Transcript LECTURE W14-15-L29-30

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
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
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, AllenUnwin/Kluwer.
Trace elements
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

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).
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
 10
Be/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 (<1 x 106 atom/g) and
10Be/9Be is <5 x 10-14
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
10Be/Be
total
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.
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
Possible sources?

Arc crust
Unlikely (too thin – in island arcs anyway)

Mantle
Unlikely (solidus too high + role of water)

Subducted crust
Possible?

Mantle + subducted fluids
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), LópezEscobar et al. (1981), Hörmann 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.
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,
1982; López-Escobar et al. 1981). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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.
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.
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.
4) Melting of basaltic 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