Subduction zone magmatism

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 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.
5
Subduction Products

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

Complexly Interrelated
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
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)
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)

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
fracturingOregon
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
• 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

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 16-3. Data compiled by Terry Plank (Plank
and Langmuir, 1988) Earth Planet. Sci. Lett., 90,
349-370.
29
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).
30
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.
31
Figure 16-6. 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
Figure 16-6. c. FeO/MgO vs. SiO2 diagram
distinguishing tholeiitic and calc-alkaline
series.
34
Figure 16-6. 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 16-5. 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 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.
37
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

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 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).
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 10-11 in the
  uppermost oceanic sediments
• In mantle-derived MORB and OIB magmas,
  continental crust, 10Be is below detection limits
  (lt1 x 106 atom/g) and 10Be/9Be is lt5 x 10-14

42

• 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

43

• 10Be/Betotal vs. B/Betotal diagram (Betotal ?
  9Be since 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.
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
Thermal structure of subduction zones
46
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?
47
Can the subducted slab melt?
48
Figure 16-12. Variation in 207Pb/204Pb vs.
206Pb/204Pb 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 207Pb/204Pb and
206Pb/204Pb in a single-stage homogeneous
reservoir. Data sources listed in Wilson (1989).
49
P-T path along the subducted slab
Subducted Crust
Figure 16-16. Subducted crust pressure-temperature
-time (P-T-t) 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).
50
Subduction zone magmatism(part II)
51
Island arc magmas

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

52
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
53

• Dehydration D and liberation of water takes
  place (mature arcs with lithosphere gt 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
3. BUT slab melting occurs (when it occurs) in
garnet stability field
Gt-in
54
Garnet stability in mafic rocks

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

55

• 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)

56

• 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 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.
57
Thermal structure of subduction zones
58
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.
59
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60

• 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

61
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

62
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)

63
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.
64
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),
López-Escobar et al. (1981), Hörmann and Pichler
(1982). Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
65
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)

66
Hornblende granodiorite
Hbl-Biotite granodiorite
67
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68
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.
69
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.
70
Figure 17-16. Schematic cross section of the
Coastal batholith of Peru. The shallow
flat-topped and steep-sided 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.
71
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?

72
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, 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.
73
Figure 17-5. 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.
74
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), deSilva (1991), Kay et al. (1991),
Davidson and deSilva (1992). Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
75
Figure 17-8. 87Sr/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 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.),
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.
76
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

77
3) Melting of the CC

• Paired S- and I-types granitic belts
• Link with convergence rate (and crust thickness)

78

•  paired  I and S type granitic belts in Peru

79
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.
80
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.
81
But

• Isotopes do not (always) match a purely crustal
  origin
• Cf. Pseudo-S types in the Cordillera Blanca
  Batholith (Pieter)

82
But
Isotopic characteristics of granites from
Peruvian batholith not consistent with a purely
juvenile ( non-crustal) source
Figure 17-19. a. Initial 87Sr/86Sr 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. 207Pb/204Pb vs. 206Pb/204Pb 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.
83
4) Melting of basaltic lithologies
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.
84
Garnet stability in mafic rocks

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

85

• Garnet must be present
• Most probable metabasalts (garnet-bearing
  crustal rocks are metasediments -gt 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)

86
Partial melting of dioritic gneisses in exhumed
arcs (N. Zealand)

• Garnet associated with leucosomes (incongruent
  melting, Hbl Pg L Grt) Daczo et al. 2001

87
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.
88
Possible sources for felsic magmas?

• Ultimate differenciation
• not consistent with isotopes etc.
• would require too many cumulates
• proportions felsic/mafic not right
• Melting of pre-existing granitic/gneissic crust
• Possible if melting of recent crust (isotopes!)
• Something of an  egg and chicken problem 
• Melting of basaltic underplates (cf. Petford
  Atherton paper)

89
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