Potential differences with respect to Island Arcs:

Thick sialic crust contrasts greatly with mantlederived 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

Rock types Subduction related lavas No big difference with island arcs (at least in terms of

minerals and majors) Tholeites less common I-type granitoids See examples in previous lectures (Himalaya) Mafic terms uncommon (mostly granites) Figure 17-9. Relative frequency of rock types in the Andes vs. SW Pacific Island arcs. Data from 397 Andean and 1484 SW Pacific

analyses in Ewart (1982) In R. S. Thorpe (ed.), Andesites. Wiley. New York, pp. 25-95. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figure 17-3. AFM and K2O vs. SiO2 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), LpezEscobar et al. (1981), Hrmann and Pichler (1982). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Rock types Subduction related lavas

No big difference with island arcs (at least in terms of minerals and majors) Tholeites less common I-type granitoids See examples in previous lectures (Himalaya)

Mafic terms uncommon (mostly granites) Hornblende granodiorite Hbl-Biotite granodiorite Figure 17-15b. Major plutons of the South American Cordillera, a principal segment of a continuous Mesozoic-Tertiary belt from the Aleutians to Antarctica. After USGS.

Figure 17-15a. Major plutons of the North American Cordillera, a principal segment of a continuous Mesozoic-Tertiary belt from the Aleutians to Antarctica. After Anderson (1990, preface to The Nature and Origin of Cordilleran Magmatism. Geol. Soc. Amer. Memoir, 174. The Sr 0.706 line in N. America is after Kistler (1990), Miller and Barton (1990) and Armstrong (1988). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 17-16. Schematic cross section of the Coastal batholith of Peru. The shallow flat-topped and steepsided bell-jar-shaped plutons are stoped into place. Successive pulses may be nested at a single locality. The heavy line is the present erosion surface. From Myers (1975) Geol. Soc. Amer. Bull., 86, 1209-1220. Continental arc magmas: why are they more silicic?

Crustal contamination of andesitic magmas Extreme differenciation of andesitic magmas Melting of the continental crust Melting of less basic lithologies (i.e., basalts rather than peridotites) Slab? Lower crust/underplated basalts?

1) Crustal influence 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, 5370; 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. Figure 17-5. MORB-normalized spider diagram (Pearce, 1983) for selected Andean volcanics. NVZ (6 samples, average SiO 2 = 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, Figure 17-6. Sr vs. Nd isotopic ratios for the three zones of the Andes. Data from James et al. (1976), Hawkesworth et al. (1979), James (1982), Harmon et al. (1984), Frey et al. (1984), Thorpe et al. (1984), Hickey et al. (1986), Hildreth and Moorbath (1988), Geist (pers. comm), Davidson (pers. comm.), Wrner et al. (1988), Walker et al. (1991), deSilva (1991), Kay et al. (1991), Davidson and deSilva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 17-8. 87Sr/86Sr, D7/4, D8/4, and d18O vs. Latitude for the Andean volcanics. 7/4 and 8/4 are indices of 207Pb and 208Pb enrichment over the NHRL values of Figure 17-7 (see Rollinson, 1993, p. 240). Shaded areas are estimates for mantle and MORB isotopic ranges from Chapter 10. Data from James et al. (1976),

Hawkesworth et al. (1979), James (1982), Harmon et al. (1984), Frey et al. (1984), Thorpe et al. (1984), Hickey et al. (1986), Hildreth and Moorbath (1988), Geist (pers. comm), Davidson (pers. comm.), Wrner et al. (1988), Walker et al. (1991), deSilva (1991), Kay et al. (1991), Davidson and deSilva (1992). Winter (2001) An Introduction to Igneous and

Metamorphic Petrology. Prentice Hall. 2) Differenciation Horblendite cumulates Thick crust leaves time for fractionnation (FC)

But not always consistent with isotopes etc. would require too many cumulates proportions felsic/mafic not right 3) Melting of the CC

Paired S- and I-types granitic belts Link with convergence rate (and crust thickness) paired I and S type granitic belts in Peru Figure 17-12. Time-averaged rates of extrusion of mafic (basalt and basaltic

andesite), andesitic, and silicic (dacite and rhyolite) volcanics (Priest, 1990, J. Geophys. Res., 95, 19583-19599) and Juan de Fuca-North American plate convergence rates (Verplanck and Duncan, 1987 Tectonics, 6, 197-209) for the past 35 Ma. The volcanics are poorly exposed and sampled, so the timing should be considered tentative. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Figure 17-11. Schematic cross sections of a volcanic arc showing an initial state (a) followed by trench migration toward the continent (b), resulting in a destructive boundary and subduction erosion of the overlying crust. Alternatively, trench migration away from the continent (c) results in extension and a constructive boundary. In this case the extension in (c) is accomplished by roll-back of the subducting plate. An alternative method involves a jump of the subduction zone away from the continent, leaving a

segment of oceanic crust (original dashed) on the left of the new trench. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. But Isotopes do not (always) match a purely crustal origin

Cf. Pseudo-S types in the Cordillera Blanca Batholith (Pieter) 4) Melting of more silicic lithologies KD Gt/melt Yb = 10 - 20

(other minerals 1) Fractionated HREE Figure 17-22. Range and average chondrite-normalized rare earth element patterns for tonalites from the three zones of the Peninsular Ranges batholith. Data from Gromet and Silver (1987) J. Petrol., 28, 75-125. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Garnet stability in mafic rocks

From a dozen of experimental studies Well-constrained grt-in line at about 10-12 kbar KD Gt/melt Yb

= 10 - 20 (other minerals 1) Garnet must be present

Most probable: metabasalts (garnet-bearing crustal rocks are metasediments -> granites should be Stypes) Slab melts or underplated basalts? Slab melt thermally unlikely at least in this case Underplated basalts: possible from seismic, gravi studies + gabbro outcrops

Occasionally: partially molten mafic lower crust in exhumed arcs (Fjordland, New Zealand) Partial melting of dioritic gneisses in exhumed arcs (N. Zealand) Garnet associated with leucosomes (incongruent melting, Hbl + Pg = L + Grt) Daczo et al. 2001 Two stage model

Figure 17-20. Schematic diagram illustrating (a) the formation of a gabbroic crustal underplate at an continental arc and (b) the remelting of the underplate to generate tonalitic plutons. After Cobbing and Pitcher (1983) in J. A. Roddick (ed.), Circum-Pacific Plutonic Terranes. Geol. Soc. Amer. Memoir, 159. pp. 277-291. Continental arc magmas

Multiple sources: Normal andesites (hydrated mantle) Re-melting of the continental crust Melting of basalts Slab melts (unlikely except in special cases cf adakites) Underplated basalts

Differenciation (FC) Mixing between these types of magmas Contamination by the CC

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