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Title: Volcanic Arcs, Chapters 16 and 17


1
Volcanic Arcs, Chapters 16 and 17
2
  • 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.
3
Arcs are
  • Arcuate volcanic chains above subduction zones
  • Distinctly different from mainly basaltic
    provinces thus far
  • Compositions more diverse
  • Basalt generally subordinate
  • More explosive viscous, cool, magmas trap gas
  • Strato-volcanoes most common volcanic landform

4
Chapter 16. Island Arc Magmatism
5
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)
6
Volcanic Rocks of Island Arcs
  • Complex tectonic situation and broad spectrum of
    rock types
  • High proportion of Basaltic - andesite and
    Andesite
  • Most Andesites occur in subduction zone settings

7
Recall Major Magma Series
  • Alkaline series (OIA ocean island alkaline)
  • 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

8
Major Magma Series visualized with Major Elements
  • a. Alkali vs. silica all
  • b. AFM for subalkaline
  • c. FeO/MgO 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.
9
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.
10
Sub-series Calc-Alkaline
  • K2O 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 RKOC76 (Carnegie
Institute of Washington).
11
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.
12
If partition on basis of K versus
Tholeiitic/calc-alkaline, most common samples are
  • 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).
13
Tholeiitic vs. Calc-alkaline differentiationfor
our three examples
Figure 16-6. From Winter (2001) An Introduction
to Igneous and Metamorphic Petrology. Prentice
Hall.
14
Tholeiitic vs. Calc-alkaline differentiationseems
to depend on K
  • C-A shows continually increasing SiO2 and lacks
    dramatic Fe enrichment

High K
15
Calc-alkaline differentiation WHY?
http//www.springerlink.com/content/u383118http//
www.springerlink.com/content/u38311872w004w16/72w0
04w16/
  • 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

16
Other Trends
  • Spatial
  • 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?
  • Temporal
  • Early Tholeiitic ? later C-A and often latest
    alkaline is common

17
Trace Elements
  • REEs
  • 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
18
  • MORB-normalized Spider diagrams
  • IA high LIL (LIL are hydrophilic), low HFS

What is it about subduction zone setting that
causes fluid-assisted enrichment?
HFSHigh Field-strength
Intraplate OIB has similar hump
Most incompatible
Figure 16-11a. 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.
Figure 14-4. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
Data from Sun and McDonough (1989) In A. D.
Saunders and M. J. Norry (eds.), Magmatism in the
Ocean Basins. Geol. Soc. London Spec. Publ., 42.
pp. 313-345.
19
Isotopes
  • 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 87Sr/86Sr around 0.715and
143Nd/144Nd 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 10-15. After Wilson (1989),
Arculus and Powell (1986), Gill (1981), and
McCulloch et al. (1994). Atlantic sediment data
from White et al. (1985).
20
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 (207Pb and 206Pb),
trending toward the appropriate oceanic marine
sedimentary reservoir
Figure 16-13. 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).
21
  • 10Be 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 10Be is no longer detectable.
    9Be is stable, natural.
  • 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

22
  • 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

23
  • 10Be/Betotal vs. B/Betotal diagram (Betotal ?
    9Be since 10Be 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. 10Be/Be(total) vs. B/Be for six
arcs. After Morris (1989) Carnegie Inst. of
Washington Yearb., 88, 111-123.
24
The potential source components ? IA magmas
  • 1. The crustal portion of the subducted slab
  • 1a Altered oceanic crust (hydrated by circulating
    seawater, and metamorphosed in large part to
    greenschist facies)
  • 1b Subducted oceanic and forearc sediments
  • 1c 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, 8309-8319) and mantle flow
lines (yellow- after Tatsumi and Eggins, 1995,
Subduction Zone Magmatism. Blackwell. Oxford).
25
  • Not 1a the subducted basalt fide flat HREEs
  • The trace element and isotopic data suggest that
    both 1b and 1c, 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 the thermal
    regime shown
  • LIL/HFS ratios of arc magmas ? water plays a
    significant role in arc magmatism

26
Freezing Point Depression always occurs in a
mixture
27
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.
28
  • Amphibole-bearing hydrated peridotite should melt
    at 120 km
  • Phlogopite-bearing hydrated peridotite should
    melt at 200 km
  • ? second arc behind first?

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.
29
  • 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
  • The flat HREE pattern argues against a
    garnet-bearing (eclogite) source
  • Thus modern opinion has swung toward a
    non-melting subducted lithosphere slab model for
    most cases of IA genesis

30
Island Arc Petrogenesis Model
Mantle here is too shallow to have Garnet.
Subducted slab turns to Eclogite with Garnet at
110 km.
  • Phlogopite is stable in ultramafic rocks beyond
    the conditions at which amphibole breaks down
  • P-T-t paths for the wedge reach the
    phlogopite-2-pyroxene dehydration reaction at
    about 200 km depth

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.
31
Chapter 17 Continental Arc Magmatism
Figure 17-1. NVZ, CVZ, and SVZ are the northern,
central, and southern volcanic zones.
32
Continental Volcanic Arcs
  • 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

33
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.
34
SVZ has a flat HREE which suggests a shallow
garnet-free source
NVZ and CVZ have a steep slope with depleted HREE
which suggests a deep garnet rich source, (the
garnets dont 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 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.
35
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 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.
36
Assimilation
Recall low 143Nd/144Nd and high 87Sr/86Sr 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), deSilva (1991), Kay et al. (1991),
Davidson and deSilva (1992). Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
37
Andean Pb enrichments are not much greater than
OIBs, and could be derived almost solely from a
subducted sediment
Figure 17-7. 208Pb/204Pb vs. 206Pb/204Pb and
207Pb/204Pb vs. 206Pb/204Pb 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), deSilva (1991), Kay et al. (1991),
Davidson and deSilva (1992). Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
38
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.
39
  • Figure 17-11. Schematic cross sections of a
    volcanic arc showing
  • an initial state followed by
  • trench migration toward the continent resulting
    in a destructive boundary and subduction erosion
    of the overlying crust.
  • 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_P
rinceton.pdf
40
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.
41
  • Hundreds to thousands of individual intrusions
  • The range of volcanics from basalts to rhyolites
    is matched by the plutonics
  • Gabbro -gt diorite -gt tonalite -gt granodiorite -gt
    granite

Q
Quartzolite
90
90
Quartz-rich
Granitoid
60
60
Grano-
Tonalite
Granite
diorite
Alkali Feldspar Granite
20
20
Quartz
Quartz
Quartz
Syenite
Monzonite
Monzodiorite
5
Syenite
Monzodiorite
Monzonite
10
35
65
90
A
P
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.
42
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.
43
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 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.
44
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.
45
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.
46
Lima segment intruded into younger, thinner crust
so radiogenic 87Sr low, reflecting the mantle
derived parent. Arequipa intrudes and
assimilated old thick crust so 87Sr high. Lima
segment has high 206Pb reflecting minor
assimilation of Pacific sediments
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.
47
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.
  • Up-arched mantle results in partial melting and
    underplate gabbros.
  • 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.
48
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.
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