1. Subduction zone magmatism

2. 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

3. 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

4. Ocean-ocean  Island Arc (IA) Ocean-continent  Continental Arc or
Active Continental Margin (ACM)
Figure 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.

5. Subduction Products Characteristic igneous associations
Distinctive patterns of metamorphism
Orogeny and mountain belts
Complexly
Interrelated
Orogenic and Subduction-related synonyms when referring to the common association of basalts, basaltic andesites, andesites, dacites, and rhyolites produced at subduction zones
= “orogenic suite”

6. 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

7. Structure of an Island Arc
Note mantle flow directions (induced drag), isolated wedge, and upwelling to  back-arc basin spreading system
Benioff-Wadati seismic zone (x x x x)
Volcanic Front
h is relatively constant  depth is important
Figure 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)

8. Location of the volcanic arc
Whatever the dip of the Benioff plane, the (main) arc is 100 km above the slab

9. 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

10. Major Elements and Magma Series
Tholeiitic (MORB, OIT)
Alkaline (OIA)
Calc-Alkaline (~ restricted to subduction zones)
All three series occur in SZ setting, yet something about SZ is different that  CALC-ALKALINE
Calc-alkaline magma series is used as yet another synonym to orogenic suite by some workers
Since other magma series can occur at subduction zones, I recommend that we use the term calc-alkaline strictly to denote a chemical magma series, not a tectonic association

11. Arc alkaline series
Arc calc-alkaline
(B-BA-A-D-R)
Arc
tholeites

12. Island-arc subalkaline series

13. Fresh Andesite, note black color, and fracturing Oregon

14. Andesite, note amp -120 cleavage, biotite - brown, augite green, plag zoned

15. Andesite subhedral phenocryst of plag and pyroxene in fine grained
Matrix

16. Zoned plag in andesite

17. Dacite, with zoned plag, quartz (untwinned), in fine grained matrix

18. Perlitic cracks in rhyolite, magnetite, and alkaline feldspar

19. Rhyolite in glass alkaline phenocrysts with glass inclusions, mag crystals
Perlitic cracks.

20. Flow texture in rhyolite brown color due to devitrification

21. Welded tuff

22. Devitrification in rhyolite, spherulites

23. Island arc alkaline series

24. Trachyte, alkaline felspar, no twinning, in fine matrix, gas vesicles dark patches

25. Trachytic texture (aligned feldspars caused flow in a viscose melt)

26. Trachyte, K-spar untwinned

27. Other Trends Spatial Temporal
“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
Many exceptions to any trend!

28. 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 Data compiled by Terry Plank (Plank and Langmuir, 1988) Earth Planet. Sci. Lett., 90,

29. Sub-series of Calc-Alkaline
K2O is an important discriminator  3 sub-series
Figure 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).
The three andesite series of Gill (1981)
A fourth very high K shoshonite series is rare

30. Figure 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 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.

31. Figure b. AFM diagram distinguishing tholeiitic and calc-alkaline series. Arrows represent differentiation trends within a series.

32. Figure 16-6. c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.

33. Can determine C-A vs. Tholeiite for a particular value of SiO2 (Gill chose 57.5) -> FeO/MgO = ~ 2.3
Figure c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.

34. Similarly, 57. 5 wt. % SiO2 -> K2O at Low-K vs. Med-K = ~ 0
Similarly, 57.5 wt. % SiO2 -> K2O at Low-K vs. Med-K = ~ and Med-K vs. Hi-K = ~ 1.8
Figure c. FeO*/MgO vs. SiO2 diagram distinguishing tholeiitic and calc-alkaline series.

35. 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 Combined K2O - FeO*/MgO diagram in which the Low-K to High-K series are combined with the tholeiitic vs. calc-alkaline 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).

36. Figure 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.

37. Trace elements Decoupling of LIL and HFS (compare OIB)
Nb-Ta « anomaly »
No fractionnation MREE/HREE
Role of fluids (as opposed to unifromally enriched source)
Nb-Ta rich phases in the residuum (Ti-oxides: rutile)
No Garnet in the residuum

38. Volatile rich andesite, Oregon

39. Bombs in Andesite

40. Isotopes
New Britain, Marianas, Aleutians, and South Sandwich volcanics plot within a surprisingly limited range of DM
Figure Nd-Sr isotopic variation in some island arc volcanics. MORB and mantle array from Figures and After Wilson (1989), Arculus and Powell (1986), Gill (1981), and McCulloch et al. (1994). Atlantic sediment data from White et al. (1985).
The principal source of island arc magmas is very similar to MORB source
Although the trace element data still require enriched components
The data for the other arcs extend along 2 enrichment trends, one for the Banda arc and the other for the Lesser Antilles extend beyond the OIB field
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 87Sr/86Sr around and 143Nd/144Nd around 0.715)
The increasing N-S Antilles enrichment probably related to the increasing proximity of the southern end to the South American sediment source of the Amazon

41. 10Be created by cosmic rays + oxygen and nitrogen in upper atmos.
 Earth by precipitation & readily  clay-rich oceanic seds
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
10Be/9Be averages about 5000 x  in the uppermost oceanic sediments
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
9Be is a stable natural isotope & used as a normalization factor

42. Very brief residence time deep in subduction zones
B is a stable element
Very brief residence time deep in subduction zones
B in recent sediments is high ( ppm), but has a greater affinity for altered oceanic crust ( ppm)
In MORB and OIB it rarely exceeds 2-3 ppm
Very brief residence time deep in subduction zone source areas & cycles quickly through to shallow crustal and hydrospheric systems
B concentrations in recent sediments is high ( ppm), but has a greater affinity for altered oceanic crust ( ppm)
In MORB and OIB it rarely exceeds 2-3 ppm

43. 10Be/Betotal vs. B/Betotal diagram (Betotal  9Be since 10Be is so rare)
Figure Be/Be(total) vs. B/Be for six arcs. After Morris (1989) Carnegie Inst. of Washington Yearb., 88,
Each arc studied formed linear arrays, each arc having a unique slope
Also shown are other known reservoirs, including typical mantle (virtually no 10Be or B), hydrated and altered oceanic crust (high B, low 10Be), and young pelagic oceanic sediments (low B and 10Be/Be extending off the diagram up to 2000)
The simplest explanation: each arc represents a mixing line between a mantle reservoir (near the origin) and a fluid (or melt) reservoir, that is specific for each arc and itself a mixture of slab crust and sediment
Hypothetical fields for each arc are illustrated, but the exact location along the extrapolated line is unknown

44. 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

45. Possible sources? Arc crust Mantle Subducted crust
Mantle + subducted fluids
Unlikely (too thin – in island arcs anyway)
Unlikely (solidus too high + role of water)
Possible?

46. 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 Mg- rich andesites directly.
Subducted Crust
What happens to intermediate cases, such as the curve for the 5 Ma old arc?
If dehydration occurs before the subducted material reaches the wet melting point, will it melt?
Depends upon the fate of the water released by dehydration
If it remains in place, melting will occur when the wet solidus is reached
Many workers believe that the water rises and leaves the system, and the remaining crust will no longer have free water, and no melting will occur at the wet solidus
An anhydrous eclogite soon forms
Will it melt, or will the water escape first? This is the subject of great controversy, and models diverge at this point into those that perceive the melting of the oceanic crust (as eclogite plus water) as the dominant source of arc magma, and those that favor the mantle wedge.
3. BUT slab melting occurs (when it occurs) in garnet stability field…
Gt-in

47. Garnet stability in mafic rocks
From a dozen of experimental studies
Well-constrained grt-in line at about kbar

48. 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)
McCulloch (1993) suggests that young, warm subducted crust is more likely to melt before it dehydrates. Thus slab melting may be more common where the a subduction zone is close to a ridge, or during the Archean, when heat production was greater

49. Amphibole-bearing hydrated peridotite should melt at ~ 120 km
Phlogopite-bearing hydrated peridotite should melt at ~ 200 km
 second arc behind first? (K-richer)
Crust and Mantle Wedge
Figure Some calculated P-T-t paths for peridotite in the mantle wedge as it follows a path similar to the flow lines in Figure 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 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.
Areas in which the dehydration curves are crossed by the P-T-t paths below the wet solidus for peridotite are blue and labeled D for dehydration
Areas in which the dehydration curves are crossed above the wet solidus are purple and labeled M for melting.
Note that although the slab crust usually dehydrates, the wedge peridotite melts as amphibole dehydrates above the wet solidus
A second melting may also occur as phlogopite dehydrates in the presence of two pyroxenes.

50. 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, and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford.
Altered oceanic crust begins to dehydrate at depths ~ 50 km or less, as chlorite, phengite, and other hydrous phyllosilicates decompose
Further dehydration takes place at greater depths as other hydrous phases become unstable, including amphibole at about 3 GPa.
The slab crust is successively converted to blueschist, amphibolite, and finally anhydrous eclogite as it reaches about km depth
In most (mature) arcs, the temperature in the subducted crust is below the wet solidus for basalt, so the released water cannot cause melting, and most of the water is believed to rise into the overlying mantle wedge, where it reacts with the lherzolite to form a pargasitic amphibole and probably phlogopite (yellowish area)
Slightly hydrous mantle immediately above the slab is carried downward by induced convective flow where it heats up, dehydrates, and melts at A (120 km)

52. 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
Mass-balance calculations suggest that the contribution of the subducted sediments is only a few percent in most arc systems, but the result is the high LIL/HFS patterns
The nearly closed-cell induced flow in the wedge may result in progressive depletion of the wedge as arc magmas are extracted. This provides an explanation of some HREE and more compatible trace element data, which in many cases is more depleted than MORB

53. 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

54. Rock types Subduction related lavas I-type granitoids
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)

55. Figure 17-9. Relative frequency of rock types in the Andes vs
Figure 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 Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

56. Figure 17-3. AFM and K2O vs. SiO2 diagrams (including Hi-K, Med
Figure AFM and K2O vs. SiO2 diagrams (including Hi-K, Med.-K and Low-K types of Gill, 1981; see Figs 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.

57. Rock types Subduction related lavas I-type granitoids
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)

58. Hornblende granodiorite
Hbl-Biotite granodiorite

60. 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.

61. 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 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.

62. 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?

63. 1) Crustal influence
Figure 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 ; and Harmon et al. (1984) J. Geol. Soc. London, 141, Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

64. Figure MORB-normalized spider diagram (Pearce, 1983) 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; López-Escobar et al. 1981). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

65. Figure 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), deSilva (1991), Kay et al. (1991), Davidson and deSilva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

66. Figure 17-8. 87Sr/86Sr, D7/4, D8/4, and d18O vs
Figure Sr/86Sr, D7/4, D8/4, and d18O vs. Latitude for the Andean volcanics. D7/4 and D8/4 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 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), deSilva (1991), Kay et al. (1991), Davidson and deSilva (1992). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

67. 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

68. 3) Melting of the CC Paired S- and I-types granitic belts
Link with convergence rate (and crust thickness)

69. « paired » I and S type granitic belts in Peru

70. Figure Time-averaged rates of extrusion of mafic (basalt and basaltic andesite), andesitic, and silicic (dacite and rhyolite) volcanics (Priest, 1990, J. Geophys. Res., 95, ) and Juan de Fuca-North American plate convergence rates (Verplanck and Duncan, 1987 Tectonics, 6, ) 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.

71. Figure 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.

72. 4) Melting of basaltic lithologiesKD
Gt/melt =
(other minerals ≤ 1) Yb
Fractionated HREE
Figure 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, Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

73. Garnet stability in mafic rocks
From a dozen of experimental studies
Well-constrained grt-in line at about kbar KD
Gt/melt =
(other minerals ≤ 1) Yb

74. Slab melts or underplated basalts?
Garnet must be present
Most probable: metabasalts (garnet-bearing crustal rocks are metasediments -> granites should be S-types)
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)

75. Partial melting of dioritic gneisses in exhumed arcs (N. Zealand)
Garnet associated with leucosomes (incongruent melting, Hbl + Pg = L + Grt) – Daczo et al. 2001

76. Two stage model
Figure 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

77. 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