Volcanic Arcs Chapters 16 and 17 Oceanocean convergence

Volcanic Arcs, Chapters 16 and 17

Ocean-ocean convergence Island Arc (IA) Ocean-continent convergence Continental Arc 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.

Arcs are: Arcuate volcanic chains above subduction zones n Distinctly different from mainly basaltic provinces thus far n – – Compositions more diverse Basalt generally subordinate More explosive: viscous, cool, magmas trap gas Strato-volcanoes most common volcanic landform

Chapter 16. Island Arc Magmatism

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 of rock types n High proportion of Basaltic - andesite and Andesite – Most Andesites occur in subduction zone settings n

Recall Major Magma Series n Alkaline series (OIA ocean island alkaline) n Sub-alkaline types: – Tholeiitic series (MORB, OIT) – Calc-Alkaline series (IA island arcs) C-A ~ restricted to magmas generated near subduction zones, but keep in mind other series occur there too

Major Magma Series visualized with Major Elements a. Alkali vs. silica all b. AFM for subalkaline c. Fe. O*/Mg. O vs. silica Diagrams for 1, 946 analyses from ~ 30 volcanic island arcs and continental arcs Figure 16 -3. Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett. , 90, 349 -370.

Not all volcanic arcs above a subduction zone are calc-alkaline. Figure 16 -6. b. AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows represent differentiation trends within a series.

Sub-series Calc-Alkaline n K 2 O is an important discriminator Gill (1981) recognized three Andesite 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 RKOC 76 (Carnegie Institute of Washington).

Figure 16 -6. a. K 2 O-Si. O 2 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 K 2 O at low Si. O 2. After Gill, 1981, Orogenic Andesites and Plate Tectonics. Springer-Verlag.

If partition on basis of K versus Tholeiitic/calcalkaline, most common samples are: – Low-K tholeiitic – Med-K C-A – Hi-K mixed Figure 16 -5. Combined K 2 O - Fe. O*/Mg. O 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).

Tholeiitic vs. Calc-alkaline differentiation for our three examples Figure 16 -6. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Tholeiitic vs. Calc-alkaline differentiation seems to depend on K C-A shows continually increasing Si. O 2 and lacks dramatic Fe enrichment High K

Calc-alkaline differentiation WHY? http: //www. springerlink. com/content/u 38311872 w 004 w 16/ – Early (as opposed to late in Tholeiites) crystallization of an Fe-Ti oxide phase. Probably related to the high water content of calc-alkaline magmas in arcs – Iron is removed early so a middle fractionation high iron composition cannot occur as it does in Tholeiites

n Spatial Other Trends § Antilles more alkaline N S § Aleutians segmented with C-A prevalent in center and tholeiite prevalent at ends § IDEA: source/collection points for high K clays (Illite) near trench? n Temporal – Early Tholeiitic later C-A and often latest alkaline is common

n REEs Trace Elements – HREE flat in all, – so garnet, which sequesters the HREEs, not in equilibrium with the melt – Garnet last to go in partial melting of Lherzolite. If melted, HREE would be high. – also not from subducted basalt, which becomes eclogite with garnet at 110 km. The HREE are flat, implying that garnet, which strongly partitions (holds) the HREE, was not in equilibrium with the melt. Melts derived from eclogite are depleted in HREE (abundant garnet in residue). This causes the characteristic low HREE Figure 16 -10

n MORB-normalized Spider diagrams – IA: high LIL (LIL are hydrophilic), low HFS What is it about subduction zone setting that causes fluid-assisted enrichment? Intraplate OIB has similar hump HFS=High Field-strength Most incompatible Figure 14 -4. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and Mc. Donough (1989) In A. D. Saunders and M. J. Norry (eds. ), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ. , 42. pp. 313 -345. Figure 16 -11 a. 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.

Isotopes n New Britain, Marianas, Aleutians, and South Sandwich volcanics plot show sediment contamination of DM Antilles (Atlantic) and Banda and New Zealand (Pacific) can be explained by partial melting of a MORB-type source + the addition of the type of sediment that exist on the subducting plate (Pacific sediment has 87 Sr/86 Sr around 0. 715 and 143 Nd/144 Nd around 0. 5123) The increasing N-S Antilles Nd enrichment probably related to the increasing proximity of the southern end to the South American sediment source of the Amazon Figure 16 -12. Nd-Sr isotopic variation in some island arc volcanics. MORB and mantle array from Figures 13 -11 and 1015. After Wilson (1989), Arculus and Powell (1986), Gill (1981), and Mc. Culloch et al. (1994). Atlantic sediment data from White et al. (1985).

Pb in some arcs overlap with the MORB data; depleted mantle component is a major reservoir for subduction zone magmas Majority of data enriched in radiogenic lead (207 Pb and 206 Pb), trending toward the appropriate oceanic marine sedimentary reservoir Figure 16 -13. Variation in 207 Pb/204 Pb vs. 206 Pb/204 Pb for oceanic island arc volcanics. Included are the isotopic reservoirs and the Northern Hemisphere Reference Line (NHRL) proposed in Chapter 14. The geochron represents the mutual evolution of 207 Pb/204 Pb and 206 Pb/204 Pb in a single-stage homogeneous reservoir. Data sources listed in Wilson (1989).

10 Be created by cosmic rays + oxygen and nitrogen in upper atmos. – Earth by precipitation & readily clay-rich oceanic sediments – Half-life of only 1. 5 Ma (long enough to be subducted, but quickly lost to mantle systems). After about 10 Ma 10 Be is no longer detectable. 9 Be is stable, natural. – 10 Be/9 Be averages about 5000 x 10 -11 in the uppermost oceanic sediments – In mantle-derived MORB and OIB magmas, & continental crust, 10 Be is below detection limits (<1 x 106 atom/g) and 10 Be/9 Be is <5 x 10 -14

Boron 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

10 Be/Be 9 10 total vs. B/Betotal diagram (Betotal Be since Be is so rare). This is the smoking gun, the evidence for the fluids (mostly ion-rich water) squeezed out of the sediments. Figure 16 -14. 10 Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb. , 88, 111 -123.

The potential source components IA magmas 1. The crustal portion of the subducted slab 1 a Altered oceanic crust (hydrated by circulating seawater, and metamorphosed in large part to greenschist facies) 1 b Subducted oceanic and forearc sediments 1 c Seawater trapped in pore spaces 2. The mantle wedge between the slab and the arc crust Figure 16 -15. Cross section of a subduction zone showing isotherms (red-after Furukawa, 1993, J. Geophys. Res. , 98, 83098319) and mantle flow lines (yellow- after Tatsumi and Eggins, 1995, Subduction Zone Magmatism. Blackwell. Oxford).

Not 1 a the subducted basalt fide flat HREEs n The trace element and isotopic data suggest that both 1 b and 1 c, the subducted sediments and water and 2, the mantle wedge contribute to arc magmatism. How, and to what extent? – Dry peridotite solidus too high for melting of anhydrous mantle to occur anywhere in thermal regime shown – LIL/HFS ratios of arc magmas water plays a significant role in arc magmatism n

Freezing Point Depression always occurs in a mixture

Even small amounts of water (0. 5%) and carbon dioxide (0. 5%) strongly depress the temperatures of the solidus, moving it below the geotherm at all depths. This effect dominates in subduction environments, where arc magmas are generated. (Modified from B. M. Wilson (1989) Igneous petrogenesis: a global tectonic approach. Chapman and Hall, London. ) An upside -down PT diagram Effects of the addition of small amounts of volatiles to mantle Iherzolite. A mantle adiabat with potential temperature of 1280 °C is shown for reference.

Amphibole-bearing hydrated peridotite should melt at ~ 120 km n Phlogopite-bearing hydrated peridotite should melt at ~ 200 km second arc behind first? n 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

n The data from LIL Large Ion Lithophiles and HFS High Field Strength trace elements underscore the importance of slab-derived water and a MORB-like mantle wedge source n The flat HREE pattern argues against a garnet-bearing (eclogite) source n Thus modern opinion has swung toward a non-melting subducted lithosphere slab model for most cases of IA genesis

Island Arc Petrogenesis Model Phlogopite is stable in ultramafic rocks beyond the conditions at which amphibole breaks down n P-T-t paths for the wedge reach the phlogopite-2 -pyroxene dehydration reaction at about 200 km depth n Figure 16 -11 b. 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. Mantle here is too shallow to have Garnet. Subducted slab turns to Eclogite with Garnet at 110 km.

Chapter 17: Continental Arc Magmatism NVZ, CVZ, and SVZ are the northern, central, and southern volcanic zones. Figure 17 -1.

Continental Volcanic Arcs n Potential differences with respect to Island Arcs: – Assimilation of thick silica-rich crust versus mantle-derived partial melts more pronounced effects of contamination – Low density of crust may slow magma ascent more potential for differentiation – Low melting point of crust allows for partial melting and some crust-derived melts

A subducting slab with shallow dip can pinch out the asthenosphere from the overlying mantle wedge Lithospheric Mantle too shallow to have garnet 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.

SVZ has a flat HREE which suggests a SVZ shallow garnet-free source NVZ and CVZ have a steep slope with CVZ depleted HREE which suggests a deep garnet rich source, (the garnets don’t melt) consistent with a steep slab dip angle and aesthenosphere source. Figure 17 -4. Chondrite-normalized REE diagram for selected Andean volcanics. NVZ (6 samples, average Si. O 2 = 60. 7, K 2 O = 0. 66, data from Thorpe et al. 1984; Geist, pers. comm. ). CVZ (10 samples, ave. Si. O 2 = 54. 8, K 2 O = 2. 77, data from Deruelle, 1982; Davidson, pers. comm. ; Thorpe et al. , 1984). SVZ (49 samples, average Si. O 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.

LILs are very soluble in aqueous fluids. LIL enrichment of the mantle wedge via aqueous fluids from dehydration of the subducting slab and sediments. Similar to Island Arcs Figure 17 -5. MORB-normalized spider diagram (Pearce, 1983) for selected Andean volcanics. NVZ (6 samples, average Si. O 2 = 60. 7, K 2 O = 0. 66, data from Thorpe et al. 1984; Geist, pers. comm. ). CVZ (10 samples, ave. Si. O 2 = 54. 8, K 2 O = 2. 77, data from Deruelle, 1982; Davidson, pers. comm. ; Thorpe et al. , 1984). SVZ (49 samples, average Si. O 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.

Assimilation Recall low 143 Nd/144 Nd and high 87 Sr/86 Sr is due to an isotopically enriched source such as continental crust contamination. The CVZ exhibits substantial crustal contamination 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. ), Wörner et al. (1988), Walker et al. (1991), de. Silva (1991), Kay et al. (1991), Davidson and de. Silva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Andean Pb enrichments are not much greater than OIBs, and could be derived almost solely from a subducted sediment Figure 17 -7. 208 Pb/204 Pb vs. 206 Pb/204 Pb and 207 Pb/204 Pb vs. 206 Pb/204 Pb for Andean volcanics plotted over the OIB fields from Figures 14 -7 and 14 -8. 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. ), Wörner et al. (1988), Walker et al. (1991), de. Silva (1991), Kay et al. (1991), Davidson and de. Silva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Andean chemistry is similar to Island Arcs. They also have as their main source the depleted mantle above the subducted slab. However, Andean volcanics are more evolved, as they must pass through continental lithosphere, which has a lower melting point than the rising magma. 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 -11. Schematic cross sections of a volcanic arc showing (a) an initial state followed by (b) trench migration toward the continent resulting in a destructive boundary and subduction erosion of the overlying crust. (c) Alternatively, trench migration away from the continent 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. http: //geoweb. princeton. edu/events/abstract_talk_Princeton. pdf

Figure 17 -10. Map of the Juan de Fuca plate-Cascade Arc system Also shown are the approximate locations of the subduction zone as it migrated westward to its present location.

§ Hundreds to thousands of individual intrusions §The range of volcanics from basalts to rhyolites is matched by the plutonics: §Gabbro -> diorite -> tonalite -> granodiorite -> granite Q Quartzolite 90 90 Quartz-rich Granitoid 60 par eld s 20 Alk ali F lite Granodiorite na Granite To Gra n ite 60 20 Quartz Monzonite Quartz Syenite A 10 Syenite 35 Monzonite Quartz Monzodiorite 65 Monzodiorite 5 90 P Figure 17 -15 a. 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 -15 b. Major plutons of the South American Cordillera, a principal segment of a continuous Mesozoic. Tertiary belt from the Aleutians to Antarctica. After USGS.

Granitoid magmas rise to, and freeze at, similar shallow subvolcanic levels of the crust. 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.

Consistent with fractional crystallization of plagioclase and pyroxene +/- magnetite, later giving away to hornblende and biotite , from initial gabbroic, tonalitic, or quartz diorite parental material Notice that the great majority of Peruvian samples are calc-alcaline Figure 17 -17. Harker-type and AFM variation diagrams for the Coastal batholith of Peru. Data span several suites from W. S. Pitcher, M. P. Atherton, E. J. Cobbing, and R. D. Beckensale (eds. ), Magmatism at a Plate Edge. The Peruvian Andes. Blackie. Glasgow.

Coastal Peru batholiths have the same REE profiles as coastal Peru volcanics Figure 17 -18. Chondrite-normalized REE abundances for the Linga and Tiybaya super-units of the Coastal batholith of Peru and associated volcanics. From Atherton et al. (1979) In M. P. Atherton and J. Tarney (eds. ), Origin of Granite Batholiths: Geochemical Evidence. Shiva. Kent. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Lima segment intruded into younger, thinner crust so radiogenic 87 Sr low, reflecting the mantle derived parent. Arequipa intrudes and assimilated old thick crust so 87 Sr high. Lima segment has high 206 Pb reflecting minor assimilation of Pacific sediments Figure 17 -19. a. Initial 87 Sr/86 Sr ranges for three principal segments of the Coastal batholith of Peru (after Beckinsale et al. , 1985) in W. S Pitcher, M. P. Atherton, E. J. Cobbing, and R. D. Beckensale (eds. ), Magmatism at a Plate Edge. The Peruvian Andes. Blackie. Glasgow, pp. 177 -202. . b. 207 Pb/204 Pb vs. 206 Pb/204 Pb data for the plutons (after Mukasa and Tilton, 1984) in R. S. Harmon and B. A. Barreiro (eds. ), Andean Magmatism: Chemical and Isotopic Constraints. Shiva. Nantwich, pp. 235 -238. ORL = Ocean Regression Line for depleted mantle sources (similar to oceanic crust). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Why are granitoids so abundant? Experiments show Tonalites (granitoids with low K-spar) can be formed by the partial fusion remelting of gabbroic magmas under hydrous conditions. a. Up-arched mantle results in partial melting and underplate gabbros. b. During later compression, heat added by more underplate magmas remelts the underplate gabbros to produce tonalites. 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.

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.