Transcript Chapter 16. Island Arc Magmatism

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
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
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/cm2/sec)
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
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
A
55
4
49
42
85
D
9
18
12
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
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
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.
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
 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).
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.
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 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).
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 SiO2 and
lacks dramatic Fe
enrichment
High K
Calc-alkaline differentiation WHY?
http://www.springerlink.com/content/u383118http://www.springerlink.com/content/u38311872w004w16/72w004w16/
– 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

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?

Temporal
– Early Tholeiitic  later C-A and often latest
alkaline is common

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

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 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.
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.
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 1015. After Wilson (1989), Arculus
and Powell (1986), Gill (1981),
and McCulloch 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
(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).
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 (<1 x
106 atom/g) and 10Be/9Be 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
10Be/Be
total
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.
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, 83098319) and mantle flow lines
(yellow- after Tatsumi and
Eggins, 1995, Subduction Zone
Magmatism. Blackwell. Oxford).


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
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
upsidedown 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
Phlogopite-bearing hydrated peridotite should melt at ~ 200 km
 second arc behind first?
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
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

Island Arc Petrogenesis Model
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.
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

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
shallow garnet-free source
NVZ and CVZ have a steep slope with
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 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; LópezEscobar 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 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.
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.
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.
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
60
Granite
Granodiorite
20
20
Quartz
Monzonite
Quartz
Syenite
A
10
Syenite
35
Monzonite
Quartz
Monzodiorite
65
Monzodiorite
5
90
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.
Figure 17-15b. Major
plutons of the South
American Cordillera, a
principal segment of a
continuous MesozoicTertiary 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 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.
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.